[PATCH 4/4] tap: Drop frames from guest whose length is more than remaining buffer

[Stefano Brivio <sbrivio@redhat.com>]

This was reported by Matej a while ago, but we forgot to fix it. Even
if the hypervisor is necessarily trusted by passt, as it can in any
case terminate the guest or disrupt guest connectivity, it's a good
idea to be robust against possible issues.

Instead of resetting the connection to the hypervisor, just skip the
offending frame, as we had a few cases where QEMU would get the
length descriptor wrong, in the past.

Reported-by: Matej Hrica <mhrica@redhat.com>
Suggested-by: Matej Hrica <mhrica@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tap.c b/tap.c
index 7f8c26d..bb993e0 100644
--- a/tap.c
+++ b/tap.c
@@ -1026,6 +1026,9 @@ redo:
 		p += sizeof(uint32_t);
 		n -= sizeof(uint32_t);
 
+		if (l2len > (ssize_t)TAP_BUF_BYTES - n)
+			goto next;
+
 		/* At most one packet might not fit in a single read, and this
 		 * needs to be blocking.
 		 */
-- 
@@ -1026,6 +1026,9 @@ redo:
 		p += sizeof(uint32_t);
 		n -= sizeof(uint32_t);
 
+		if (l2len > (ssize_t)TAP_BUF_BYTES - n)
+			goto next;
+
 		/* At most one packet might not fit in a single read, and this
 		 * needs to be blocking.
 		 */
-- 
2.43.0