hw/net/net_tx_pkt: fix assertion failure in net_tx_pkt_add_raw_fragment()

Git-commit: 035e69b063
References: bsc#1174641, CVE-2020-16092

An assertion failure issue was found in the code that processes network packets
while adding data fragments into the packet context. It could be abused by a
malicious guest to abort the QEMU process on the host. This patch replaces the
affected assert() with a conditional statement, returning false if the current
data fragment exceeds max_raw_frags.

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Reported-by: Ziming Zhang <ezrakiez@gmail.com>
Reviewed-by: Dmitry Fleytman <dmitry.fleytman@gmail.com>
Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Bruce Rogers <brogers@suse.com>

hw/net/net_tx_pkt.c

@@ -379,7 +379,10 @@ bool net_tx_pkt_add_raw_fragment(struct NetTxPkt *pkt, hwaddr pa,
     hwaddr mapped_len = 0;
     struct iovec *ventry;
     assert(pkt);
-    assert(pkt->max_raw_frags > pkt->raw_frags);
+
+    if (pkt->raw_frags >= pkt->max_raw_frags) {
+        return false;
+    }
 
     if (!len) {
         return true;