tap: Don't attempt to carry on if we get a bad frame length from qemu

If we receive a too-short or too-long frame from the QEMU socket, currently
we try to skip it and carry on.  That sounds sensible on first blush, but
probably isn't wise in practice.  If this happens, either (a) qemu has done
something seriously unexpected, or (b) we've received corrupt data over a
Unix socket.  Or more likely (c), we have a bug elswhere which has put us
out of sync with the stream, so we're trying to read something that's not a
frame length as a frame length.

Neither (b) nor (c) is really salvageable with the same stream.  Case (a)
might be ok, but we can no longer be confident qemu won't do something else
we can't cope with.

So, instead of just skipping the frame and trying to carry on, log an error
and close the socket.  As a bonus, establishing firm bounds on l2len early
will allow simplifications to how we deal with the case where a partial
frame is recv()ed.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Change error message: it's not necessarily QEMU, and mention
 that we are resetting the connection]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 7 insertions, 9 deletions

diff --git a/tap.c b/tap.c
index ff2b668..18aad9a 100644
--- a/tap.c
+++ b/tap.c
@@ -1013,7 +1013,13 @@ redo:
 	}
 
 	while (n > (ssize_t)sizeof(uint32_t)) {
-		ssize_t l2len = ntohl(*(uint32_t *)p);
+		uint32_t l2len = ntohl(*(uint32_t *)p);
+
+		if (l2len < sizeof(struct ethhdr) || l2len > ETH_MAX_MTU) {
+			err("Bad frame size from guest, resetting connection");
+			tap_sock_reset(c);
+			return;
+		}
 
 		p += sizeof(uint32_t);
 		n -= sizeof(uint32_t);
@@ -1027,16 +1033,8 @@ redo:
 				return;
 		}
 
-		/* Complete the partial read above before discarding a malformed
-		 * frame, otherwise the stream will be inconsistent.
-		 */
-		if (l2len < (ssize_t)sizeof(struct ethhdr) ||
-		    l2len > (ssize_t)ETH_MAX_MTU)
-			goto next;
-
 		tap_add_packet(c, l2len, p);
 
-next:
 		p += l2len;
 		n -= l2len;
 	}