Messages in this thread | | | Date | Mon, 12 Mar 2012 13:31:31 +0000 | From | "Jan Beulich" <> | Subject | Re: recent x86-64 nested NMI adjustments |
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>>> On 12.03.12 at 14:16, Steven Rostedt <rostedt@goodmis.org> wrote: > On Mon, 2012-03-12 at 12:10 +0000, Jan Beulich wrote: >> Hi Steven, >> >> the explanation of 45d5a1683c04be28abdf5c04c27b1417e0374486 >> seems bogus to me: When arriving from user mode, %rsp won't point >> to the user stack anymore, as it gets switched away from during the >> processing of the exception (the more that the IDT entry specifies a >> separate stack anyway, which even guarantees this for kernel mode >> entries). > > No it is real, and I had a test program that exploited it. I'm not > worried about the current %rsp, I'm worried about what %rsp is saved on > the stack. Two things are used to check if the incoming NMI is nested or > not. > > 1) if the on-stack "in-nmi" variable is set > > 2) if the saved %rsp is pointing to the NMI stack.
Ah, right - the way the new check got placed I (wrongly) implied it to guard the immediately succeeding check of the "special variable", whereas it really guards (only) test_in_nmi.
Thanks for the explanation, and sorry for the noise then.
Jan
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