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    SubjectRe: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()
    On Wed, Mar 29, 2017 at 6:41 PM, Kees Cook <keescook@chromium.org> wrote:
    > On Wed, Mar 29, 2017 at 3:38 PM, Andy Lutomirski <luto@amacapital.net> wrote:
    >> On Wed, Mar 29, 2017 at 11:15 AM, Kees Cook <keescook@chromium.org> wrote:
    >>> Based on PaX's x86 pax_{open,close}_kernel() implementation, this
    >>> allows HAVE_ARCH_RARE_WRITE to work on x86.
    >>>
    >>
    >>> +
    >>> +static __always_inline unsigned long __arch_rare_write_begin(void)
    >>> +{
    >>> + unsigned long cr0;
    >>> +
    >>> + preempt_disable();
    >>
    >> This looks wrong. DEBUG_LOCKS_WARN_ON(!irqs_disabled()) would work,
    >> as would local_irq_disable(). There's no way that just disabling
    >> preemption is enough.
    >>
    >> (Also, how does this interact with perf nmis?)
    >
    > Do you mean preempt_disable() isn't strong enough here? I'm open to
    > suggestions. The goal would be to make sure nothing between _begin and
    > _end would get executed without interruption...
    >

    Sorry for the very slow response.

    preempt_disable() isn't strong enough to prevent interrupts, and an
    interrupt here would run with WP off, causing unknown havoc. I tend
    to think that the caller should be responsible for turning off
    interrupts.

    --Andy

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