Fix missing sanity check in /dev/sg

Al Viro · torvalds · commit 137d01df511b · 2017-02-19T09:54:31.000-08:00
What happens is that a write to /dev/sg is given a request with non-zero
-&gt;iovec_count combined with zero -&gt;dxfer_len.  Or with -&gt;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 &lt;viro@zeniv.linux.org.uk&gt;
Reported-by: Dmitry Vyukov &lt;dvyukov@google.com&gt;
Reviewed-by: Christoph Hellwig &lt;hch@lst.de&gt;
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
@@ -1753,6 +1753,10 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 			return res;
 
 		iov_iter_truncate(&i, hp->dxfer_len);
+		if (!iov_iter_count(&i)) {
+			kfree(iov);
+			return -EINVAL;
+		}
 
 		res = blk_rq_map_user_iov(q, rq, md, &i, GFP_ATOMIC);
 		kfree(iov);
