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 15:29:14 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Stefan Seyfried <stefan.seyfried@...glemail.com>,
	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 Wed, Mar 18, 2015 at 3:22 PM, Jiri Kosina <jkosina@...e.cz> wrote:
> On Wed, 18 Mar 2015, Andy Lutomirski wrote:
>
>> sysret64 can only fail with #GP, and we're totally screwed if that
>> happens,
>
> But what if the GPF handler pagefaults afterwards? It'd be operating on
> user stack already.

Good point.

Stefan, can you try changing the first "jne
opportunistic_sysret_failed" to "jmp opportunistic_sysret_failed" in
entry_64.S and seeing if you can reproduce this?  (Is it easy enough
to reproduce that this would tell us anything?)

It's a shame that double_fault doesn't record what gs was on entry.
If we did sysret -> general_protection -> page_fault -> double_fault,
then we'd enter double_fault with usergs, whereas syscall ->
page_fault -> double_fault would enter double_fault with kernelgs.

Hmm.  We may be able to answer this more directly.  Stefan, can you
dump a couple hundred bytes starting at 0x00007fffa55eafb8 (i.e. your
page_fault stack at the time of the failure)?  That will tell us the
faulting address.  If that fails, try starting at 00007fffa55eb000
instead.

--Andy
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