linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <20150724081326.GO25159@twins.programming.kicks-ass.net>
Date:	Fri, 24 Jul 2015 10:13:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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>,
	Willy Tarreau <w@....eu>, Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

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.

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