Messages in this thread |  | | Date | Tue, 25 Feb 2014 09:07:51 -0500 (EST) | From | Vince Weaver <> | Subject | Re: perf_fuzzer compiled for x32 causes reboot |
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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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