Re: [PATCH net] ipv6: the entire IPv6 header chain must fit the first fragment

[Eric Dumazet <eric.dumazet@gmail.com>]

On 03/23/2018 06:05 AM, Paolo Abeni wrote:
> While building ipv6 datagram we currently allow arbitrary large
> extheaders, even beyond pmtu size. The syzbot has found a way
> to exploit the above to trigger the following splat:
> 
... 
> As stated by RFC 7112 section 5:
> 
>    When a host fragments an IPv6 datagram, it MUST include the entire
>    IPv6 Header Chain in the First Fragment.
> 
> So this patch addresses the issue dropping datagrams with excessive
> extheader length. It also updates the error path to report to the
> calling socket nonnegative pmtu values.
> 
> The issue apparently predates git history.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: syzbot+91e6f9932ff122fa4410@syzkaller.appspotmail.com
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
>  net/ipv6/ip6_output.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 

> -				 sizeof(struct ipv6hdr));
> +		/* with large extheader pmtu can be negative, cap the reported
> +		 * value to 0, since it is unsigned
> +		 */
> +		pmtu = mtu + sizeof(struct ipv6hdr) > headersize ?
> +				  mtu - headersize + sizeof(struct ipv6hdr) : 0;

I would suggest :

pmtu = max_t(int, mtu - headersize + sizeof(struct ipv6hdr), 0);

And you can omit the comment, since the max_t() intent is obvious.

Thanks for working on this syzbot report.