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

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Date:	Wed, 18 Mar 2015 21:06:20 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Stefan Seyfried <stefan.seyfried@...glemail.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Takashi Iwai <tiwai@...e.de>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: PANIC: double fault, error_code: 0x0 in 4.0.0-rc3-2, kvm related?

On 03/18/2015 08:26 PM, Andy Lutomirski wrote:
> Hi Linus-
> 
> You seem to enjoy debugging these things.  Want to give this a shot?
> My guess is a vmalloc fault accessing either old_rsp or kernel_stack
> right after swapgs in syscall entry.

The code is:

ENTRY(system_call)
        SWAPGS_UNSAFE_STACK
GLOBAL(system_call_after_swapgs)
        movq    %rsp,PER_CPU_VAR(rsp_scratch)
	movq    PER_CPU_VAR(kernel_stack),%rsp

If PER_CPU_VAR(var) memory access can page fault
(I was thinking this is ensured to never fault),
then on these two instructions such page fault
will be fatal: we will still have userspace %rsp.

I thought we can only get a NMI or debug interrupt here,
and they are both set up to use IST stacks
to prevent this scenario (among other reasons).

-- 
vda
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