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.christopher...@intel.com> Cc: Dave Hansen <dave.han...@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
