[PATCH 5.15 099/110] watch_queue: Fix lack of barrier/sync/lock between post and read

Greg Kroah-Hartman
2022-03-14T12:20:44Z

From: David Howells <dhowells@redhat.com>

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: c73be61cede5 ("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>
---
fs/pipe.c | 3 ++-
kernel/watch_queue.c | 2 +-
2 files changed, 3 insertions(+), 2 deletions(-)

--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -252,7 +252,8 @@ pipe_read(struct kiocb *iocb, struct iov
*/
was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
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 mask = pipe->ring_size - 1;

--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -113,7 +113,7 @@ static bool post_one_notification(struct
buf->offset = offset;
buf->len = len;
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)) {
spin_unlock_irq(&pipe->rd_wait.lock);