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    SubjectRe: Dealing with the NMI mess
    On Fri, Jul 24, 2015 at 10:13:26AM +0200, Peter Zijlstra wrote:
    > On Thu, Jul 23, 2015 at 02:59:56PM -0700, Linus Torvalds wrote:
    > > Hmmm. I thought watchpoints were "before the instruction" too, but
    > > that's just because I haven't used them in ages, and I didn't remember
    > > the details. I just looked it up.
    > >
    > > You're right - the memory watchpoints trigger after the instruction
    > > has executed, so RF isn't an issue. So yes, the only issue is
    > > instruction breakpoints, and those are the only ones we need to clear.
    > >
    > > And that makes it really easy.
    > >
    > > So yes, I agree. We only need to clear all kernel breakpoints.
    >
    > But but but, we can access userspace with !IF, imagine someone doing:
    >
    > local_irq_disable();
    > copy_from_user_inatomic();
    >
    > and as luck would have it, there's a breakpoint on the user memory we
    > just touched. And we go and disable a user breakpoint.

    Then shouldn't we use !IF && RSP matches NMI's stack ?
    User-space cannot control the two at once.

    Willy



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