pipe: Fix bogus dereference in iov_iter_alignment()

We cannot look at 'i->pipe' unless we know the iter is a pipe. Move the
ring_size load to a branch in iov_iter_alignment() where we've already
checked the iter is a pipe to avoid bogus dereference.

Reported-by: syzbot+bea68382bae9490e7dd6@syzkaller.appspotmail.com
Fixes: 8cefc10 ("pipe: Use head and tail pointers for the ring, not cursor and length")
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -1222,11 +1222,12 @@ EXPORT_SYMBOL(iov_iter_discard);
 
 unsigned long iov_iter_alignment(const struct iov_iter *i)
 {
-	unsigned int p_mask = i->pipe->ring_size - 1;
 	unsigned long res = 0;
 	size_t size = i->count;
 
 	if (unlikely(iov_iter_is_pipe(i))) {
+		unsigned int p_mask = i->pipe->ring_size - 1;
+
 		if (size && i->iov_offset && allocated(&i->pipe->bufs[i->head & p_mask]))
 			return size | i->iov_offset;
 		return size;