netdev - Re: [PATCH] netfilter: ebtables: handle string from userspace with care

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Date:   Fri, 27 Apr 2018 11:46:19 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Paolo Abeni <pabeni@...hat.com>, netfilter-devel@...r.kernel.org,
        syzbot <syzbot+4e42a04e0bc33cb6c087@...kaller.appspotmail.com>,
        coreteam@...filter.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] netfilter: ebtables: handle string from userspace with care

On Fri, Apr 27, 2018 at 11:26 AM, Florian Westphal <fw@...len.de> wrote:
> Paolo Abeni <pabeni@...hat.com> wrote:
>> strlcpy() can't be safely used on a user-space provided string,
>> as it can try to read beyond the buffer's end, if the latter is
>> not NULL terminated.
>
> Yes.
>
>> Leveraging the above, syzbot has been able to trigger the following
>> splat:
>>
>> BUG: KASAN: stack-out-of-bounds in strlcpy include/linux/string.h:300
>> [inline]
>> BUG: KASAN: stack-out-of-bounds in compat_mtw_from_user
>> net/bridge/netfilter/ebtables.c:1957 [inline]
>> BUG: KASAN: stack-out-of-bounds in ebt_size_mwt
>> net/bridge/netfilter/ebtables.c:2059 [inline]
>> BUG: KASAN: stack-out-of-bounds in size_entry_mwt
>> net/bridge/netfilter/ebtables.c:2155 [inline]
>> BUG: KASAN: stack-out-of-bounds in compat_copy_entries+0x96c/0x14a0
>> net/bridge/netfilter/ebtables.c:2194
>> Write of size 33 at addr ffff8801b0abf888 by task syz-executor0/4504
>
> Which is weird, I don't understand this report.
> The code IS wrong, but it should cause out-of-bounds read (strlen on
> src), but not out-of-bounds write.

Please see this for explanation:
https://groups.google.com/d/msg/syzkaller-bugs/-Jyti8zBWjU/6n-fkmXeBAAJ
The stack overwrite actually happens here.

> Yes, I sent a recent patch (dceb48d86b4871984b8ce9ad5057fb2c01aa33de in
> nf.git) that would now allow to get rid of the strlcpy and use the
> source directly.