Re: [PATCH net 2/3] selftests: net: tuntap: fix UDP checksum length in build_udp_packet_csum()

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Eric Dumazet wrote:
> build_udp_packet_csum() passes udph->len, which is in network byte
> order, as the number of bytes to checksum. On little-endian hosts this
> reads up to 64KB past the UDP header and computes a wrong checksum.
> 
> Existing callers did not crash only because they use packet buffers
> of MAX_VNET_TUNNEL_PACKET_SZ bytes.
> 
> Use ntohs(udph->len), as tap.c does in its own build_udp_packet().
> 
> Fixes: a942fcd72e97 ("selftest: tun: Introduce tuntap_helpers.h header for TUN/TAP testing")
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>

Reviewed-by: Willem de Bruijn <willemb@google.com>

> ---
>  tools/testing/selftests/net/tuntap_helpers.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/net/tuntap_helpers.h b/tools/testing/selftests/net/tuntap_helpers.h
> index d6c0437136ec..ac9b2da57a6d 100644
> --- a/tools/testing/selftests/net/tuntap_helpers.h
> +++ b/tools/testing/selftests/net/tuntap_helpers.h
> @@ -335,7 +335,7 @@ static inline void build_udp_packet_csum(uint8_t *buf, int family,
>  	sum += htons(IPPROTO_UDP) + udph->len;
>  
>  	if (!csum_off)
> -		sum += add_csum(buf, udph->len);
> +		sum += add_csum(buf, ntohs(udph->len));
>  
>  	udph->check = finish_ip_csum(sum);

If respinning: we probably should also do CSUM_MANGLED_0 conversion here.

And to zero udph->check first. All existing callers do. But the calloc in
patch 3/3 does not.

>  }
> -- 
> 2.53.0
>