Re: [Qemu-devel] [PATCH v2 1/2] net: check packet payload length

[Jason Wang <jasowang@redhat.com>]

On 02/24/2016 12:08 AM, P J P wrote:
> From: Prasad J Pandit <pjp@fedoraproject.org>
>
> While computing IP checksum, 'net_checksum_calculate' reads
> payload length from the packet. It could exceed the given 'data'
> buffer size. Add a check to avoid it.
>
> Reported-by: Liu Ling <liuling-it@360.cn>
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> ---
>  net/checksum.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> Update as per review:
>   -> https://lists.gnu.org/archive/html/qemu-devel/2016-02/msg04062.html
>
> diff --git a/net/checksum.c b/net/checksum.c
> index 14c0855..bd89083 100644
> --- a/net/checksum.c
> +++ b/net/checksum.c
> @@ -59,6 +59,11 @@ void net_checksum_calculate(uint8_t *data, int length)
>      int hlen, plen, proto, csum_offset;
>      uint16_t csum;
>  

I must say this is still far from perfect, since it has too assumptions
. But I agree we can fix OOB first.

> +    /* Ensure data has complete L2 & L3 headers. */
> +    if (length < 14 + 20) {
> +        return;
> +    }
> +
>      if ((data[14] & 0xf0) != 0x40)
>  	return; /* not IPv4 */
>      hlen  = (data[14] & 0x0f) * 4;
> @@ -76,8 +81,9 @@ void net_checksum_calculate(uint8_t *data, int length)
>  	return;
>      }
>  
> -    if (plen < csum_offset+2)
> -	return;
> +    if (plen < csum_offset + 2 || plen + hlen >= length) {
> +        return;
> +    }

Should we count mac header here? Did "plen + hlen >= length" imply "14 +
hlen + csum_offset + 1" < length?

Looks not. Consider a TCP packet can report evil plen (e.g 20) but just
have 10 bytes payload in fact. In this case:

hlen = 20, plen = 20, csum_offset = 16, length = 44 which can pass all
the above tests, but 14 + hlen + csum_offset = 50 which is greater than
length.

>  
>      data[14+hlen+csum_offset]   = 0;
>      data[14+hlen+csum_offset+1] = 0;