Re: [PATCH net] net: sit: fix UBSAN Undefined behaviour in check_6rd

[David Miller]

From: linmiaohe <linmiaohe@xxxxxxxxxx>
Date: Mon, 11 Mar 2019 16:29:32 +0800

> @@ -778,8 +778,9 @@ static bool check_6rd(struct ip_tunnel *tunnel, const struct in6_addr *v6dst,
>  		pbw0 = tunnel->ip6rd.prefixlen >> 5;
>  		pbi0 = tunnel->ip6rd.prefixlen & 0x1f;
> 
> -		d = (ntohl(v6dst->s6_addr32[pbw0]) << pbi0) >>
> -		    tunnel->ip6rd.relay_prefixlen;
> +		d = tunnel->ip6rd.relay_prefixlen < 32 ?
> +			(ntohl(v6dst->s6_addr32[pbw0]) << pbi0) >>
> +		    tunnel->ip6rd.relay_prefixlen : 0;
> 

I hate the fact that we have to guard against something which the rest
of the code makes sure NEVER EVER happens.

Every assignment of ->relay_prefixlen is guarded by a check against 32.

I don't like this at all, and I have to put my foot down somehow.

So I'm not applying this, sorry.