linux-kernel - Re: Dealing with the NMI mess

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Date:	Thu, 23 Jul 2015 23:46:03 +0200
From:	Willy Tarreau <w@....eu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Thu, Jul 23, 2015 at 05:31:05PM -0400, Steven Rostedt wrote:
> On Thu, 23 Jul 2015 14:08:59 -0700
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > On Thu, Jul 23, 2015 at 1:49 PM, Andy Lutomirski <luto@...capital.net> wrote:
> > >
> > > Issue A: to return with RF clear, we need to disarm the breakpoint.
> > > If it's limited to the duration of the NMI, that's easy.  If not, when
> > > do we re-arm?  New prepare_exit_to_usermode hook?  Hmm, setting ti
> > > flags during context switch may target the wrong task.
> > 
> > We don't re-arm it.
> > 
> 
> Let me get this straight. The idea is in the #DB handler to detect that
> it was triggered in NMI context, and if so, simply disarm that
> breakpoint permanently, right?
> 
> Nothing should be adding hw breakpoints to NMI code anyway. Sounds
> perfectly reasonable to me. Of course, how we tell we are in NMI
> brings back all the races as we had in the nesting code. We can check
> the per-cpu variable that is set with nmi_enter() and cleared at
> nmi_exit() but what happens if the breakpoint is outside those calls.
> We can check the stack pointer, but then we are back to userspace
> fooling us. Maybe add the DF trick again?

Can't the back link of the TSS tell us where we come from ? At least
it should not be manipulable from user-space.

Willy

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