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fs: consistently deref the files table with rcu_dereference_raw() 

[ Upstream commit f381640e1b ]

... except when the table is known to be only used by one thread.

A file pointer can get installed at any moment despite the ->file_lock
being held since the following:
8a81252b77 ("fs/file.c: don't acquire files->file_lock in fd_install()")

Accesses subject to such a race can in principle suffer load tearing.

While here redo the comment in dup_fd -- it only covered a race against
files showing up, still assuming fd_install() takes the lock.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250313135725.1320914-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 323960a187a9622d9006484ff6635680d3a0118f)
This commit is contained in:
Mateusz Guzik 2025-03-13 14:57:25 +01:00 committed by Wentao Guan
parent 83017988bd
commit 52538be9b3
1 changed files with 17 additions and 9 deletions
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26
fs/file.c
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@ -364,17 +364,25 @@ struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_ho
old_fds = old_fdt->fd;
new_fds = new_fdt->fd;
/*
* We may be racing against fd allocation from other threads using this
* files_struct, despite holding ->file_lock.
*
* alloc_fd() might have already claimed a slot, while fd_install()
* did not populate it yet. Note the latter operates locklessly, so
* the file can show up as we are walking the array below.
*
* At the same time we know no files will disappear as all other
* operations take the lock.
*
* Instead of trying to placate userspace racing with itself, we
* ref the file if we see it and mark the fd slot as unused otherwise.
*/
for (i = open_files; i != 0; i--) {
struct file *f = *old_fds++;
struct file *f = rcu_dereference_raw(*old_fds++);
if (f) {
get_file(f);
} else {
/*
* The fd may be claimed in the fd bitmap but not yet
* instantiated in the files array if a sibling thread
* is partway through open(). So make sure that this
* fd is available to the new process.
*/
__clear_open_fd(open_files - i, new_fdt);
}
rcu_assign_pointer(*new_fds++, f);
@ -631,7 +639,7 @@ static struct file *pick_file(struct files_struct *files, unsigned fd)
return NULL;
fd = array_index_nospec(fd, fdt->max_fds);
file = fdt->fd[fd];
file = rcu_dereference_raw(fdt->fd[fd]);
if (file) {
rcu_assign_pointer(fdt->fd[fd], NULL);
__put_unused_fd(files, fd);
@ -1102,7 +1110,7 @@ __releases(&files->file_lock)
*/
fdt = files_fdtable(files);
fd = array_index_nospec(fd, fdt->max_fds);
tofree = fdt->fd[fd];
tofree = rcu_dereference_raw(fdt->fd[fd]);
if (!tofree && fd_is_open(fd, fdt))
goto Ebusy;
get_file(file);