Re: [Xen-devel] [PATCH 7/9] x86: skip check for spurious faults for non-present faults

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On 04/15/2014 07:15 AM, David Vrabel wrote:
> If a fault on a kernel address is due to a non-present page, then it
> cannot be the result of stale TLB entry from a protection change (RO
> to RW or NX to X).  Thus the pagetable walk in spurious_fault() can be
> skipped.

Erk... this code is screaming WTF to me.  The x86 architecture is such
that the CPU is responsible for avoiding these faults.

<dig> <dig> <dig>

5b727a3b0158a129827c21ce3bfb0ba997e8ddd0

    x86: ignore spurious faults

    When changing a kernel page from RO->RW, it's OK to leave stale TLB
    entries around, since doing a global flush is expensive and they
    pose no security problem.  They can, however, generate a spurious
    fault, which we should catch and simply return from (which will
    have the side-effect of reloading the TLB to the current PTE).

    This can occur when running under Xen, because it frequently changes
    kernel pages from RW->RO->RW to implement Xen's pagetable semantics.
    It could also occur when using CONFIG_DEBUG_PAGEALLOC, since it
    avoids doing a global TLB flush after changing page permissions.

    Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>
    Cc: Harvey Harrison <harvey.harrison@xxxxxxxxx>
    Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
    Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Again WTF?

Are we chasing hardware errata here?  Or did someone go off and *assume*
that the x86 hardware architecture work a certain way?  Or is there
something way more subtle going on?

I guess next step is mailing list archaeology...

Does anyone still have contacts with Jeremy, and if so, could they poke
him perhaps?

        -hpa


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