linux-kernel - Re: [PATCH 10/19] x86/dumpstack: add get_stack

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Message-ID: <20160726135127.5e3daf44@gandalf.local.home>
Date:	Tue, 26 Jul 2016 13:51:27 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface

On Tue, 26 Jul 2016 11:26:42 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:


> Ok, I think that makes sense to me now.  As I understand it, the
> "outermost" RIP is the authoritative one, because it was written by the
> original NMI.  Any nested NMIs will update the original and/or iret
> RIPs, which will only ever point to NMI entry code, and so they should
> be ignored.

Just to confirm:

  -- top-of-stack --
  [ hardware written stack ] <- what the NMI hardware mechanism wrote
  [ internal variables ] <- you don't need to know what this is
  [ where to go next ] <- the stack to use to return on current NMI
  [ original copy of hardware stack ] <- the stack of the first NMI

IIRC, the original version had the "where to go next" stack last, but
to keep pt_regs in line with the stack, it made sense to have the
original NMI stack at the bottom, just above pt_regs, like a real
interrupt would.

> 
> But I think there's a case where this wouldn't work:
> 
> task stack
> NMI
> IST
> stack dump
> 
> If the IST interrupt hits before the NMI has a chance to update the
> outermost regs, the authoritative RIP would be the original one written
> by HW, right?

The only IST interrupt that would hit there is MCE and it would
probably be a critical error. Do we really need to worry about such an
unlikely scenario? The system is probably doomed anyway.

-- Steve