linux-kernel - Re: [PATCH v2] decnet: dn_rtmsg: Improve input length sanitization in dnrmg_receive_user

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Message-ID: <CAPr8fOQNP+y78xJqx5BNUfY6Rmjj2R8+jrz3Ett1SgdY+a5WLw@mail.gmail.com>
Date:   Wed, 7 Jun 2017 16:41:57 +0200
From:   Mateusz Jurczyk <mjurczyk@...gle.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        "David S. Miller" <davem@...emloft.net>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        linux-decnet-user@...ts.sourceforge.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] decnet: dn_rtmsg: Improve input length sanitization in dnrmg_receive_user_skb

On Wed, Jun 7, 2017 at 4:18 PM, Florian Westphal <fw@...len.de> wrote:
> Mateusz Jurczyk <mjurczyk@...gle.com> wrote:
>> Verify that the length of the socket buffer is sufficient to cover the
>> nlmsghdr structure before accessing the nlh->nlmsg_len field for further
>> input sanitization. If the client only supplies 1-3 bytes of data in
>> sk_buff, then nlh->nlmsg_len remains partially uninitialized and
>> contains leftover memory from the corresponding kernel allocation.
>> Operating on such data may result in indeterminate evaluation of the
>> nlmsg_len < sizeof(*nlh) expression.
>>
>> The bug was discovered by a runtime instrumentation designed to detect
>> use of uninitialized memory in the kernel. The patch prevents this and
>> other similar tools (e.g. KMSAN) from flagging this behavior in the future.
>
> Instead of changing all the internal users wouldn't it be better
> to add this check once in netlink_unicast_kernel?
>

Perhaps. I must admit I'm not very familiar with this code
area/interface, so I preferred to fix the few specific cases instead
of submitting a general patch, which might have some unexpected side
effects, e.g. behavior different from one of the internal clients etc.

If you think one check in netlink_unicast_kernel is a better way to do
it, I'm happy to implement it like that.

Thanks,
Mateusz