#2 is what I really don't understand.
I worry something else is going on there
On February 25, 2014 6:07:51 AM PST, Vince Weaver <vincent.wea...@maine.edu>
wrote:
>On Mon, 24 Feb 2014, H. Peter Anvin wrote:
>
>> On 02/24/2014 11:30 AM, Peter Zijlstra wrote:
>> > On Mon, Feb 24, 2014 at 02:13:29PM -0500, Steven Rostedt wrote:
>> >> Ah, and x86_64 saves off the cr2 register when entering NMI and
>restores
>> >> it before returning. But it seems to be missing from the i386
>code.
>> >
>> > arch/x86/kernel/nmi.c:
>> >
>> > #define nmi_nesting_preprocess(regs)
>> > \
>> > do { \
>> > if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) { \
>> > this_cpu_write(nmi_state, NMI_LATCHED); \
>> > return; \
>> > } \
>> > this_cpu_write(nmi_state, NMI_EXECUTING); \
>> > this_cpu_write(nmi_cr2, read_cr2()); \
>> > } while (0); \
>> > nmi_restart:
>> >
>> > #define nmi_nesting_postprocess() \
>> > do { \
>> > if (unlikely(this_cpu_read(nmi_cr2) != read_cr2())) \
>> > write_cr2(this_cpu_read(nmi_cr2)); \
>> > if (this_cpu_dec_return(nmi_state)) \
>> > goto nmi_restart; \
>> > } while (0)
>> >
>> > That very much looks like saving/restoring CR2 to me.
>> >
>> > FWIW; I hate how the x86_64 and i386 versions of this NMI nesting
>magic
>> > are so completely different.
>>
>> Is there any way that nmi_cr2 can end up getting overwritten by
>multiple
>> nestings of some kind? I would have thought it would have made more
>> sense to spill cr2 onto the stack after the stack has been properly
>set up.
>
>So how can I help with debugging this?
>
>While the missing cr2 issue made debugging frustrating, I find the
>other
>aspects of the bug more serious:
>
> 1. Programs that are doing valid memory accesses can segfault
>and worse
>2. This bug can cause an instant-reboot of the system (I assume
>somehow
> with the right combination of memory accesses it causes a
> triple-fault?)
>
>#2 is why I spent all of this time tracking this down, I couldn't leave
>a
>machine fuzzing overnight without the machine rebooting.
>
>Vince
--
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