Re: [Qemu-devel] [PATCH for 2.5 1/2] net: pcnet: add check to validate receive data size(CVE-2015-7504)

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 30, 2015 at 03:38:22PM +0800, Jason Wang wrote:
> From: Prasad J Pandit <pjp@fedoraproject.org>
> 
> 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
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> Signed-off-by: Jason Wang <jasowang@redhat.com>

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  hw/net/pcnet.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/net/pcnet.c b/hw/net/pcnet.c
> index 0eb3cc4..309c40b 100644
> --- a/hw/net/pcnet.c
> +++ b/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;
>          }
> -- 
> 2.5.0
>