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

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Dec 10, 2015 at 11:09:21AM -0800, Andy Lutomirski wrote:
> On Thu, Dec 10, 2015 at 10:55 AM, Thomas Gleixner <tglx@linutronix.de> 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@gmail.com
> >> 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.

seems like the issue we hit some time ago:
  http://marc.info/?l=linux-kernel&m=143976421117070&w=2

jirka