linux-kernel - Re: Dealing with the NMI mess

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Date:	Fri, 24 Jul 2015 12:06:56 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.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 Fri, 24 Jul 2015 08:48:57 -0700
Andy Lutomirski <luto@...capital.net> wrote:

> So by the time we detect that we've hit a watchpoint, the instruction
> that tripped it is done and we don't need RF.  Furthermore, after
> reading 17.3.1.1: I *think* that regs->flags withh have RF *clear* if
> we hit a watchpoint.  So this might be as simple as:
> 
> if ((dr6 && (0xf * DR_TRAP0) && (regs->flags & (X86_EFLAGS_RF |
> X86_EFLAGS_IF)) == X86_EFLAGS_RF && !user_mode(regs))

Also, wouldn't !(regs->X86_EFLAGS_IF) && !user_mode(regs) be a bug?
When do we allow coming from userspace with interrupts disabled?

-- Steve

>   for (i = 0; i < 4; i++)
>     if (dr6 & (DR_TRAP0<<i)) {
>       /* hit a kernel breakpoint with IF clear */
>       dr7 &= ~(DR_GLOBAL_ENABLE << (i * DR_ENABLE_SHIFT));
>     }
> 
> I'm not saying that your code is wrong, but I think this is simpler
> and avoids poking at yet more per-cpu state from NMI context, which is
> kind of nice.
> 
> If you don't like the RF games above, it would also be straightforward
> to parse dr0..dr3 for each DR_TRAP bit that's set and see if it's a
> breakpoint.
> 
> --Andy

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