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 23:27:34 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>
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 11:20 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> There is an easy way to test the theory that SYSRET is to blame.
>>
>> Just replace
>>
>>         movq RCX(%rsp),%rcx
>>         cmpq %rcx,RIP(%rsp)             /* RCX == RIP */
>>         jne opportunistic_sysret_failed
>>
>> this "jne" with "jmp", and try to reproduce.
>>
>
> This is a classic root exploit, and it's why we check for
> non-canonical RIP.  In theory, that's the only way this can happen.
> Intel screwed up -- AMD never fails SYSRET.

I'm not saying the code needs to be changed.

I'm saying that *people who see the crash* can make this change,
run the modified kernel, and if crash disappears -
then it is caused by "opportunistic SYSRET".
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