Messages in this thread |  | | Subject | Re: perf_fuzzer compiled for x32 causes reboot | From | "H. Peter Anvin" <> | Date | Tue, 25 Feb 2014 06:34:55 -0800 |
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#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.weaver@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
-- Sent from my mobile phone. Please pardon brevity and lack of formatting.
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