linux-kernel - Re: KASAN, xt_TCPMSS finally found nasty use-after-free bug? 4.10.8

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Message-ID: <ddb4ab6cee309a79f4d8e7b8597e5a8f@nuclearcat.com>
Date:   Sun, 02 Apr 2017 14:51:25 +0300
From:   Denys Fedoryshchenko <nuclearcat@...learcat.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Patrick McHardy <kaber@...sh.net>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        linux-kernel@...r.kernel.org
Subject: Re: KASAN, xt_TCPMSS  finally found nasty use-after-free bug? 4.10.8

On 2017-04-02 14:45, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@...il.com> wrote:
>> -	for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += 
>> optlen(opt, i)) {
>> +	for (i = sizeof(struct tcphdr); i < tcp_hdrlen - TCPOLEN_MSS; i += 
>> optlen(opt, i)) {
>>  		if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
>>  			u_int16_t oldmss;
> 
> maybe I am low on caffeeine but this looks fine, for tcp header with
> only tcpmss this boils down to "20 <= 24 - 4" so we acccess offsets
> 20-23 which seems ok.
It seems some non-standard(or corrupted) packets are passing, because 
even on ~1G server it might cause corruption once per several days, 
KASAN seems need less time to trigger.

I am not aware how things working, but:
[25181.875696] Memory state around the buggy address:
[25181.875919]  ffff8802975fff80: 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00
[25181.876275]  ffff880297600000: 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00
[25181.876628] >ffff880297600080: 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00
[25181.876984]
^
[25181.877203]  ffff880297600100: 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00
[25181.877569]  ffff880297600180: 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00

Why all data here is zero? I guess it should be some packet data?