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

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Date:	Tue, 26 Jul 2016 13:59:41 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Brian Gerst <brgerst@...il.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>,
	Steven Rostedt <rostedt@...dmis.org>,
	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, Jul 26, 2016 at 01:49:06PM -0400, Brian Gerst wrote:
> On Tue, Jul 26, 2016 at 12:47 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Mon, Jul 25, 2016 at 05:09:44PM -0700, Andy Lutomirski wrote:
> >> On Sat, Jul 23, 2016 at 7:04 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >> > Am I correct in understanding that there can only be one level of NMI
> >> > nesting at any given time?  If so, could we make it easier on the
> >> > unwinder by putting the nested NMI on a separate software stack, so the
> >> > "next stack" pointers are always in the same place?  Or am I just being
> >> > naive?
> >>
> >> I think you're being naive :)
> >
> > Another dumb question: since NMIs are reentrant, have you considered
> > removing the NMI IST entry, and instead just have NMIs keep using the
> > current stack?
> >
> > The first NMI could then be switched to an NMI software stack, like IRQs
> > (assuming there's a way to do that atomically!).  And then determining
> > the context of subsequent NMIs would be straightforward, and we'd no
> > longer need to jump through all those horrible hoops in the entry code
> > to deal with NMI nesting.
> >
> > Now you can tell me what else I'm missing...
> 
> There are several places (most notably SYSCALL entry) where the kernel
> stack pointer is unsafe/user controlled for a brief time.  Since an
> NMI can interrupt anywhere in the kernel, you have to use an IST to
> protect against that case.

Ah, that makes sense.  Thanks.

-- 
Josh