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Subject: [PATCH 1/2] pipe: Remove assertion from pipe_poll() [ver #2]
From:   David Howells <dhowells@redhat.com>
To:     torvalds@linux-foundation.org, ebiggers@kernel.org
Cc:     dhowells@redhat.com, viro@zeniv.linux.org.uk,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Date:   Thu, 05 Dec 2019 22:30:30 +0000
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An assertion check was added to pipe_poll() to make sure that the ring
occupancy isn't seen to overflow the ring size.  However, since no locks
are held when the three values are read, it is possible for F_SETPIPE_SZ to
intervene and muck up the calculation, thereby causing the oops.

Fix this by simply removing the assertion and accepting that the
calculation might be approximate.

Note that the previous code also had a similar issue, though there was no
assertion check, since the occupancy counter and the ring size were not
read with a lock held, so it's possible that the poll check might have
malfunctioned then too.

Also wake up all the waiters so that they can reissue their checks if there
was a competing read or write.

Fixes: 8cefc107ca54 ("pipe: Use head and tail pointers for the ring, not cursor and length")
Reported-by: syzbot+d37abaade33a934f16f2@syzkaller.appspotmail.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Eric Biggers <ebiggers@kernel.org>
---

 fs/pipe.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 648ce440ca85..da782ee251d2 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -579,8 +579,6 @@ pipe_poll(struct file *filp, poll_table *wait)
 
 	poll_wait(filp, &pipe->wait, wait);
 
-	BUG_ON(pipe_occupancy(head, tail) > pipe->ring_size);
-
 	/* Reading only -- no need for acquiring the semaphore.  */
 	mask = 0;
 	if (filp->f_mode & FMODE_READ) {
@@ -1176,6 +1174,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
 	pipe->max_usage = nr_slots;
 	pipe->tail = tail;
 	pipe->head = head;
+	wake_up_interruptible_all(&pipe->wait);
 	return pipe->max_usage * PAGE_SIZE;
 
 out_revert_acct: