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Date:   Tue, 7 Mar 2017 09:29:11 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>, linux-audit@...hat.com,
        David Miller <davem@...emloft.net>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: netlink: GPF in netlink_unicast

On Mon, Mar 6, 2017 at 11:03 PM, Richard Guy Briggs <rgb@...hat.com> wrote:
> On 2017-03-06 10:10, Cong Wang wrote:
>> On Mon, Mar 6, 2017 at 2:54 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> > Hello,
>> >
>> > I've got the following crash while running syzkaller fuzzer on
>> > net-next/8d70eeb84ab277377c017af6a21d0a337025dede:
>> >
>> > kasan: GPF could be caused by NULL-ptr deref or user memory access
>> > general protection fault: 0000 [#1] SMP KASAN
>> > Dumping ftrace buffer:
>> >    (ftrace buffer empty)
>> > Modules linked in:
>> > CPU: 0 PID: 883 Comm: kauditd Not tainted 4.10.0+ #6
>> > Hardware name: Google Google Compute Engine/Google Compute Engine,
>> > BIOS Google 01/01/2011
>> > task: ffff8801d79f0240 task.stack: ffff8801d7a20000
>> > RIP: 0010:sock_sndtimeo include/net/sock.h:2162 [inline]
>> > RIP: 0010:netlink_unicast+0xdd/0x730 net/netlink/af_netlink.c:1249
>> > RSP: 0018:ffff8801d7a27c38 EFLAGS: 00010206
>> > RAX: 0000000000000056 RBX: ffff8801d7a27cd0 RCX: 0000000000000000
>> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000000002b0
>> > RBP: ffff8801d7a27cf8 R08: ffffed00385cf286 R09: ffffed00385cf286
>> > R10: 0000000000000006 R11: ffffed00385cf285 R12: 0000000000000000
>> > R13: dffffc0000000000 R14: ffff8801c2fc3c80 R15: 00000000014000c0
>> > FS:  0000000000000000(0000) GS:ffff8801dbe00000(0000) knlGS:0000000000000000
>> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> > CR2: 0000000020cfd000 CR3: 00000001c758f000 CR4: 00000000001406f0
>> > Call Trace:
>> >  kauditd_send_unicast_skb+0x3c/0x70 kernel/audit.c:482
>> >  kauditd_thread+0x174/0xb00 kernel/audit.c:599
>> >  kthread+0x326/0x3f0 kernel/kthread.c:229
>> >  ret_from_fork+0x31/0x40 arch/x86/entry/entry_64.S:430
>> > Code: 44 89 fe e8 56 15 ff ff 8b 8d 70 ff ff ff 49 89 c6 31 c0 85 c9
>> > 75 27 e8 b2 b2 f4 fd 49 8d bc 24 b0 02 00 00 48 89 f8 48 c1 e8 03 <42>
>> > 80 3c 28 00 0f 85 37 06 00 00 49 8b 84 24 b0 02 00 00 4c 8d
>> > RIP: sock_sndtimeo include/net/sock.h:2162 [inline] RSP: ffff8801d7a27c38
>> > RIP: netlink_unicast+0xdd/0x730 net/netlink/af_netlink.c:1249 RSP:
>> > ffff8801d7a27c38
>> > ---[ end trace ad1bba9d457430b6 ]---
>> > Kernel panic - not syncing: Fatal exception
>> >
>> >
>> > This is not reproducible and seems to be caused by an elusive race.
>> > However, looking at the code I don't see any proper protection of
>> > audit_sock (other than the if (!audit_pid) which is obviously not
>> > enough to protect against races).
>>
>> audit_cmd_mutex is supposed to protect it, I think.
>> But kauditd_send_unicast_skb() seems not holding this mutex.
>
> Hmmmm, I wonder if it makes sense to wrap most of the contents of the
> outer while loop in kauditd_thread in the audit_cmd_mutex, or around the
> first two innter while loops and the "if (auditd)" condition after the
> "quick_loop:" label.  The condition on auditd is supposed to catch that
> case.  We don't want it locked while playing with the scheduler at the
> bottom of that function.

Let me look into this and play around with a few things.  I suspected
there might be a problem here, so I've got thoughts on how we might
resolve it; I just need to see code them up and see what option sucks
the least.

FWIW Richard, yes wrapping most of kauditd_thread *should* resolve
this but it's pretty heavy handed and not my first choice.

-- 
paul moore
www.paul-moore.com

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