linux-kernel - Re: [RFC] x86_64: A real proposal for iret-less return to kernel

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Date:	Wed, 21 May 2014 15:52:16 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	"Luck, Tony" <tony.luck@...el.com>, Jiri Kosina <jkosina@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC] x86_64: A real proposal for iret-less return to kernel

On Wed, May 21, 2014 at 3:48 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, May 21, 2014 at 03:39:11PM -0700, Andy Lutomirski wrote:
>> But if we get a new MCE in here, it will be an MCE from kernel context
>> and it's fatal. So, yes, we'll clobber the stack, but we'll never
>> return (unless tolerant is set to something insane), so who cares?
>
> Ok, but we still have to do the work before returning to the process. So
> if not mce_notify_process() how else are you suggesting we do this?

I'm suggesting that you re-enable interrupts and do the work in
do_machine_check.  I think it'll just work.  It might pay to set a
flag so that you panic very loudly if do_machine_check recurses.

I suspect that, if the hardware is generating machine checks while
doing memory poisoning, the hardware is broken enough that even
panicking might not work, though :)

--Andy
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