Messages in this thread | | | Date | Thu, 15 May 2014 11:42:30 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 7/9] x86: skip check for spurious faults for non-present faults |
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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.
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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@xensource.com> Cc: Harvey Harrison <harvey.harrison@gmail.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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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