On Tue, Oct 04, 2011 at 08:34:40AM +0200, Borislav Petkov wrote:
> On Mon, Oct 03, 2011 at 05:33:36PM +0530, K.Prasad wrote:
> > It's interesting...according to Intel's Software Developer Manual
> > (quoting from Volume 3A, Chapter 15), the MCIP bit in IA32_MCG_STATUS
> > MSR behaves as described below.
> >
> > "MCIP (machine check in progress) flag, bit 2 Indicates (when set)
> > that a machine-check exception was generated. Software can set or clear this
> > flag. The occurrence of a second Machine-Check Event while MCIP is set will
> > cause the processor to enter a shutdown state."
> >
> > While in do_machine_check function, we enter the panic path (for
> > unrecoverable errors) much before the IA32_MCG_STATUS MSR is reset and
> > this is likely to dangerous.
> >
> > 911 void do_machine_check(struct pt_regs *regs, long error_code)
> > 912 {
> > .............
> > ................
> > 1055 if (no_way_out && tolerant < 3)
> > 1056 mce_panic("Fatal machine check on current CPU", final, msg);
> > .............
> > ................
> > 1073 mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
> > 1074 out:
> >
> > It'd be interesting to know the type of memory error (as classified by
> > the processor) for which you're able to capture the memory dump.
> > Maybe a dump of the various MCE status registers (and struct mce) would
> > help us understand the behaviour on your system better.
>
> Well, there are MCE types for which we need to panic but we don't
> necessarily corrupt memory. Your approach is to unconditionally avoid
> dumping core whenever we panic while you should look at the MCE
> signature and decide then whether to capture crashed kernel memory or
> not.
>
> For example, if the MCE signature says UC DRAM error, then you can
> be pretty sure that there is a landmine somewhere in the DRAM region
> mapping the crashed kernel. If it is, say, a UC when doing data fills
> from L2 to L1, that doesn't necessarily mean that DRAM is corrupted. But
> even in the first case, you can evaluate the MCi_ADDR reported with the
> UC DRAM error and simply skip that particular cacheline when dumping the
> core instead of not capturing anything at all.
>
True. Like stated by me earlier, there could be two possible outcomes
from capturing memory dump in such cases - they're either dangerous or
doesn't make sense. It is best to avoid a normal kdump in both cases,
although the elf-note doesn't distinguish between the two.
NT_NOCOREDUMP, in my opinion, is just the first step towards introducing
a framework where different code paths that lead to panic() can
'opt-out' from kdump by adding an elf-note.
We can modify this to add more fine-grained messages using different elf-note
types (or use the elf-note name under the NT_NOCOREDUMP type) to
indicate the cause/type of crash.
I'd like to hear further from you and the rest of the community to see if
there's a need felt for such a change.
> Btw, the doublefault example you give above - is this something you
> experience on real hardware or just a theoretical thing?
>
Unfortunately, I still haven't been able to try injecting memory errors
and study the behaviour (trying to get access to machine with
appropriate firmware). I'll have a reply to this after some experiments
with memory error injection.
Thanks,
K.Prasad
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