Re: Slab Corruption with ipv6 and tcp6fuzz

[David Miller <davem@davemloft.net>]

From: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Date: Sat, 26 Apr 2008 20:05:20 +0400

Patrick, please look at this, your defer-accept changes are causing
crashes.  See below.

Eric, I'm CC:'ing you because you ACK'd some of this stuff :-))

> On Fri, Apr 25, 2008 at 05:09:46PM +0400, Evgeniy Polyakov (johnpol@2ka.mipt.ru) wrote:
> > On Fri, Apr 25, 2008 at 02:52:30PM +0200, Eric Sesterhenn (snakebyte@gmx.de) wrote:
> > > > > [  215.103103]  [<c0671e3b>] tcp_v6_do_rcv+0x33b/0xcd0
> > > > 
> > > > So far can you run kernel with debug turned on and provide output of
> > > > gdb ./vmlinux
> > > > l *(tcp_v6_do_rcv+0x33b)
> > > 
> > > l *(tcp_v6_do_rcv+0x33b)
> > > Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
> > > (gdb) l *(tcp_v6_do_rcv+0x33b)
> > > 0xc0671e3b is in tcp_v6_do_rcv (net/ipv6/tcp_ipv6.c:1670).
> > > 1665    reset:
> > > 1666            tcp_v6_send_reset(sk, skb);
> > > 1667    discard:
> > > 1668            if (opt_skb)
> > > 1669                    __kfree_skb(opt_skb);
> > > 1670            kfree_skb(skb);
> > > 1671            return 0;
> > > 1672    csum_err:
> > > 1673            TCP_INC_STATS_BH(TCP_MIB_INERRS);
> > > 1674            goto discard;
> > 
> > Well, first possible error path:
> > 
> > tcp_v6_do_rcv()->tcp_rcv_established(), the latter goes to step5, where
> > eventually skb can be freed via tcp_data_queue() (drop: label), then if
> > check for tcp_defer_accept_check() returns true and thus
> > tcp_rcv_established() returns -1, which forces tcp_v6_do_rcv() to jump
> > to reset: label, which in turn will pass through discard: label and free
> > the same skb again.
> >  
> > > Here is the programm itself...
> > 
> > I will work on this this weekend, thank you.
>
> Ok, I can not reproduce it, so lets try hard way.
> Can you test attached patch, its idea is described above and I checked
> multiple times, that it is forbidden to free skb and return non-zero
> value from tcp_rcv_established(). This behaviour was introduced with TCP
> defer accept changes in 2.6.25 with
> ec3c0982a2dd1e671bad8e9d26c28dcba0039d87 commit.
> 
> Please test. This bug affects both ipv6 and ipv4 code actually.
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index ac9b848..0298f80 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4925,8 +4925,7 @@ step5:
>  	tcp_data_snd_check(sk);
>  	tcp_ack_snd_check(sk);
>  
> -	if (tcp_defer_accept_check(sk))
> -		return -1;
> +	tcp_defer_accept_check(sk);
>  	return 0;
>  
>  csum_error:
> 
> 
> -- 
> 	Evgeniy Polyakov