linux-kernel - Re: netlink: GPF in sock

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Date:   Tue, 13 Dec 2016 23:17:01 -0500
From:   Richard Guy Briggs <rgb@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Johannes Berg <johannes.berg@...el.com>,
        netdev <netdev@...r.kernel.org>, Florian Westphal <fw@...len.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>, linux-audit@...hat.com,
        syzkaller <syzkaller@...glegroups.com>,
        David Miller <davem@...emloft.net>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: netlink: GPF in sock_sndtimeo

On 2016-12-13 16:17, Cong Wang wrote:
> On Tue, Dec 13, 2016 at 2:52 AM, Richard Guy Briggs <rgb@...hat.com> wrote:
> > It is actually the audit_pid and audit_nlk_portid that I care about
> > more.  The audit daemon could vanish or close the socket while the
> > kernel sock to which it was attached is still quite valid.  Accessing
> > the set of three atomically is the urge.  I wonder if it makes more
> > sense to test for the presence of auditd using audit_sock rather than
> > audit_pid, but still keep audit_pid for our reporting and replacement
> > strategy.  Another idea would be to put the three in one struct.
> 
> Note, the process has audit_pid should hold a refcnt to the netns too,
> so the netns can't be gone until that process is gone.

I noted that.  I did wonder if there might be a problem if all the
processes were moved to another netns with the struct sock stuck in the
now process-void netns.

This is alluded-to in 6f285b19d09f ("audit: Send replies in the proper
network namespace.").

> > Can someone explain how they think the original test was able to trigger
> > this GPF?  Network namespace shutdown while something pretended to set
> > up a new auditd?  That's impressive for a fuzzer if that's the case...
> > Is there an strace?  I guess it is all in test().
> 
> I am surprised you still don't get the race condition even when you
> are now working on v2...
> 
> The race happens in this scenarios :
> 
> 1) Create a new netns
> 
> 2) In the new netns, communicate with kauditd to set audit_sock
> 
> 3) Generate some audit messages, so kauditd will keep sending them
> via audit_sock
> 
> 4) exit the netns
> 
> 5) the previous audit_sock is now going away, but kaudit_sock could still
> access it in this small window.

Ah ok that fits...

- RGB

--
Richard Guy Briggs <rgb@...hat.com>
Kernel Security Engineering, Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635