fs: use acquire ordering in __fget_light()

[ 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>

diff --git a/fs/file.c b/fs/file.c
index ee931734670257393aa9ee1383596e233c69d05b..214364e19d76f45b87509eb945a93d759fab8570 100644 (file)
--- a/fs/file.c
+++ b/fs/file.c
@@ -1029,7 +1029,16 @@ 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;