Re: Slab Corruption with ipv6 and tcp6fuzz

[Evgeniy Polyakov <johnpol@2ka.mipt.ru>]

Hi.

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