NetRxPkt: Fix memory corruption on VLAN header stripping

This patch fixed a problem that was introduced in commit eb70002.

When net_rx_pkt_attach_iovec() calls eth_strip_vlan()
this can result in pkt->ehdr_buf being overflowed, because
ehdr_buf is only sizeof(struct eth_header) bytes large
but eth_strip_vlan() can write
sizeof(struct eth_header) + sizeof(struct vlan_header)
bytes into it.

Devices affected by this problem: vmxnet3.

Cc: qemu-stable@nongnu.org
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/net_rx_pkt.c

@@ -23,13 +23,13 @@
 
 struct NetRxPkt {
     struct virtio_net_hdr virt_hdr;
-    uint8_t ehdr_buf[sizeof(struct eth_header)];
+    uint8_t ehdr_buf[sizeof(struct eth_header) + sizeof(struct vlan_header)];
     struct iovec *vec;
     uint16_t vec_len_total;
     uint16_t vec_len;
     uint32_t tot_len;
     uint16_t tci;
-    bool vlan_stripped;
+    size_t ehdr_buf_len;
     bool has_virt_hdr;
     eth_pkt_types_e packet_type;
 
@@ -88,15 +88,13 @@ net_rx_pkt_pull_data(struct NetRxPkt *pkt,
                         const struct iovec *iov, int iovcnt,
                         size_t ploff)
 {
-    if (pkt->vlan_stripped) {
+    if (pkt->ehdr_buf_len) {
         net_rx_pkt_iovec_realloc(pkt, iovcnt + 1);
 
         pkt->vec[0].iov_base = pkt->ehdr_buf;
-        pkt->vec[0].iov_len = sizeof(pkt->ehdr_buf);
-
-        pkt->tot_len =
-            iov_size(iov, iovcnt) - ploff + sizeof(struct eth_header);
+        pkt->vec[0].iov_len = pkt->ehdr_buf_len;
 
+        pkt->tot_len = iov_size(iov, iovcnt) - ploff + pkt->ehdr_buf_len;
         pkt->vec_len = iov_copy(pkt->vec + 1, pkt->vec_len_total - 1,
                                 iov, iovcnt, ploff, pkt->tot_len);
     } else {
@@ -123,11 +121,12 @@ void net_rx_pkt_attach_iovec(struct NetRxPkt *pkt,
     uint16_t tci = 0;
     uint16_t ploff = iovoff;
     assert(pkt);
-    pkt->vlan_stripped = false;
 
     if (strip_vlan) {
-        pkt->vlan_stripped = eth_strip_vlan(iov, iovcnt, iovoff, pkt->ehdr_buf,
-                                            &ploff, &tci);
+        pkt->ehdr_buf_len = eth_strip_vlan(iov, iovcnt, iovoff, pkt->ehdr_buf,
+                                           &ploff, &tci);
+    } else {
+        pkt->ehdr_buf_len = 0;
     }
 
     pkt->tci = tci;
@@ -143,12 +142,13 @@ void net_rx_pkt_attach_iovec_ex(struct NetRxPkt *pkt,
     uint16_t tci = 0;
     uint16_t ploff = iovoff;
     assert(pkt);
-    pkt->vlan_stripped = false;
 
     if (strip_vlan) {
-        pkt->vlan_stripped = eth_strip_vlan_ex(iov, iovcnt, iovoff, vet,
-                                               pkt->ehdr_buf,
-                                               &ploff, &tci);
+        pkt->ehdr_buf_len = eth_strip_vlan_ex(iov, iovcnt, iovoff, vet,
+                                              pkt->ehdr_buf,
+                                              &ploff, &tci);
+    } else {
+        pkt->ehdr_buf_len = 0;
     }
 
     pkt->tci = tci;
@@ -161,8 +161,8 @@ void net_rx_pkt_dump(struct NetRxPkt *pkt)
 #ifdef NET_RX_PKT_DEBUG
     assert(pkt);
 
-    printf("RX PKT: tot_len: %d, vlan_stripped: %d, vlan_tag: %d\n",
-              pkt->tot_len, pkt->vlan_stripped, pkt->tci);
+    printf("RX PKT: tot_len: %d, ehdr_buf_len: %lu, vlan_tag: %d\n",
+              pkt->tot_len, pkt->ehdr_buf_len, pkt->tci);
 #endif
 }
 
@@ -425,7 +425,7 @@ bool net_rx_pkt_is_vlan_stripped(struct NetRxPkt *pkt)
 {
     assert(pkt);
 
-    return pkt->vlan_stripped;
+    return pkt->ehdr_buf_len ? true : false;
 }
 
 bool net_rx_pkt_has_virt_hdr(struct NetRxPkt *pkt)