rtl8139: avoid nested ifs in IP header parsing (CVE-2015-5165)

Transmit offload needs to parse packet headers.  If header fields have
unexpected values the offload processing is skipped.

The code currently uses nested ifs because there is relatively little
input validation.  The next patches will add missing input validation
and a goto label is more appropriate to avoid deep if statement nesting.

Reported-by: 朱东海(启路) <donghai.zdh@alibaba-inc.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c
index e0db4727ae650c8939b79c83f6a3768c463b0db0..8731a30b714937f760c601f94b281455fc8dd073 100644 (file)
--- a/hw/net/rtl8139.c
+++ b/hw/net/rtl8139.c
@@ -2160,28 +2160,30 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s)
             size_t   eth_payload_len  = 0;
 
             int proto = be16_to_cpu(*(uint16_t *)(saved_buffer + 12));
-            if (proto == ETH_P_IP)
+            if (proto != ETH_P_IP)
             {
-                DPRINTF("+++ C+ mode has IP packet\n");
-
-                /* not aligned */
-                eth_payload_data = saved_buffer + ETH_HLEN;
-                eth_payload_len  = saved_size   - ETH_HLEN;
-
-                ip = (ip_header*)eth_payload_data;
-
-                if (IP_HEADER_VERSION(ip) != IP_HEADER_VERSION_4) {
-                    DPRINTF("+++ C+ mode packet has bad IP version %d "
-                        "expected %d\n", IP_HEADER_VERSION(ip),
-                        IP_HEADER_VERSION_4);
-                    ip = NULL;
-                } else {
-                    hlen = IP_HEADER_LENGTH(ip);
-                    ip_protocol = ip->ip_p;
-                    ip_data_len = be16_to_cpu(ip->ip_len) - hlen;
-                }
+                goto skip_offload;
             }
 
+            DPRINTF("+++ C+ mode has IP packet\n");
+
+            /* not aligned */
+            eth_payload_data = saved_buffer + ETH_HLEN;
+            eth_payload_len  = saved_size   - ETH_HLEN;
+
+            ip = (ip_header*)eth_payload_data;
+
+            if (IP_HEADER_VERSION(ip) != IP_HEADER_VERSION_4) {
+                DPRINTF("+++ C+ mode packet has bad IP version %d "
+                    "expected %d\n", IP_HEADER_VERSION(ip),
+                    IP_HEADER_VERSION_4);
+                goto skip_offload;
+            }
+
+            hlen = IP_HEADER_LENGTH(ip);
+            ip_protocol = ip->ip_p;
+            ip_data_len = be16_to_cpu(ip->ip_len) - hlen;
+
             if (ip)
             {
                 if (txdw0 & CP_TX_IPCS)
@@ -2377,6 +2379,7 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s)
             }
         }
 
+skip_offload:
         /* update tally counter */
         ++s->tally_counters.TxOk;