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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>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
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Prasad J Pandit authored and jasowang committed Nov 20, 2015
1 parent 9596ef7 commit 837f21aacf5a714c23ddaadbbc5212f9b661e3f7
Showing with 5 additions and 3 deletions.
  1. +5 −3 hw/net/pcnet.c
@@ -1084,7 +1084,7 @@ 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));
}
@@ -1233,8 +1233,10 @@ 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;
}

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