net: pcnet: add check to validate receive data size(CVE-2015-7504)

In loopback mode, pcnet_receive routine appends CRC code to the
receive buffer. If the data size given is same as the buffer size,
the appended CRC code overwrites 4 bytes after s->buffer. Added a
check to avoid that.

Reported-by: Qinghao Tang <luodalongde@gmail.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
---
 tools/qemu-xen/hw/net/pcnet.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/tools/qemu-xen/hw/net/pcnet.c b/tools/qemu-xen/hw/net/pcnet.c
index 3437376..5f55591 100644
--- a/tools/qemu-xen/hw/net/pcnet.c
+++ b/tools/qemu-xen/hw/net/pcnet.c
@@ -1085,7 +1085,7 @@ ssize_t pcnet_receive(NetClientState *nc, const uint8_t *buf, size_t size_)
                 uint32_t fcs = ~0;
                 uint8_t *p = src;
 
-                while (p != &src[size-4])
+                while (p != &src[size])
                     CRC(fcs, *p++);
                 crc_err = (*(uint32_t *)p != htonl(fcs));
             }
@@ -1234,8 +1234,10 @@ static void pcnet_transmit(PCNetState *s)
         bcnt = 4096 - GET_FIELD(tmd.length, TMDL, BCNT);
 
         /* if multi-tmd packet outsizes s->buffer then skip it silently.
-           Note: this is not what real hw does */
-        if (s->xmit_pos + bcnt > sizeof(s->buffer)) {
+         * Note: this is not what real hw does.
+         * Last four bytes of s->buffer are used to store CRC FCS code.
+         */
+        if (s->xmit_pos + bcnt > sizeof(s->buffer) - 4) {
             s->xmit_pos = -1;
             goto txdone;
         }
-- 
2.4.3