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    SubjectRe: [PATCH] x86/pkeys: Explicitly treat PK #PF on kernel address as a bad area
    Cc-ing Jann and Andy.

    On Tue, Aug 07, 2018 at 10:29:20AM -0700, Sean Christopherson wrote:
    > Kernel addresses are always mapped with _PAGE_USER=0, i.e. PKRU
    > isn't enforced, and so we should never see X86_PF_PK set on a
    > kernel address fault. WARN once to capture the issue in case we
    > somehow don't die, e.g. the access originated in userspace.
    >
    > Remove a similar check and its comment from spurious_fault_check().
    > The intent of the comment (and later code[1]) was simply to document
    > that spurious faults due to protection keys should be impossible, but
    > that's irrelevant and a bit of a red herring since we should never
    > get a protection keys fault on a kernel address regardless of the
    > kernel's TLB flushing behavior.
    >
    > [1] http://lists-archives.com/linux-kernel/28407455-x86-pkeys-new-page-fault-error-code-bit-pf_pk.html
    >
    > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
    > Cc: Dave Hansen <dave.hansen@intel.com>
    > ---
    > There's no indication that this condition has ever been encountered.
    > I came across the code in spurious_fault_check() and was confused as
    > to why we would unconditionally treat a protection keys fault as
    > spurious when the comment explicitly stated that such a case should
    > be impossible.
    >
    > Dave Hansen suggested adding a WARN_ON_ONCE in spurious_fault_check(),
    > but it seemed more appropriate to freak out on any protection keys
    > fault on a kernel address since that would imply a hardware issue or
    > kernel bug. I omitted a Suggested-by since this isn't necessarily
    > what Dave had in mind.
    >
    > arch/x86/mm/fault.c | 16 ++++++++++------
    > 1 file changed, 10 insertions(+), 6 deletions(-)
    >
    > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
    > index 2aafa6ab6103..f19a55972136 100644
    > --- a/arch/x86/mm/fault.c
    > +++ b/arch/x86/mm/fault.c
    > @@ -1040,12 +1040,6 @@ static int spurious_fault_check(unsigned long error_code, pte_t *pte)
    >
    > if ((error_code & X86_PF_INSTR) && !pte_exec(*pte))
    > return 0;
    > - /*
    > - * Note: We do not do lazy flushing on protection key
    > - * changes, so no spurious fault will ever set X86_PF_PK.
    > - */
    > - if ((error_code & X86_PF_PK))
    > - return 1;
    >
    > return 1;
    > }
    > @@ -1241,6 +1235,14 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
    > * protection error (error_code & 9) == 0.
    > */
    > if (unlikely(fault_in_kernel_space(address))) {
    > + /*
    > + * We should never encounter a protection keys fault on a
    > + * kernel address as kernel address are always mapped with
    > + * _PAGE_USER=0, i.e. PKRU isn't enforced.
    > + */
    > + if (WARN_ON_ONCE(error_code & X86_PF_PK))
    > + goto bad_kernel_address;
    > +
    > if (!(error_code & (X86_PF_RSVD | X86_PF_USER | X86_PF_PROT))) {
    > if (vmalloc_fault(address) >= 0)
    > return;
    > @@ -1253,6 +1255,8 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
    > /* kprobes don't want to hook the spurious faults: */
    > if (kprobes_fault(regs))
    > return;
    > +
    > +bad_kernel_address:
    > /*
    > * Don't take the mm semaphore here. If we fixup a prefetch
    > * fault we could otherwise deadlock:
    > --
    > 2.18.0
    >

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