pipe: Remove assertion from pipe_poll()

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: 8cefc107 ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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) {
@@ -1174,6 +1172,7 @@ 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: