linux-kernel - Re: netlink: GPF in sock

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Date:   Fri, 9 Dec 2016 20:13:44 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     linux-audit@...hat.com, Paul Moore <pmoore@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.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>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: netlink: GPF in sock_sndtimeo

On Fri, Dec 9, 2016 at 3:01 AM, Richard Guy Briggs <rgb@...hat.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.

I will send a formal patch.

Thanks.