Re: [PATCH] ipv6: fix a potential use after free in sit.c

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Sat, 18 Oct 2014 08:52:20 -0700

> On Sat, 2014-10-18 at 17:33 +0800, roy.qing.li@gmail.com wrote:
>> From: Li RongQing <roy.qing.li@gmail.com>
>> 
>> pskb_may_pull() maybe change skb->data and make iph pointer oboslete,
>> fix it by geting ip header length directly.
>> 
>> Fixes: ca15a078 (sit: generate icmpv6 error when receiving icmpv4 error)
>> Cc: Oussama Ghorbel <ghorbel@pivasoftware.com>
>> Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
>> ---
>>  net/ipv6/sit.c |    6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> Acked-by: Eric Dumazet <edumazet@google.com>
> 
> Thanks for doing all these checks !

Indeed, I wish we could somehow automate this.

Yes, I'm sure we could construct some sparse et al. rules but
I mean at run time.  For example, having a special pointer type
that you can't dereference directly.  At the time of assignment
the pointer gets some kind of state, and pskb_may_pull() et al.
calls invalidate that "state".

It probably could just be a 2-bit counter which is incremented
every time skb->data is reallocated.

The captured 2-bit generation count could be stored in the low
bits of the pointer.

Anyways, just throwing out ideas...