Re: netlink: GPF in netlink_unicast

[Paul Moore]

On Tue, Mar 7, 2017 at 1:44 PM, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> On Tue, Mar 7, 2017 at 10:55 AM, Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
>> On 2017-03-07 09:29, Paul Moore wrote:
>>> On Mon, Mar 6, 2017 at 11:03 PM, Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
>>> > On 2017-03-06 10:10, Cong Wang wrote:
>>> >> On Mon, Mar 6, 2017 at 2:54 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> 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.
>>
>> That's why the inner loops made a bit more sense since it wasn't really
>> necessary and ran afoul of the scheduler anyways.
>
> One of my preferred options was to get us away from protecting
> everything with the audit_cmd_mutex by creating a new locking approach
> for the auditd connection state (using RCU/spinlocks since it rarely
> changes in practice) and leaving the audit_cmd_mutex for it's
> traditional role.  This should minimize the performance impact of the
> lock and clean things up a bit.  I'm also moving all the auditd
> connection state into a single struct (instead of several variables
> associated only by convention) which moves us oh so slightly closer to
> allowing multiple auditd connections (hey, it's something).
>
> It's taking a bit longer than expected as I'm dealing with a bit of a
> head cold (or something) and my mind is far less than 100% at the
> moment ...

Ooof.  I just noticed something, and maybe this is the fever talking,
but why do we ever NULL out audit_sock and why are we bothering with
those holds/puts?  We create the audit netlink socket in
audit_net_init() and it should remain valid until we kill it in
audit_next_exit(); we sorta cheat on this now because we track the
socket both in the per-netns audit_net struct as well as audit_sock,
but that doesn't make our audit_sock manipulations right ...

Man I hate this code.  I *really* hate this code.

-- 
paul moore
www.paul-moore.com