netdev - Re: netlink: GPF in sock

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Date:   Mon, 12 Dec 2016 11:07:41 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     syzkaller <syzkaller@...glegroups.com>
Cc:     Richard Guy Briggs <rgb@...hat.com>, linux-audit@...hat.com,
        Paul Moore <pmoore@...hat.com>,
        David Miller <davem@...emloft.net>,
        Johannes Berg <johannes.berg@...el.com>,
        Florian Westphal <fw@...len.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: netlink: GPF in sock_sndtimeo

On Sat, Dec 10, 2016 at 8:40 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>>> On 2016-12-08 22:57, Cong Wang wrote:
>>>> On Thu, Dec 8, 2016 at 10:02 PM, Richard Guy Briggs <rgb@...hat.com> wrote:
>>>> > I also tried to extend Cong Wang's idea to attempt to proactively respond to a
>>>> > NETLINK_URELEASE on the audit_sock and reset it, but ran into a locking error
>>>> > stack dump using mutex_lock(&audit_cmd_mutex) in the notifier callback.
>>>> > Eliminating the lock since the sock is dead anways eliminates the error.
>>>> >
>>>> > Is it safe?  I'll resubmit if this looks remotely sane.  Meanwhile I'll try to
>>>> > get the test case to compile.
>>>>
>>>> It doesn't look safe, because 'audit_sock', 'audit_nlk_portid' and 'audit_pid'
>>>> are updated as a whole and race between audit_receive_msg() and
>>>> NETLINK_URELEASE.
>>>
>>> This is what I expected and why I originally added the mutex lock in the
>>> callback...  The dumps I got were bare with no wrapper identifying the
>>> process context or specific error, so I'm at a bit of a loss how to
>>> solve this (without thinking more about it) other than instinctively
>>> removing the mutex.
>>
>> Netlink notifier can safely be converted to blocking one, I will send
>> a patch.
>>
>> But I seriously doubt you really need NETLINK_URELEASE here,
>> it adds nothing but overhead, b/c the netlink notifier is called on
>> every netlink socket in the system, but for net exit path, that is
>> relatively a slow path.
>>
>> Also, kauditd_send_skb() needs audit_cmd_mutex too.
>
> Please let me know what you think about the attached patch?

Applied the patch locally and have not seen the bug since then (~24
hours of testing).