From: Jann Horn jannh@google.com
[ Upstream commit 7ee47dcfff1835ff75a794d1075b6b5f5462cfed ]
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 --- fs/file.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/fs/file.c b/fs/file.c index 3bcc1ecc314a..57af5f8375fd 100644 --- a/fs/file.c +++ b/fs/file.c @@ -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;