Re: PANIC: double fault, error_code: 0x0 in 4.0.0-rc3-2, kvm related?

[Takashi Iwai]

At Mon, 23 Mar 2015 11:38:30 -0700,
Andy Lutomirski wrote:
> 
> On Mon, Mar 23, 2015 at 9:07 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> > On 03/23/2015 02:22 PM, Takashi Iwai wrote:
> >> At Mon, 23 Mar 2015 10:35:41 +0100,
> >> Takashi Iwai wrote:
> >>>
> >>> At Mon, 23 Mar 2015 10:02:52 +0100,
> >>> Takashi Iwai wrote:
> >>>>
> >>>> At Fri, 20 Mar 2015 19:16:53 +0100,
> >>>> Denys Vlasenko wrote:
> 
> >> I'm really puzzled now.  We have a few pieces of information:
> >>
> >> - git bisection pointed the commit 96b6352c1271:
> >>     x86_64, entry: Remove the syscall exit audit and schedule optimizations
> >>   and reverting this "fixes" the problem indeed.  Even just moving two
> >>   lines
> >>     LOCKDEP_SYS_EXIT
> >>     DISABLE_INTERRUPTS(CLBR_NONE)
> >>   at the beginning of ret_from_sys_call already fixes.  (Of course I
> >>   can't prove the fix but it stabilizes for a day without crash while
> >>   usually I hit the bug in 10 minutes in full test running.)
> >
> > The commit 96b6352c1271 moved TIF_ALLWORK_MASK check from
> > interrupt-disabled region to interrupt-enabled:
> >
> >         cmpq $__NR_syscall_max,%rax
> >         ja ret_from_sys_call
> >         movq %r10,%rcx
> >         call *sys_call_table(,%rax,8)  # XXX:    rip relative
> >         movq %rax,RAX-ARGOFFSET(%rsp)
> > ret_from_sys_call:
> >         testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >         jnz int_ret_from_sys_call_fixup /* Go the the slow path */
> >         LOCKDEP_SYS_EXIT
> >         DISABLE_INTERRUPTS(CLBR_NONE)
> >         TRACE_IRQS_OFF
> > ...
> > ...
> > int_ret_from_sys_call_fixup:
> >         FIXUP_TOP_OF_STACK %r11, -ARGOFFSET
> >         jmp int_ret_from_sys_call
> > ...
> > ...
> > GLOBAL(int_ret_from_sys_call)
> >         DISABLE_INTERRUPTS(CLBR_NONE)
> >         TRACE_IRQS_OFF
> >
> > You reverted that by moving this insn to be after first DISABLE_INTERRUPTS(CLBR_NONE).
> >
> > I also don't see how moving that check (even if it is wrong in a more
> > benign way) can have such a drastic effect.
> 
> I bet I see it.  I have the advantage of having stared at KVM code and
> cursed at it more recently than you, I suspect.  KVM does awful, awful
> things to CPU state, and, as an optimization, it allows kernel code to
> run with CPU state that would be totally invalid in user mode.  This
> happens through a bunch of hooks, including this bit in __switch_to:
> 
>     /*
>      * Now maybe reload the debug registers and handle I/O bitmaps
>      */
>     if (unlikely(task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT ||
>              task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV))
>         __switch_to_xtra(prev_p, next_p, tss);
> 
> IOW, we *change* tif during context switches.
> 
> 
> The race looks like this:
> 
>     testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP)
>     jnz int_ret_from_sys_call_fixup    /* Go the the slow path */
> 
> --- preempted here, switch to KVM guest ---
> 
> KVM guest enters and screws up, say, MSR_SYSCALL_MASK.  This wouldn't
> happen to be a *32-bit* KVM guest, perhaps?
> 
> Now KVM schedules, calling __switch_to.  __switch_to sets
> _TIF_USER_RETURN_NOTIFY.  We IRET back to the syscall exit code, turn
> off interrupts, and do sysret.  We are now screwed.

Thanks for enlightening!  That looks like a feasible scenario.
(I tested only a 64bit KVM guest, BTW.)

> I don't know why this manifests in this particular failure, but any
> number of terrible things could happen now.
> 
> FWIW, this will affect things other than KVM.  For example, SIGKILL
> sent while a process is sleeping in that two-instruction window won't
> work.
> 
> Takashi, can you re-send your patch so we can review it for real in
> light of this race?

The patch below worked.  I'll double-check tomorrow whether this
really cures reliably.


thanks,

Takashi

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 1d74d161687c..5340ac7f88a9 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -364,12 +364,12 @@ system_call_fastpath:
  * Has incomplete stack frame and undefined top of stack.
  */
 ret_from_sys_call:
-	testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
-	jnz int_ret_from_sys_call_fixup	/* Go the the slow path */
-
 	LOCKDEP_SYS_EXIT
 	DISABLE_INTERRUPTS(CLBR_NONE)
 	TRACE_IRQS_OFF
+	testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
+	jnz int_ret_from_sys_call_fixup	/* Go the the slow path */
+
 	CFI_REMEMBER_STATE
 	/*
 	 * sysretq will re-enable interrupts:
--
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