watch_queue: Fix lack of barrier/sync/lock between post and read

commit 2ed147f015af2b48f41c6f0b6746aa9ea85c19f3 upstream.

There's nothing to synchronise post_one_notification() versus
pipe_read().  Whilst posting is done under pipe->rd_wait.lock, the
reader only takes pipe->mutex which cannot bar notification posting as
that may need to be made from contexts that cannot sleep.

Fix this by setting pipe->head with a barrier in post_one_notification()
and reading pipe->head with a barrier in pipe_read().

If that's not sufficient, the rd_wait.lock will need to be taken,
possibly in a ->confirm() op so that it only applies to notifications.
The lock would, however, have to be dropped before copy_page_to_iter()
is invoked.

Fixes: c73be61ced ("pipe: Add general notification queue support")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
David Howells 2022-03-11 13:24:36 +00:00 committed by Greg Kroah-Hartman
parent 82ff8a2243
commit eb38c2e9fc
2 changed files with 3 additions and 2 deletions

View File

@ -252,7 +252,8 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
*/ */
was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage); was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
for (;;) { for (;;) {
unsigned int head = pipe->head; /* Read ->head with a barrier vs post_one_notification() */
unsigned int head = smp_load_acquire(&pipe->head);
unsigned int tail = pipe->tail; unsigned int tail = pipe->tail;
unsigned int mask = pipe->ring_size - 1; unsigned int mask = pipe->ring_size - 1;

View File

@ -113,7 +113,7 @@ static bool post_one_notification(struct watch_queue *wqueue,
buf->offset = offset; buf->offset = offset;
buf->len = len; buf->len = len;
buf->flags = PIPE_BUF_FLAG_WHOLE; buf->flags = PIPE_BUF_FLAG_WHOLE;
pipe->head = head + 1; smp_store_release(&pipe->head, head + 1); /* vs pipe_read() */
if (!test_and_clear_bit(note, wqueue->notes_bitmap)) { if (!test_and_clear_bit(note, wqueue->notes_bitmap)) {
spin_unlock_irq(&pipe->rd_wait.lock); spin_unlock_irq(&pipe->rd_wait.lock);