linux-kernel - Re: Dealing with the NMI mess

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Date:	Fri, 24 Jul 2015 11:02:51 +0200
From:	Willy Tarreau <w@....eu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>, 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>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Fri, Jul 24, 2015 at 10:13:26AM +0200, Peter Zijlstra wrote:
> On Thu, Jul 23, 2015 at 02:59:56PM -0700, Linus Torvalds wrote:
> > Hmmm. I thought watchpoints were "before the instruction" too, but
> > that's just because I haven't used them in ages, and I didn't remember
> > the details. I just looked it up.
> > 
> > You're right - the memory watchpoints trigger after the instruction
> > has executed, so RF isn't an issue. So yes, the only issue is
> > instruction breakpoints, and those are the only ones we need to clear.
> > 
> > And that makes it really easy.
> > 
> > So yes, I agree. We only need to clear all kernel breakpoints.
> 
> But but but, we can access userspace with !IF, imagine someone doing:
> 
>   local_irq_disable();
>   copy_from_user_inatomic();
> 
> and as luck would have it, there's a breakpoint on the user memory we
> just touched. And we go and disable a user breakpoint.

Then shouldn't we use !IF && RSP matches NMI's stack ?
User-space cannot control the two at once.

Willy

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