ftgmac100: Fix integer overflow in ftgmac100_do_tx()

When inserting the VLAN tag in packets, memmove() can generate an
integer overflow for packets whose length is less than 12 bytes.

Move the VLAN insertion when the last segment of the frame is reached
and check length against the size of the ethernet header (14 bytes) to
avoid the crash. Return FTGMAC100_INT_XPKT_LOST status if the frame is
too small. This seems like a good modeling choice even if Aspeed does
not specify anything in that case.

Cc: Frederic Konrad <konrad.frederic@yahoo.fr>
Cc: Mauro Matteo Cascella <mcascell@redhat.com>
Reported-by: Ziming Zhang <ezrakiez@gmail.com>
Message-Id: <20200819100956.2216690-15-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

hw/net/ftgmac100.c

@@ -481,6 +481,37 @@ static int ftgmac100_write_bd(FTGMAC100Desc *bd, dma_addr_t addr)
     return 0;
 }
 
+static int ftgmac100_insert_vlan(FTGMAC100State *s, int frame_size,
+                                  uint8_t vlan_tci)
+{
+    uint8_t *vlan_hdr = s->frame + (ETH_ALEN * 2);
+    uint8_t *payload = vlan_hdr + sizeof(struct vlan_header);
+
+    if (frame_size < sizeof(struct eth_header)) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: frame too small for VLAN insertion : %d bytes\n",
+                      __func__, frame_size);
+        s->isr |= FTGMAC100_INT_XPKT_LOST;
+        goto out;
+    }
+
+    if (frame_size + sizeof(struct vlan_header) > sizeof(s->frame)) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: frame too big : %d bytes\n",
+                      __func__, frame_size);
+        s->isr |= FTGMAC100_INT_XPKT_LOST;
+        frame_size -= sizeof(struct vlan_header);
+    }
+
+    memmove(payload, vlan_hdr, frame_size - (ETH_ALEN * 2));
+    stw_be_p(vlan_hdr, ETH_P_VLAN);
+    stw_be_p(vlan_hdr + 2, vlan_tci);
+    frame_size += sizeof(struct vlan_header);
+
+out:
+    return frame_size;
+}
+
 static void ftgmac100_do_tx(FTGMAC100State *s, uint32_t tx_ring,
                             uint32_t tx_descriptor)
 {
@@ -530,25 +561,17 @@ static void ftgmac100_do_tx(FTGMAC100State *s, uint32_t tx_ring,
             break;
         }
 
-        /* Check for VLAN */
-        if (bd.des0 & FTGMAC100_TXDES0_FTS &&
-            bd.des1 & FTGMAC100_TXDES1_INS_VLANTAG &&
-            be16_to_cpu(PKT_GET_ETH_HDR(ptr)->h_proto) != ETH_P_VLAN) {
-            if (frame_size + len + 4 > sizeof(s->frame)) {
-                qemu_log_mask(LOG_GUEST_ERROR, "%s: frame too big : %d bytes\n",
-                              __func__, len);
-                s->isr |= FTGMAC100_INT_XPKT_LOST;
-                len =  sizeof(s->frame) - frame_size - 4;
-            }
-            memmove(ptr + 16, ptr + 12, len - 12);
-            stw_be_p(ptr + 12, ETH_P_VLAN);
-            stw_be_p(ptr + 14, bd.des1);
-            len += 4;
-        }
-
         ptr += len;
         frame_size += len;
         if (bd.des0 & FTGMAC100_TXDES0_LTS) {
+
+            /* Check for VLAN */
+            if (flags & FTGMAC100_TXDES1_INS_VLANTAG &&
+                be16_to_cpu(PKT_GET_ETH_HDR(s->frame)->h_proto) != ETH_P_VLAN) {
+                frame_size = ftgmac100_insert_vlan(s, frame_size,
+                                            FTGMAC100_TXDES1_VLANTAG_CI(flags));
+            }
+
             if (flags & FTGMAC100_TXDES1_IP_CHKSUM) {
                 net_checksum_calculate(s->frame, frame_size);
             }