netdev - Re: [PATCH] tcp: Don't access TCP_SKB

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Message-ID: <84cc037d-44c9-195b-ea38-3447dd64ff90@gmail.com>
Date:   Mon, 11 Mar 2019 15:19:20 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Christoph Paasch <cpaasch@...le.com>,
        David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] tcp: Don't access TCP_SKB_CB before initializing it



On 03/11/2019 11:41 AM, Christoph Paasch wrote:
> Since commit eeea10b83a13 ("tcp: add
> tcp_v4_fill_cb()/tcp_v4_restore_cb()"), tcp_vX_fill_cb is only called
> after tcp_filter(). That means, TCP_SKB_CB(skb)->end_seq still points to
> the IP-part of the cb.
> 
> We thus should not mock with it, as this can trigger bugs (thanks
> syzkaller):
> [   12.349396] ==================================================================
> [   12.350188] BUG: KASAN: slab-out-of-bounds in ip6_datagram_recv_specific_ctl+0x19b3/0x1a20
> [   12.351035] Read of size 1 at addr ffff88006adbc208 by task test_ip6_datagr/1799
> 
> Setting end_seq is actually no more necessary in tcp_filter as it gets
> initialized later on in tcp_vX_fill_cb.
> 
> Cc: Eric Dumazet <edumazet@...gle.com>
> Fixes: eeea10b83a13 ("tcp: add tcp_v4_fill_cb()/tcp_v4_restore_cb()")
> Signed-off-by: Christoph Paasch <cpaasch@...le.com>
> ---
>  net/ipv4/tcp_ipv4.c | 9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)

Good catch, thanks !

Signed-off-by: Eric Dumazet <edumazet@...gle.com>