[PATCH 2/5] tap: Don't attempt to carry on if we get a bad frame length from qemu

[David Gibson <david@gibson.dropbear.id.au>]

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>
---
 tap.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/tap.c b/tap.c
index a2ae7c2a..08700f65 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("Invalid frame size from QEMU (corrupt stream?)");
+			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;
 	}
-- 
@@ -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("Invalid frame size from QEMU (corrupt stream?)");
+			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;
 	}
-- 
2.45.2