linux-kernel - Re: [PATCH] x86/pkeys: Explicitly treat PK #PF on kernel address as a bad area

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Message-ID: <CALCETrU-HXfr_eAfeS1t7JH+-gOh0ZfAoGxjcOckxEX1fN_YDA@mail.gmail.com>
Date:   Tue, 4 Sep 2018 12:56:13 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] x86/pkeys: Explicitly treat PK #PF on kernel address as a
 bad area

On Tue, Sep 4, 2018 at 12:50 PM, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
> 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@...el.com>
>> Cc: Dave Hansen <dave.hansen@...el.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:

I have no objection to this patch.

Dave, why did you think that we could get a PK fault on the vsyscall
page, even on kernels that still marked it executable?  Sure, you
could get an instruction in the vsyscall page to get a PK fault, but
CR2 wouldn't point to the vsyscall page, right?

>> --
>> 2.18.0
>>