Fix missing sanity check in /dev/sg

commit 137d01df upstream.

What happens is that a write to /dev/sg is given a request with non-zero
->iovec_count combined with zero ->dxfer_len.  Or with ->dxferp pointing
to an array full of empty iovecs.

Having write permission to /dev/sg shouldn't be equivalent to the
ability to trigger BUG_ON() while holding spinlocks...

Found by Dmitry Vyukov and syzkaller.

[ The BUG_ON() got changed to a WARN_ON_ONCE(), but this fixes the
  underlying issue.  - Linus ]
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.16: we're not using iov_iter, but can check the
 byte length after truncation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/scsi/sg.c

@@ -1716,6 +1716,10 @@ static int sg_start_req(Sg_request *srp, unsigned char *cmd)
 			iov_count = iov_shorten(iov, iov_count, hp->dxfer_len);
 			len = hp->dxfer_len;
 		}
+		if (len == 0) {
+			kfree(iov);
+			return -EINVAL;
+		}
 
 		res = blk_rq_map_user_iov(q, rq, md, (struct sg_iovec *)iov,
 					  iov_count,