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

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Date:	Mon, 28 Mar 2016 13:46:03 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jan Engelhardt <jengelh@...i.de>
Cc:	David Miller <davem@...emloft.net>, 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 22:20 +0200, Jan Engelhardt wrote:
> On Monday 2016-03-28 21:29, David Miller wrote:
> >>> > > @@ -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.
> 
> Is that because the skbuff is always padded to a multiple of (at
> least) two? Maybe such padding is explicitly foregone when ASAN is in
> place. After all, glibc, in userspace, is likely to do padding as
> well for malloc, and yet, ASAN catches these cases.

We have at least 384 bytes of padding in skb->head (this is struct
skb_shared_info).

Whatever garbage we might read, current code is fine.

We have to deal with a false positive here.