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    SubjectRe: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

    * Linus Torvalds <torvalds@linux-foundation.org> wrote:

    > On Mon, 15 Jun 2009, tip-bot for Peter Zijlstra wrote:
    > >
    > > __copy_from_user_inatomic() isn't NMI safe in that it can trigger
    > > the page fault handler which is another trap and its return path
    > > invokes IRET which will also close the NMI context.
    >
    > That's not the only problem.
    >
    > An even more fundamental problem is that the page fault handler is
    > not re-entrant because of simple the value in %cr2. So regardless
    > of any 'iret' issues, you *CANNOT* take a page fault in an NMI,
    > because the NMI might happen while we're in the critical region of
    > having taken another page fault, but before we've saved off the
    > value of %cr2 in that old page fault.
    >
    > If the NMI handler causes a page fault, it will corrupt the %cr2
    > of the outer page fault. That's why the page fault is done with an
    > interrupt gate, and why we have that conditional
    > local_irq_enable() in it.
    >
    > So page faults are fundamentally only safe wrt normal interrupts,
    > not NMI.

    ahhh ... a light goes up. Indeed.

    I was suspecting something much more complex: like the CPU somehow
    having shadow state for attempted-fault which gets confused by
    NMI->fault.

    A simple cr2 corruption would explain all those cc1 SIGSEGVs and
    other user-space crashes i saw, with sufficiently intense sampling -
    easily.

    The thing is, that __copy_user_inatomic() has been in
    arch/x86/oprofile/backtrace.c for years, i didnt even suspect some
    simple, fundamental flaw like this. Apparently nobody uses it.

    This is really good news in a sense: i really hate that additional
    entry*.S mucking in the exception path in the dont-IRET patch. We
    want less entry*.S magic, not more.

    Ingo


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