Re: Dealing with the NMI mess

[Andy Lutomirski]

On Fri, Jul 24, 2015 at 10:10 AM, Willy Tarreau <w@xxxxxx> wrote:
> On Fri, Jul 24, 2015 at 08:48:57AM -0700, Andy Lutomirski 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))
>>   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, section 5.8 of the SDM makes me think we could possibly abuse SYSRET
> to emulate IRET, and then possibly simplify the flags processing. It says
> that it takes the CPL3 code segment but nowhere it says that the target is
> validated for effectively being userland, and further it suggests that it
> doesn't validate anything :
>
>   "It is the responsibility of the OS to ensure the descriptors in
>    the GDT/LDT correspond to the selectors loaded by SYSCALL/SYSRET
>    (consistent with the base, limit, and attribute values forced by
>    the instructions)."

You are an evil bastard.  I seriously doubt that this will work.
SYSRET goes to CPL3 no matter what.  Also, I don't think you want to
start poking at MSRs to return.

--Andy
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