804 lines
31 KiB
ReStructuredText
804 lines
31 KiB
ReStructuredText
===========================================
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Control Flow Integrity Design Documentation
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===========================================
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This page documents the design of the :doc:`ControlFlowIntegrity` schemes
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supported by Clang.
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Forward-Edge CFI for Virtual Calls
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==================================
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This scheme works by allocating, for each static type used to make a virtual
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call, a region of read-only storage in the object file holding a bit vector
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that maps onto to the region of storage used for those virtual tables. Each
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set bit in the bit vector corresponds to the `address point`_ for a virtual
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table compatible with the static type for which the bit vector is being built.
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For example, consider the following three C++ classes:
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.. code-block:: c++
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struct A {
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virtual void f1();
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virtual void f2();
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virtual void f3();
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};
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struct B : A {
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virtual void f1();
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virtual void f2();
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virtual void f3();
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};
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struct C : A {
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virtual void f1();
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virtual void f2();
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virtual void f3();
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};
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The scheme will cause the virtual tables for A, B and C to be laid out
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consecutively:
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.. csv-table:: Virtual Table Layout for A, B, C
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:header: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
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A::offset-to-top, &A::rtti, &A::f1, &A::f2, &A::f3, B::offset-to-top, &B::rtti, &B::f1, &B::f2, &B::f3, C::offset-to-top, &C::rtti, &C::f1, &C::f2, &C::f3
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The bit vector for static types A, B and C will look like this:
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.. csv-table:: Bit Vectors for A, B, C
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:header: Class, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
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A, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0
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B, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0
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C, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0
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Bit vectors are represented in the object file as byte arrays. By loading
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from indexed offsets into the byte array and applying a mask, a program can
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test bits from the bit set with a relatively short instruction sequence. Bit
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vectors may overlap so long as they use different bits. For the full details,
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see the `ByteArrayBuilder`_ class.
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In this case, assuming A is laid out at offset 0 in bit 0, B at offset 0 in
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bit 1 and C at offset 0 in bit 2, the byte array would look like this:
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.. code-block:: c++
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char bits[] = { 0, 0, 1, 0, 0, 0, 3, 0, 0, 0, 0, 5, 0, 0 };
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To emit a virtual call, the compiler will assemble code that checks that
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the object's virtual table pointer is in-bounds and aligned and that the
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relevant bit is set in the bit vector.
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For example on x86 a typical virtual call may look like this:
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.. code-block:: none
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ca7fbb: 48 8b 0f mov (%rdi),%rcx
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ca7fbe: 48 8d 15 c3 42 fb 07 lea 0x7fb42c3(%rip),%rdx
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ca7fc5: 48 89 c8 mov %rcx,%rax
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ca7fc8: 48 29 d0 sub %rdx,%rax
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ca7fcb: 48 c1 c0 3d rol $0x3d,%rax
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ca7fcf: 48 3d 7f 01 00 00 cmp $0x17f,%rax
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ca7fd5: 0f 87 36 05 00 00 ja ca8511
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ca7fdb: 48 8d 15 c0 0b f7 06 lea 0x6f70bc0(%rip),%rdx
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ca7fe2: f6 04 10 10 testb $0x10,(%rax,%rdx,1)
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ca7fe6: 0f 84 25 05 00 00 je ca8511
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ca7fec: ff 91 98 00 00 00 callq *0x98(%rcx)
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[...]
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ca8511: 0f 0b ud2
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The compiler relies on co-operation from the linker in order to assemble
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the bit vectors for the whole program. It currently does this using LLVM's
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`type metadata`_ mechanism together with link-time optimization.
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.. _address point: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#vtable-general
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.. _type metadata: https://llvm.org/docs/TypeMetadata.html
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.. _ByteArrayBuilder: https://llvm.org/docs/doxygen/html/structllvm_1_1ByteArrayBuilder.html
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Optimizations
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-------------
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The scheme as described above is the fully general variant of the scheme.
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Most of the time we are able to apply one or more of the following
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optimizations to improve binary size or performance.
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In fact, if you try the above example with the current version of the
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compiler, you will probably find that it will not use the described virtual
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table layout or machine instructions. Some of the optimizations we are about
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to introduce cause the compiler to use a different layout or a different
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sequence of machine instructions.
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Stripping Leading/Trailing Zeros in Bit Vectors
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If a bit vector contains leading or trailing zeros, we can strip them from
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the vector. The compiler will emit code to check if the pointer is in range
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of the region covered by ones, and perform the bit vector check using a
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truncated version of the bit vector. For example, the bit vectors for our
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example class hierarchy will be emitted like this:
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.. csv-table:: Bit Vectors for A, B, C
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:header: Class, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
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A, , , 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, ,
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B, , , , , , , , 1, , , , , , ,
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C, , , , , , , , , , , , , 1, ,
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Short Inline Bit Vectors
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~~~~~~~~~~~~~~~~~~~~~~~~
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If the vector is sufficiently short, we can represent it as an inline constant
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on x86. This saves us a few instructions when reading the correct element
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of the bit vector.
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If the bit vector fits in 32 bits, the code looks like this:
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.. code-block:: none
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dc2: 48 8b 03 mov (%rbx),%rax
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dc5: 48 8d 15 14 1e 00 00 lea 0x1e14(%rip),%rdx
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dcc: 48 89 c1 mov %rax,%rcx
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dcf: 48 29 d1 sub %rdx,%rcx
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dd2: 48 c1 c1 3d rol $0x3d,%rcx
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dd6: 48 83 f9 03 cmp $0x3,%rcx
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dda: 77 2f ja e0b <main+0x9b>
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ddc: ba 09 00 00 00 mov $0x9,%edx
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de1: 0f a3 ca bt %ecx,%edx
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de4: 73 25 jae e0b <main+0x9b>
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de6: 48 89 df mov %rbx,%rdi
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de9: ff 10 callq *(%rax)
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[...]
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e0b: 0f 0b ud2
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Or if the bit vector fits in 64 bits:
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.. code-block:: none
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11a6: 48 8b 03 mov (%rbx),%rax
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11a9: 48 8d 15 d0 28 00 00 lea 0x28d0(%rip),%rdx
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11b0: 48 89 c1 mov %rax,%rcx
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11b3: 48 29 d1 sub %rdx,%rcx
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11b6: 48 c1 c1 3d rol $0x3d,%rcx
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11ba: 48 83 f9 2a cmp $0x2a,%rcx
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11be: 77 35 ja 11f5 <main+0xb5>
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11c0: 48 ba 09 00 00 00 00 movabs $0x40000000009,%rdx
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11c7: 04 00 00
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11ca: 48 0f a3 ca bt %rcx,%rdx
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11ce: 73 25 jae 11f5 <main+0xb5>
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11d0: 48 89 df mov %rbx,%rdi
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11d3: ff 10 callq *(%rax)
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[...]
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11f5: 0f 0b ud2
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If the bit vector consists of a single bit, there is only one possible
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virtual table, and the check can consist of a single equality comparison:
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.. code-block:: none
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9a2: 48 8b 03 mov (%rbx),%rax
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9a5: 48 8d 0d a4 13 00 00 lea 0x13a4(%rip),%rcx
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9ac: 48 39 c8 cmp %rcx,%rax
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9af: 75 25 jne 9d6 <main+0x86>
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9b1: 48 89 df mov %rbx,%rdi
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9b4: ff 10 callq *(%rax)
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[...]
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9d6: 0f 0b ud2
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Virtual Table Layout
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~~~~~~~~~~~~~~~~~~~~
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The compiler lays out classes of disjoint hierarchies in separate regions
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of the object file. At worst, bit vectors in disjoint hierarchies only
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need to cover their disjoint hierarchy. But the closer that classes in
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sub-hierarchies are laid out to each other, the smaller the bit vectors for
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those sub-hierarchies need to be (see "Stripping Leading/Trailing Zeros in Bit
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Vectors" above). The `GlobalLayoutBuilder`_ class is responsible for laying
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out the globals efficiently to minimize the sizes of the underlying bitsets.
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.. _GlobalLayoutBuilder: https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Transforms/IPO/LowerTypeTests.h
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Alignment
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~~~~~~~~~
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If all gaps between address points in a particular bit vector are multiples
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of powers of 2, the compiler can compress the bit vector by strengthening
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the alignment requirements of the virtual table pointer. For example, given
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this class hierarchy:
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.. code-block:: c++
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struct A {
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virtual void f1();
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virtual void f2();
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};
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struct B : A {
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virtual void f1();
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virtual void f2();
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virtual void f3();
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virtual void f4();
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virtual void f5();
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virtual void f6();
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};
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struct C : A {
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virtual void f1();
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virtual void f2();
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};
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The virtual tables will be laid out like this:
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.. csv-table:: Virtual Table Layout for A, B, C
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:header: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
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A::offset-to-top, &A::rtti, &A::f1, &A::f2, B::offset-to-top, &B::rtti, &B::f1, &B::f2, &B::f3, &B::f4, &B::f5, &B::f6, C::offset-to-top, &C::rtti, &C::f1, &C::f2
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Notice that each address point for A is separated by 4 words. This lets us
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emit a compressed bit vector for A that looks like this:
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.. csv-table::
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:header: 2, 6, 10, 14
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1, 1, 0, 1
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At call sites, the compiler will strengthen the alignment requirements by
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using a different rotate count. For example, on a 64-bit machine where the
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address points are 4-word aligned (as in A from our example), the ``rol``
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instruction may look like this:
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.. code-block:: none
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dd2: 48 c1 c1 3b rol $0x3b,%rcx
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Padding to Powers of 2
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~~~~~~~~~~~~~~~~~~~~~~
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Of course, this alignment scheme works best if the address points are
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in fact aligned correctly. To make this more likely to happen, we insert
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padding between virtual tables that in many cases aligns address points to
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a power of 2. Specifically, our padding aligns virtual tables to the next
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highest power of 2 bytes; because address points for specific base classes
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normally appear at fixed offsets within the virtual table, this normally
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has the effect of aligning the address points as well.
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This scheme introduces tradeoffs between decreased space overhead for
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instructions and bit vectors and increased overhead in the form of padding. We
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therefore limit the amount of padding so that we align to no more than 128
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bytes. This number was found experimentally to provide a good tradeoff.
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Eliminating Bit Vector Checks for All-Ones Bit Vectors
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If the bit vector is all ones, the bit vector check is redundant; we simply
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need to check that the address is in range and well aligned. This is more
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likely to occur if the virtual tables are padded.
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Forward-Edge CFI for Virtual Calls by Interleaving Virtual Tables
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-----------------------------------------------------------------
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Dimitar et. al. proposed a novel approach that interleaves virtual tables in [1]_.
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This approach is more efficient in terms of space because padding and bit vectors are no longer needed.
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At the same time, it is also more efficient in terms of performance because in the interleaved layout
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address points of the virtual tables are consecutive, thus the validity check of a virtual
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vtable pointer is always a range check.
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At a high level, the interleaving scheme consists of three steps: 1) split virtual table groups into
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separate virtual tables, 2) order virtual tables by a pre-order traversal of the class hierarchy
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and 3) interleave virtual tables.
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The interleaving scheme implemented in LLVM is inspired by [1]_ but has its own
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enhancements (more in `Interleave virtual tables`_).
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.. [1] `Protecting C++ Dynamic Dispatch Through VTable Interleaving <https://cseweb.ucsd.edu/~lerner/papers/ivtbl-ndss16.pdf>`_. Dimitar Bounov, Rami Gökhan Kıcı, Sorin Lerner.
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Split virtual table groups into separate virtual tables
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The Itanium C++ ABI glues multiple individual virtual tables for a class into a combined virtual table (virtual table group).
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The interleaving scheme, however, can only work with individual virtual tables so it must split the combined virtual tables first.
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In comparison, the old scheme does not require the splitting but it is more efficient when the combined virtual tables have been split.
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The `GlobalSplit`_ pass is responsible for splitting combined virtual tables into individual ones.
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.. _GlobalSplit: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/IPO/GlobalSplit.cpp
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Order virtual tables by a pre-order traversal of the class hierarchy
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This step is common to both the old scheme described above and the interleaving scheme.
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For the interleaving scheme, since the combined virtual tables have been split in the previous step,
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this step ensures that for any class all the compatible virtual tables will appear consecutively.
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For the old scheme, the same property may not hold since it may work on combined virtual tables.
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For example, consider the following four C++ classes:
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.. code-block:: c++
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struct A {
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virtual void f1();
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};
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struct B : A {
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virtual void f1();
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virtual void f2();
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};
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struct C : A {
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virtual void f1();
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virtual void f3();
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};
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struct D : B {
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virtual void f1();
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virtual void f2();
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};
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This step will arrange the virtual tables for A, B, C, and D in the order of *vtable-of-A, vtable-of-B, vtable-of-D, vtable-of-C*.
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Interleave virtual tables
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~~~~~~~~~~~~~~~~~~~~~~~~~
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This step is where the interleaving scheme deviates from the old scheme. Instead of laying out
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whole virtual tables in the previously computed order, the interleaving scheme lays out table
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entries of the virtual tables strategically to ensure the following properties:
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(1) offset-to-top and RTTI fields layout property
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The Itanium C++ ABI specifies that offset-to-top and RTTI fields appear at the offsets behind the
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address point. Note that libraries like libcxxabi do assume this property.
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(2) virtual function entry layout property
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For each virtual function the distance between an virtual table entry for this function and the corresponding
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address point is always the same. This property ensures that dynamic dispatch still works with the interleaving layout.
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Note that the interleaving scheme in the CFI implementation guarantees both properties above whereas the original scheme proposed
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in [1]_ only guarantees the second property.
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To illustrate how the interleaving algorithm works, let us continue with the running example.
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The algorithm first separates all the virtual table entries into two work lists. To do so,
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it starts by allocating two work lists, one initialized with all the offset-to-top entries of virtual tables in the order
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computed in the last step, one initialized with all the RTTI entries in the same order.
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.. csv-table:: Work list 1 Layout
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:header: 0, 1, 2, 3
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A::offset-to-top, B::offset-to-top, D::offset-to-top, C::offset-to-top
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.. csv-table:: Work list 2 layout
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:header: 0, 1, 2, 3,
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&A::rtti, &B::rtti, &D::rtti, &C::rtti
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Then for each virtual function the algorithm goes through all the virtual tables in the previously computed order
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to collect all the related entries into a virtual function list.
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After this step, there are the following virtual function lists:
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.. csv-table:: f1 list
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:header: 0, 1, 2, 3
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&A::f1, &B::f1, &D::f1, &C::f1
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.. csv-table:: f2 list
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:header: 0, 1
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&B::f2, &D::f2
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.. csv-table:: f3 list
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:header: 0
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&C::f3
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Next, the algorithm picks the longest remaining virtual function list and appends the whole list to the shortest work list
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until no function lists are left, and pads the shorter work list so that they are of the same length.
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In the example, f1 list will be first added to work list 1, then f2 list will be added
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to work list 2, and finally f3 list will be added to the work list 2. Since work list 1 now has one more entry than
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work list 2, a padding entry is added to the latter. After this step, the two work lists look like:
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.. csv-table:: Work list 1 Layout
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:header: 0, 1, 2, 3, 4, 5, 6, 7
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A::offset-to-top, B::offset-to-top, D::offset-to-top, C::offset-to-top, &A::f1, &B::f1, &D::f1, &C::f1
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.. csv-table:: Work list 2 layout
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:header: 0, 1, 2, 3, 4, 5, 6, 7
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&A::rtti, &B::rtti, &D::rtti, &C::rtti, &B::f2, &D::f2, &C::f3, padding
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Finally, the algorithm merges the two work lists into the interleaved layout by alternatingly
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moving the head of each list to the final layout. After this step, the final interleaved layout looks like:
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.. csv-table:: Interleaved layout
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:header: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
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A::offset-to-top, &A::rtti, B::offset-to-top, &B::rtti, D::offset-to-top, &D::rtti, C::offset-to-top, &C::rtti, &A::f1, &B::f2, &B::f1, &D::f2, &D::f1, &C::f3, &C::f1, padding
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In the above interleaved layout, each virtual table's offset-to-top and RTTI are always adjacent, which shows that the layout has the first property.
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For the second property, let us look at f2 as an example. In the interleaved layout,
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there are two entries for f2: B::f2 and D::f2. The distance between &B::f2
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and its address point D::offset-to-top (the entry immediately after &B::rtti) is 5 entry-length, so is the distance between &D::f2 and C::offset-to-top (the entry immediately after &D::rtti).
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Forward-Edge CFI for Indirect Function Calls
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============================================
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Under forward-edge CFI for indirect function calls, each unique function
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type has its own bit vector, and at each call site we need to check that the
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function pointer is a member of the function type's bit vector. This scheme
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works in a similar way to forward-edge CFI for virtual calls, the distinction
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being that we need to build bit vectors of function entry points rather than
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of virtual tables.
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Unlike when re-arranging global variables, we cannot re-arrange functions
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in a particular order and base our calculations on the layout of the
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functions' entry points, as we have no idea how large a particular function
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will end up being (the function sizes could even depend on how we arrange
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the functions). Instead, we build a jump table, which is a block of code
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consisting of one branch instruction for each of the functions in the bit
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set that branches to the target function, and redirect any taken function
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addresses to the corresponding jump table entry. In this way, the distance
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between function entry points is predictable and controllable. In the object
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file's symbol table, the symbols for the target functions also refer to the
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jump table entries, so that addresses taken outside the module will pass
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any verification done inside the module.
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In more concrete terms, suppose we have three functions ``f``, ``g``,
|
||
``h`` which are all of the same type, and a function foo that returns their
|
||
addresses:
|
||
|
||
.. code-block:: none
|
||
|
||
f:
|
||
mov 0, %eax
|
||
ret
|
||
|
||
g:
|
||
mov 1, %eax
|
||
ret
|
||
|
||
h:
|
||
mov 2, %eax
|
||
ret
|
||
|
||
foo:
|
||
mov f, %eax
|
||
mov g, %edx
|
||
mov h, %ecx
|
||
ret
|
||
|
||
Our jump table will (conceptually) look like this:
|
||
|
||
.. code-block:: none
|
||
|
||
f:
|
||
jmp .Ltmp0 ; 5 bytes
|
||
int3 ; 1 byte
|
||
int3 ; 1 byte
|
||
int3 ; 1 byte
|
||
|
||
g:
|
||
jmp .Ltmp1 ; 5 bytes
|
||
int3 ; 1 byte
|
||
int3 ; 1 byte
|
||
int3 ; 1 byte
|
||
|
||
h:
|
||
jmp .Ltmp2 ; 5 bytes
|
||
int3 ; 1 byte
|
||
int3 ; 1 byte
|
||
int3 ; 1 byte
|
||
|
||
.Ltmp0:
|
||
mov 0, %eax
|
||
ret
|
||
|
||
.Ltmp1:
|
||
mov 1, %eax
|
||
ret
|
||
|
||
.Ltmp2:
|
||
mov 2, %eax
|
||
ret
|
||
|
||
foo:
|
||
mov f, %eax
|
||
mov g, %edx
|
||
mov h, %ecx
|
||
ret
|
||
|
||
Because the addresses of ``f``, ``g``, ``h`` are evenly spaced at a power of
|
||
2, and function types do not overlap (unlike class types with base classes),
|
||
we can normally apply the `Alignment`_ and `Eliminating Bit Vector Checks
|
||
for All-Ones Bit Vectors`_ optimizations thus simplifying the check at each
|
||
call site to a range and alignment check.
|
||
|
||
Shared library support
|
||
======================
|
||
|
||
**EXPERIMENTAL**
|
||
|
||
The basic CFI mode described above assumes that the application is a
|
||
monolithic binary; at least that all possible virtual/indirect call
|
||
targets and the entire class hierarchy are known at link time. The
|
||
cross-DSO mode, enabled with **-f[no-]sanitize-cfi-cross-dso** relaxes
|
||
this requirement by allowing virtual and indirect calls to cross the
|
||
DSO boundary.
|
||
|
||
Assuming the following setup: the binary consists of several
|
||
instrumented and several uninstrumented DSOs. Some of them may be
|
||
dlopen-ed/dlclose-d periodically, even frequently.
|
||
|
||
- Calls made from uninstrumented DSOs are not checked and just work.
|
||
- Calls inside any instrumented DSO are fully protected.
|
||
- Calls between different instrumented DSOs are also protected, with
|
||
a performance penalty (in addition to the monolithic CFI
|
||
overhead).
|
||
- Calls from an instrumented DSO to an uninstrumented one are
|
||
unchecked and just work, with performance penalty.
|
||
- Calls from an instrumented DSO outside of any known DSO are
|
||
detected as CFI violations.
|
||
|
||
In the monolithic scheme a call site is instrumented as
|
||
|
||
.. code-block:: none
|
||
|
||
if (!InlinedFastCheck(f))
|
||
abort();
|
||
call *f
|
||
|
||
In the cross-DSO scheme it becomes
|
||
|
||
.. code-block:: none
|
||
|
||
if (!InlinedFastCheck(f))
|
||
__cfi_slowpath(CallSiteTypeId, f);
|
||
call *f
|
||
|
||
CallSiteTypeId
|
||
--------------
|
||
|
||
``CallSiteTypeId`` is a stable process-wide identifier of the
|
||
call-site type. For a virtual call site, the type in question is the class
|
||
type; for an indirect function call it is the function signature. The
|
||
mapping from a type to an identifier is an ABI detail. In the current,
|
||
experimental, implementation the identifier of type T is calculated as
|
||
follows:
|
||
|
||
- Obtain the mangled name for "typeinfo name for T".
|
||
- Calculate MD5 hash of the name as a string.
|
||
- Reinterpret the first 8 bytes of the hash as a little-endian
|
||
64-bit integer.
|
||
|
||
It is possible, but unlikely, that collisions in the
|
||
``CallSiteTypeId`` hashing will result in weaker CFI checks that would
|
||
still be conservatively correct.
|
||
|
||
CFI_Check
|
||
---------
|
||
|
||
In the general case, only the target DSO knows whether the call to
|
||
function ``f`` with type ``CallSiteTypeId`` is valid or not. To
|
||
export this information, every DSO implements
|
||
|
||
.. code-block:: none
|
||
|
||
void __cfi_check(uint64 CallSiteTypeId, void *TargetAddr, void *DiagData)
|
||
|
||
This function provides external modules with access to CFI checks for
|
||
the targets inside this DSO. For each known ``CallSiteTypeId``, this
|
||
function performs an ``llvm.type.test`` with the corresponding type
|
||
identifier. It reports an error if the type is unknown, or if the
|
||
check fails. Depending on the values of compiler flags
|
||
``-fsanitize-trap`` and ``-fsanitize-recover``, this function may
|
||
print an error, abort and/or return to the caller. ``DiagData`` is an
|
||
opaque pointer to the diagnostic information about the error, or
|
||
``null`` if the caller does not provide this information.
|
||
|
||
The basic implementation is a large switch statement over all values
|
||
of CallSiteTypeId supported by this DSO, and each case is similar to
|
||
the InlinedFastCheck() in the basic CFI mode.
|
||
|
||
CFI Shadow
|
||
----------
|
||
|
||
To route CFI checks to the target DSO's __cfi_check function, a
|
||
mapping from possible virtual / indirect call targets to the
|
||
corresponding __cfi_check functions is maintained. This mapping is
|
||
implemented as a sparse array of 2 bytes for every possible page (4096
|
||
bytes) of memory. The table is kept readonly most of the time.
|
||
|
||
There are 3 types of shadow values:
|
||
|
||
- Address in a CFI-instrumented DSO.
|
||
- Unchecked address (a “trusted” non-instrumented DSO). Encoded as
|
||
value 0xFFFF.
|
||
- Invalid address (everything else). Encoded as value 0.
|
||
|
||
For a CFI-instrumented DSO, a shadow value encodes the address of the
|
||
__cfi_check function for all call targets in the corresponding memory
|
||
page. If Addr is the target address, and V is the shadow value, then
|
||
the address of __cfi_check is calculated as
|
||
|
||
.. code-block:: none
|
||
|
||
__cfi_check = AlignUpTo(Addr, 4096) - (V + 1) * 4096
|
||
|
||
This works as long as __cfi_check is aligned by 4096 bytes and located
|
||
below any call targets in its DSO, but not more than 256MB apart from
|
||
them.
|
||
|
||
CFI_SlowPath
|
||
------------
|
||
|
||
The slow path check is implemented in a runtime support library as
|
||
|
||
.. code-block:: none
|
||
|
||
void __cfi_slowpath(uint64 CallSiteTypeId, void *TargetAddr)
|
||
void __cfi_slowpath_diag(uint64 CallSiteTypeId, void *TargetAddr, void *DiagData)
|
||
|
||
These functions loads a shadow value for ``TargetAddr``, finds the
|
||
address of ``__cfi_check`` as described above and calls
|
||
that. ``DiagData`` is an opaque pointer to diagnostic data which is
|
||
passed verbatim to ``__cfi_check``, and ``__cfi_slowpath`` passes
|
||
``nullptr`` instead.
|
||
|
||
Compiler-RT library contains reference implementations of slowpath
|
||
functions, but they have unresolvable issues with correctness and
|
||
performance in the handling of dlopen(). It is recommended that
|
||
platforms provide their own implementations, usually as part of libc
|
||
or libdl.
|
||
|
||
Position-independent executable requirement
|
||
-------------------------------------------
|
||
|
||
Cross-DSO CFI mode requires that the main executable is built as PIE.
|
||
In non-PIE executables the address of an external function (taken from
|
||
the main executable) is the address of that function’s PLT record in
|
||
the main executable. This would break the CFI checks.
|
||
|
||
Backward-edge CFI for return statements (RCFI)
|
||
==============================================
|
||
|
||
This section is a proposal. As of March 2017 it is not implemented.
|
||
|
||
Backward-edge control flow (`RET` instructions) can be hijacked
|
||
via overwriting the return address (`RA`) on stack.
|
||
Various mitigation techniques (e.g. `SafeStack`_, `RFG`_, `Intel CET`_)
|
||
try to detect or prevent `RA` corruption on stack.
|
||
|
||
RCFI enforces the expected control flow in several different ways described below.
|
||
RCFI heavily relies on LTO.
|
||
|
||
Leaf Functions
|
||
--------------
|
||
If `f()` is a leaf function (i.e. it has no calls
|
||
except maybe no-return calls) it can be called using a special calling convention
|
||
that stores `RA` in a dedicated register `R` before the `CALL` instruction.
|
||
`f()` does not spill `R` and does not use the `RET` instruction,
|
||
instead it uses the value in `R` to `JMP` to `RA`.
|
||
|
||
This flavour of CFI is *precise*, i.e. the function is guaranteed to return
|
||
to the point exactly following the call.
|
||
|
||
An alternative approach is to
|
||
copy `RA` from stack to `R` in the first instruction of `f()`,
|
||
then `JMP` to `R`.
|
||
This approach is simpler to implement (does not require changing the caller)
|
||
but weaker (there is a small window when `RA` is actually stored on stack).
|
||
|
||
|
||
Functions called once
|
||
---------------------
|
||
Suppose `f()` is called in just one place in the program
|
||
(assuming we can verify this in LTO mode).
|
||
In this case we can replace the `RET` instruction with a `JMP` instruction
|
||
with the immediate constant for `RA`.
|
||
This will *precisely* enforce the return control flow no matter what is stored on stack.
|
||
|
||
Another variant is to compare `RA` on stack with the known constant and abort
|
||
if they don't match; then `JMP` to the known constant address.
|
||
|
||
Functions called in a small number of call sites
|
||
------------------------------------------------
|
||
We may extend the above approach to cases where `f()`
|
||
is called more than once (but still a small number of times).
|
||
With LTO we know all possible values of `RA` and we check them
|
||
one-by-one (or using binary search) against the value on stack.
|
||
If the match is found, we `JMP` to the known constant address, otherwise abort.
|
||
|
||
This protection is *near-precise*, i.e. it guarantees that the control flow will
|
||
be transferred to one of the valid return addresses for this function,
|
||
but not necessary to the point of the most recent `CALL`.
|
||
|
||
General case
|
||
------------
|
||
For functions called multiple times a *return jump table* is constructed
|
||
in the same manner as jump tables for indirect function calls (see above).
|
||
The correct jump table entry (or its index) is passed by `CALL` to `f()`
|
||
(as an extra argument) and then spilled to stack.
|
||
The `RET` instruction is replaced with a load of the jump table entry,
|
||
jump table range check, and `JMP` to the jump table entry.
|
||
|
||
This protection is also *near-precise*.
|
||
|
||
Returns from functions called indirectly
|
||
----------------------------------------
|
||
|
||
If a function is called indirectly, the return jump table is constructed for the
|
||
equivalence class of functions instead of a single function.
|
||
|
||
Cross-DSO calls
|
||
---------------
|
||
Consider two instrumented DSOs, `A` and `B`. `A` defines `f()` and `B` calls it.
|
||
|
||
This case will be handled similarly to the cross-DSO scheme using the slow path callback.
|
||
|
||
Non-goals
|
||
---------
|
||
|
||
RCFI does not protect `RET` instructions:
|
||
* in non-instrumented DSOs,
|
||
* in instrumented DSOs for functions that are called from non-instrumented DSOs,
|
||
* embedded into other instructions (e.g. `0f4fc3 cmovg %ebx,%eax`).
|
||
|
||
.. _SafeStack: https://clang.llvm.org/docs/SafeStack.html
|
||
.. _RFG: https://xlab.tencent.com/en/2016/11/02/return-flow-guard
|
||
.. _Intel CET: https://software.intel.com/en-us/blogs/2016/06/09/intel-release-new-technology-specifications-protect-rop-attacks
|
||
|
||
Hardware support
|
||
================
|
||
|
||
We believe that the above design can be efficiently implemented in hardware.
|
||
A single new instruction added to an ISA would allow to perform the forward-edge CFI check
|
||
with fewer bytes per check (smaller code size overhead) and potentially more
|
||
efficiently. The current software-only instrumentation requires at least
|
||
32-bytes per check (on x86_64).
|
||
A hardware instruction may probably be less than ~ 12 bytes.
|
||
Such instruction would check that the argument pointer is in-bounds,
|
||
and is properly aligned, and if the checks fail it will either trap (in monolithic scheme)
|
||
or call the slow path function (cross-DSO scheme).
|
||
The bit vector lookup is probably too complex for a hardware implementation.
|
||
|
||
.. code-block:: none
|
||
|
||
// This instruction checks that 'Ptr'
|
||
// * is aligned by (1 << kAlignment) and
|
||
// * is inside [kRangeBeg, kRangeBeg+(kRangeSize<<kAlignment))
|
||
// and if the check fails it jumps to the given target (slow path).
|
||
//
|
||
// 'Ptr' is a register, pointing to the virtual function table
|
||
// or to the function which we need to check. We may require an explicit
|
||
// fixed register to be used.
|
||
// 'kAlignment' is a 4-bit constant.
|
||
// 'kRangeSize' is a ~20-bit constant.
|
||
// 'kRangeBeg' is a PC-relative constant (~28 bits)
|
||
// pointing to the beginning of the allowed range for 'Ptr'.
|
||
// 'kFailedCheckTarget': is a PC-relative constant (~28 bits)
|
||
// representing the target to branch to when the check fails.
|
||
// If kFailedCheckTarget==0, the process will trap
|
||
// (monolithic binary scheme).
|
||
// Otherwise it will jump to a handler that implements `CFI_SlowPath`
|
||
// (cross-DSO scheme).
|
||
CFI_Check(Ptr, kAlignment, kRangeSize, kRangeBeg, kFailedCheckTarget) {
|
||
if (Ptr < kRangeBeg ||
|
||
Ptr >= kRangeBeg + (kRangeSize << kAlignment) ||
|
||
Ptr & ((1 << kAlignment) - 1))
|
||
Jump(kFailedCheckTarget);
|
||
}
|
||
|
||
An alternative and more compact encoding would not use `kFailedCheckTarget`,
|
||
and will trap on check failure instead.
|
||
This will allow us to fit the instruction into **8-9 bytes**.
|
||
The cross-DSO checks will be performed by a trap handler and
|
||
performance-critical ones will have to be black-listed and checked using the
|
||
software-only scheme.
|
||
|
||
Note that such hardware extension would be complementary to checks
|
||
at the callee side, such as e.g. **Intel ENDBRANCH**.
|
||
Moreover, CFI would have two benefits over ENDBRANCH: a) precision and b)
|
||
ability to protect against invalid casts between polymorphic types.
|