tree 7238cca533c6f9744831b509d3258b7c70bf7a43
parent 06ab8444392acdbffb57869d6220fb6654a8c95e
author David Howells <dhowells@redhat.com> 1647005076 +0000
committer Greg Kroah-Hartman <gregkh@linuxfoundation.org> 1647436562 +0100

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: 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>