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

[Stefan Seyfried]

Am 23.03.2015 um 19:38 schrieb Andy Lutomirski:
> 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?

not in my case (penryn CPU), there it was 64bit guests.

> 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.
> 
> 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?
-- 
Stefan Seyfried
Linux Consultant & Developer -- GPG Key: 0x731B665B

B1 Systems GmbH
OsterfeldstraÃe 7 / 85088 Vohburg / http://www.b1-systems.de
GF: Ralph Dehner / Unternehmenssitz: Vohburg / AG: Ingolstadt,HRB 3537
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