Re: [PATCH 1/1] Fix int1 recursion when no perf_bp_event is registered

[Andy Lutomirski]

On Thu, Dec 10, 2015 at 10:55 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> Jeff,
>
> On Thu, 10 Dec 2015, Jeff Merkey wrote:
>
>> If an int1 hardware breakpoint exception is triggered, but no perf bp
>> pevent block was registered from arch_install_hw_breakpoint, the
>> system will hard hang with the CPU stuck constantly re-interrupting at
>> the same execution address because the resume flag never gets set, and
>> the NOTIFY_DONE state prevents other int1 handlers, including the
>> default handler in do_debug, from running to handle the condition.
>> Can be reproduced by writing a program that sets an execute breakpoint
>> at schedule() without calling arch_install_hw_breakpoint.
>>
>> The proposed fix checks the dr7 register and sets the resume flag in
>> pt->regs if it determines an executed breakpoint was triggered just in
>> case the check lower down fails.  I have seen this bug and its a bug.
>
>> Signed-off-by:  jeffmerkey@xxxxxxxxx
>> diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
>> index 50a3fad..6effcae 100644
>> --- a/arch/x86/kernel/hw_breakpoint.c
>> +++ b/arch/x86/kernel/hw_breakpoint.c
>> @@ -475,6 +475,14 @@ static int hw_breakpoint_handler(struct die_args *args)
>>       for (i = 0; i < HBP_NUM; ++i) {
>>               if (likely(!(dr6 & (DR_TRAP0 << i))))
>>                       continue;
>> +             /*
>> +              * Set up resume flag to avoid breakpoint recursion when
>> +              * returning back to origin in the event an int1
>> +              * exception is triggered and no event handler
>> +              * is present.
>> +              */
>> +             if ((dr7 & (3 << ((i * 4) + 16))) == 0)
>
> We have proper defines for all of this. See __encode_dr7().
>
>> +                     args->regs->flags |= X86_EFLAGS_RF;
>
> If there is a break point installed, then we do the same thing after
> calling perf_bp_event() again.

On brief inspection, this smells like a microcode bug.  Can you send
/proc/cpuinfo output?

For example, this CPU and microcode combination is known bad:

processor       : 7
vendor_id       : AuthenticAMD
cpu family      : 21
model           : 2
model name      : AMD Opteron(tm) Processor 3380
stepping        : 0
microcode       : 0x6000832

If this is the issue, I'm not sure we want to be in the business of
working around localized microcode bugs and, if we do, then I think we
should explicitly detect the bug and log about it.

--Andy
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