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

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Date:	Wed, 21 May 2014 23:05:32 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>
CC:	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 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?

Remember that machine checks are broadcast.  So some other cpu
can hit a recoverable machine check in user mode ... but that int#18
goes everywhere.  Other cpus are innocent bystanders ... they will
see MCG_STATUS.RIPV=1, MCG_STATUS.EIPV=0 and nothing important
in any of their machine check banks.

But if we are still finishing off processing the previous machine check,
this will be a nested one - and BOOM, we are dead.

-Tony

[If you peer closely at the latest edition of the SDM - you'll see the
bits are defined for a non-broadcast model ... e.g. LMCE_S bit in
MCG_STATUS .... but currently shipping silicon doesn't use that]