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fs: use acquire ordering in __fget_light() 

[ Upstream commit 7ee47dcfff ]

We must prevent the CPU from reordering the files->count read with the
FD table access like this, on architectures where read-read reordering is
possible:

    files_lookup_fd_raw()
                                  close_fd()
                                  put_files_struct()
    atomic_read(&files->count)

I would like to mark this for stable, but the stable rules explicitly say
"no theoretical races", and given that the FD table pointer and
files->count are explicitly stored in the same cacheline, this sort of
reordering seems quite unlikely in practice...

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Jann Horn 2022-10-31 18:52:56 +01:00 committed by Greg Kroah-Hartman
parent 73374ab8c1
commit d285dee5dc
1 changed files with 10 additions and 1 deletions
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11
fs/file.c
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@ -1002,7 +1002,16 @@ static unsigned long __fget_light(unsigned int fd, fmode_t mask)
struct files_struct *files = current->files;
struct file *file;
if (atomic_read(&files->count) == 1) {
/*
* If another thread is concurrently calling close_fd() followed
* by put_files_struct(), we must not observe the old table
* entry combined with the new refcount - otherwise we could
* return a file that is concurrently being freed.
*
* atomic_read_acquire() pairs with atomic_dec_and_test() in
* put_files_struct().
*/
if (atomic_read_acquire(&files->count) == 1) {
file = files_lookup_fd_raw(files, fd);
if (!file || unlikely(file->f_mode & mask))
return 0;