[PATCH] hw/net/ftgmac100: Fix integer overflow in ftgmac100_do_tx()

[Mauro Matteo Cascella <mcascell@redhat.com>]

An integer overflow issue was reported by Mr. Ziming Zhang, CC'd here. It
occurs while inserting the VLAN tag in packets whose length is less than
12 bytes, as (len-12) is passed to memmove() without proper checking.
This patch is intended to fix this issue by checking the minimum
Ethernet frame size during packet transmission.

Reported-by: Ziming Zhang <ezrakiez@gmail.com>
Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
---
 hw/net/ftgmac100.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/hw/net/ftgmac100.c b/hw/net/ftgmac100.c
index 043ba61b86..bcf4d84aea 100644
--- a/hw/net/ftgmac100.c
+++ b/hw/net/ftgmac100.c
@@ -238,6 +238,11 @@ typedef struct {
  */
 #define FTGMAC100_MAX_FRAME_SIZE    9220
 
+/*
+ * Min frame size
+ */
+#define FTGMAC100_MIN_FRAME_SIZE    64
+
 /* Limits depending on the type of the frame
  *
  *   9216 for Jumbo frames (+ 4 for VLAN)
@@ -507,6 +512,15 @@ static void ftgmac100_do_tx(FTGMAC100State *s, uint32_t tx_ring,
         }
 
         len = FTGMAC100_TXDES0_TXBUF_SIZE(bd.des0);
+
+        /* drop small packets */
+        if (bd.des0 & FTGMAC100_TXDES0_FTS &&
+            len < FTGMAC100_MIN_FRAME_SIZE) {
+            qemu_log_mask(LOG_GUEST_ERROR, "%s: frame too small: %d bytes\n",
+                          __func__, len);
+            break;
+        }
+
         if (frame_size + len > sizeof(s->frame)) {
             qemu_log_mask(LOG_GUEST_ERROR, "%s: frame too big : %d bytes\n",
                           __func__, len);
-- 
2.26.2