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

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Message-ID: <CALCETrXL1sm9GB=RWoDEcuE0_ZxG=fnwjhhkYYM15xZaQ__--w@mail.gmail.com>
Date:	Wed, 18 Mar 2015 15:29:11 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	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 3:28 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Mar 18, 2015 at 3:22 PM, Jiri Kosina <jkosina@...e.cz> wrote:
>>
>> But what if the GPF handler pagefaults afterwards? It'd be operating on
>> user stack already.
>
> So I think this might be the answer. We don't see the GP fault,
> because we don't have a backtrace, because that backtrace is on the
> user stack (which is why the stack trace dumping fails - we should
> probably fix that, btw - the second oops is just confusing and not
> helpful).
>
> Is the intel check for canonical address (that __VIRTUAL_MASK_SHIFT
> thing) perhaps wrong or not as strict as Intel CPU's do? We'd never
> notice in normal situations..

I explicitly tested that I could blow up the kernel if I intentionally
broke that test, and I couldn't blow it up with the test as written.
That doesn't prove it's correct, though.

--Andy
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