netdev - Re: BUG: net/netfilter: KASAN: stack-out-of-bounds in tcp

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Date:	Mon, 28 Mar 2016 13:07:34 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	kadlec@...ckhole.kfki.hu, sploving1@...il.com, pablo@...filter.org,
	kaber@...sh.net, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: BUG: net/netfilter: KASAN: stack-out-of-bounds in tcp_packet

On Mon, 2016-03-28 at 15:29 -0400, David Miller wrote:
> From: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
> Date: Mon, 28 Mar 2016 18:48:51 +0200 (CEST)
> 
> >> > > @@ -3716,6 +3716,8 @@ void tcp_parse_options(const struct sk_buff *skb,
> >> > >               length--;
> >> > >               continue;
> >> > >           default:
> >> > > +            if (length < 2)
> >> > > +                return;
> >> > >               opsize = *ptr++;
> >> > >               if (opsize < 2) /* "silly options" */
> >> > >                   return;
> 
> I'm trying to figure out how this can even matter.
> 
> If we are in the loop, length is at least one.
> 
> That means it is legal to read the opsize byte.
> 
> By the next check, opsize is at least 2.
> 
> And then the very next line in this code makes sure length >= opsize:
> 
> 			if (opsize > length)
> 				return;	/* don't parse partial options */
> 
> Therefore no out-of-range access is possible as far as I can see.

Maybe use kasan_disable_current() and kasan_enable_current() to silence
kasan ?

Oh wait, these are not BH safe.