linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <CALCETrWjzU79ASDK+0RJQyCy6qTdM3FPTa4ZM0d5sVW66yhcug@mail.gmail.com>
Date:	Fri, 24 Jul 2015 08:48:57 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Fri, Jul 24, 2015 at 1:13 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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.
>

The Intel SDM says:

17.3.1.2 Data Memory and I/O Breakpoint Exception Conditions

Data memory and I/O breakpoints are reported when the processor
attempts to access a memory or I/O address
specified in a breakpoint-address register (DR0 through DR3) that has
been set up to detect data or I/O accesses
(R/W flag is set to 1, 2, or 3). The processor generates the exception
after it executes the instruction that made the
access, so these breakpoint condition causes a trap-class exception to
be generated.

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))
  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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