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.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 >
