linux-kernel - Re: Dealing with the NMI mess

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Date:	Thu, 23 Jul 2015 13:53:34 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Willy Tarreau <w@....eu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.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>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Thu, Jul 23, 2015 at 1:52 PM, Willy Tarreau <w@....eu> wrote:
> On Thu, Jul 23, 2015 at 01:38:33PM -0700, Linus Torvalds wrote:
>> On Thu, Jul 23, 2015 at 1:21 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> >
>> > 2. Forbid IRET inside NMIs.  Doable but maybe not that pretty.
>> >
>> > We haven't considered:
>> >
>> > 3. Forbid faults (other than MCE) inside NMI.
>>
>> I'd really prefer #2. #3 depends on us getting many things right, and
>> never introducing new cases in the future.
>>
>> #2, in contrast, seems to be fairly localized. Yes, RF is an issue,
>> but returning to user space with RF clear doesn't really seem to be
>> all that problematic.
>
> What's the worst case that can happen with RF cleared when returing
> to user space ? My understanding is that it's just that we risk to
> break again on an instruction that had a break point set and which
> already triggered the breakpoint, right ?

I assume Linus meant returning to kernel space with RF clear.  Returns
to userspace have their own fancy logic here, and it's survived for a
couple of releases, including through an explicit test of RF handling
:)

--Andy
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