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From: Cong Wang <xiyou.wangcong@gmail.com>
Date: Tue, 13 Dec 2016 16:17:14 -0800
Message-ID: <CAM_iQpVPEJ2t29ENpT4qcBznwE83w_PEBOxStwyzDH27Si2Ppw@mail.gmail.com>
Subject: Re: netlink: GPF in sock_sndtimeo
To: Richard Guy Briggs <rgb@redhat.com>
Cc: linux-audit@redhat.com, Paul Moore <pmoore@redhat.com>,
        Dmitry Vyukov <dvyukov@google.com>, David Miller <davem@davemloft.net>,
        Johannes Berg <johannes.berg@intel.com>,
        Florian Westphal <fw@strlen.de>, Eric Dumazet <edumazet@google.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        netdev <netdev@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>
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On Tue, Dec 13, 2016 at 2:52 AM, Richard Guy Briggs <rgb@redhat.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.

>
> 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.