linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <CALCETrUJq+JsSr42dNgw9GRFtB7yzJ2ZJ7EEgU-KJ_5Z-RhK0g@mail.gmail.com>
Date:	Thu, 23 Jul 2015 14:59:46 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.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>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Thu, Jul 23, 2015 at 2:54 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jul 23, 2015 at 2:45 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> Or we just re-enable them on the way out of NMI (i.e. the very last
>> thing we do in the NMI handler).  I don't want to break regular
>> userspace gdb when perf is running.
>
> I'd really prefer it if we don't touch NMI code in those kinds of
> ways. The NMI code is fragile as hell. All the problems we have with
> it is exactly due to "where is the boundary" issues.
>
> That's why I *don't* want NMI code to do magic crap. Anything that
> says "disable this during this magic window" is broken. The problems
> we've had are exactly about atomicity of the entry/exit conditions,
> and there is no really good way to get them right.
>
> I'd be much happier with a _TIF_USER_WORK_MASK approach exactly
> because it's so *obvious* that it's not a boundary condition.
>
> I dislike the "disable and re-enable dr7 in the NMI handler" exactly
> because it smells like "we can only handle faults in _this_ region".
> It may be true, but it's also what I want us to get away from. I'd
> much rather have the "big picture" be that we can take faults anywhere
> at all (*), and that none of the core code really cares. Then we "fix
> up" user space.

OK, new proposal:

In do_debug, if we trip an instruction breakpoint while
!user_mode(regs) && ((regs->flags & X86_EFLAGS_IF) == 0), then disarm
*that breakpoint*.

Why?  It only looks at hardware state (dr6 and dr7), and it can't
break gdb, because gdb can't set a breakpoint that will cause this
problem.

All the other variants of this either need cached state or break gdb
watchpoints on stack variables with perf running.

--Andy

>
>                    Linus
>
> (*) And yes, sysenter and not having a stack at all is very special,
> and I think we will *always* have to have that magical special case of
> the first few instructions there. But that's a separate hardware
> limitation we can't get around.



-- 
Andy Lutomirski
AMA Capital Management, LLC
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