linux-kernel - Re: Dealing with the NMI mess

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Date:	Thu, 23 Jul 2015 14:08:59 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	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>,
	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 1:49 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.

We can entertain the notion *eventually* to do something clever, but
for now, just say: stability and simplicity is more important.

People can use tracepoints in interrupts-off code (they get rewritten
with 'int3', that's fine), but not instruction breakpoints.

> Issue C: #DB with invalid stack pointer (can happen due to watchpoints
> during SYSCALL entry or SYSRET exit).  I guess we need to ban such
> watchpoints.

.. but this is unrelated, to NMI, just "syscall is a nasty interface".
Don't we already ban them?

> Issue D: debug exception inside EFI (especially mixed-mode EFI).  We
> can't return using RET, so we need to catch that case.

If NMI code calls EFI code, then it's broken.

> These issues mostly go away if we preemptively disarm DR7 early in NMI
> processing and rearm it at the end.

I'm not *violently* opposed to that, but it's just a band-aid. It
doesn't *fix* anything. You aren't protecting against random DB
exceptions just because somebody put a data breakpoint on the NMI
stack, for example. You still get page faults. Etc etc.

So I thinkt he whole "use ret instead" is a pretty simple approach.
Make that "just work".

Then, if you want to play with dr7 inside NMI to make it more likely
that you can have breakpoints live in irq-off situation, I think
that's a magic special case. It shouldn't be part of the design.
Things should work without it.

             Linus
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