Re: run_timer_softirq gpf. tracing?

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 21 Mar 2017, Dave Jones wrote:
> I just hit this while fuzzing..
> 
> general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> CPU: 2 PID: 0 Comm: swapper/2 Not tainted 4.11.0-rc2-think+ #1 
> task: ffff88017f0ed440 task.stack: ffffc90000094000
> RIP: 0010:run_timer_softirq+0x15f/0x700
> RSP: 0018:ffff880507c03ec8 EFLAGS: 00010086
> RAX: dead000000000200 RBX: ffff880507dd0d00 RCX: 0000000000000002
> RDX: ffff880507c03ed0 RSI: 00000000ffffffff RDI: ffffffff8204b3a0
> RBP: ffff880507c03f48 R08: ffff880507dd12d0 R09: ffff880507c03ed8
> R10: ffff880507dd0db0 R11: 0000000000000000 R12: ffffffff8215cc38
> R13: ffff880507c03ed0 R14: ffffffff82005188 R15: ffff8804b55491a8
> FS:  0000000000000000(0000) GS:ffff880507c00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000004 CR3: 0000000005011000 CR4: 00000000001406e0
> Call Trace:
>  <IRQ>
>  ? clockevents_program_event+0x47/0x120
>  __do_softirq+0xbf/0x5b1
>  irq_exit+0xb5/0xc0
>  smp_apic_timer_interrupt+0x3d/0x50
>  apic_timer_interrupt+0x97/0xa0
> RIP: 0010:cpuidle_enter_state+0x12e/0x400
> RSP: 0018:ffffc90000097e40 EFLAGS: 00000202
> [CONT START]  ORIG_RAX: ffffffffffffff10
> RAX: ffff88017f0ed440 RBX: ffffe8ffffa03cc8 RCX: 0000000000000001
> RDX: 20c49ba5e353f7cf RSI: 0000000000000001 RDI: ffff88017f0ed440
> RBP: ffffc90000097e80 R08: 00000000ffffffff R09: 0000000000000008
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000005
> R13: ffffffff820b9338 R14: 0000000000000005 R15: ffffffff820b9320
>  </IRQ>
>  cpuidle_enter+0x17/0x20
>  call_cpuidle+0x23/0x40
>  do_idle+0xfb/0x200
>  cpu_startup_entry+0x71/0x80
>  start_secondary+0x16a/0x210
>  start_cpu+0x14/0x14
> Code: 8b 05 ce 1b ef 7e 83 f8 03 0f 87 4e 01 00 00 89 c0 49 0f a3 04 24 0f 82 0a 01 00 00 49 8b 07 49 8b 57 08 48 85 c0 48 89 02 74 04 <48> 89 50 08 41 f6 47 2a 20 49 c7 47 08 00 00 00 00 48 89 df 48 
> 
> 
> RAX looks like list poison, and CR2 = 4, which is likely the ->next of a list,
> with a NULL pointer.

Certainly not on 64 bit. that would be 8. And CR2 is irrelevant here
because that's a #GP not a #PF.

> Here's the disassembly of timer.o
> https://codemonkey.org.uk/junk/timer.o.txt
> 
> Sure enough, run_timer_softirq+0x15f (1ccf) is..
> 
> 
> 	entry->next = LIST_POISON2;
>     1cc8:	48 b8 00 02 00 00 00 	movabs $0xdead000000000200,%rax

The code above says: <48> 89 50 08 which is

      mov %rdx, 0x8(%rax).

and that is:

   next->pprev = pprev

inside of detach_timer().

So this is actually inside

__hlist_del(struct hlist_node *n)
{
   struct hlist_node *next = n->next;
   struct hlist_node *pprev = n->pprev;

   WRITE_ONCE(*pprev, next);
   if (next)
      next->pprev = pprev;
}

The timer which expires has timer->entry.next == POISON2 !

This has nothing to do with tracing, it's a classic list corruption.  The
bad news is that there is no trace of the culprit because that happens when
some other timer expires after some random amount of time.

If that is reproducible, then please enable debugobjects. That should
pinpoint the culprit.

Thanks,

	tglx