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

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Date:   Wed, 5 Sep 2018 14:35:41 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] x86/pkeys: Explicitly treat PK #PF on kernel address as a
 bad area

On 09/04/2018 02:27 PM, Andy Lutomirski wrote:
> Also, I doubt it matters right here, but !X86_PF_USER isn't quite the
> same thing as "originating from kernel code" -- it can also be user
> code that does a CPL0 access due to exception delivery or access to a
> descriptor table.  Which you saw plenty of times while debugging
> PTI... :)  I doubt any of those should be spurious, though.

Yeah, you're talking about "implicit supervisor access".  Right?

I definitely saw those during KAISER/KPTI development.  But, it's
probably worth noting that the code that we have that _looks_ for these:

>         /*
>          * It's safe to allow irq's after cr2 has been saved and the
>          * vmalloc fault has been handled.
>          *
>          * User-mode registers count as a user access even for any
>          * potential system fault or CPU buglet:
>          */
>         if (user_mode(regs)) {
>                 local_irq_enable();
>                 error_code |= X86_PF_USER;
>                 flags |= FAULT_FLAG_USER;

is in the user address space handling portion of the code.  So, it's
really not useful for any of the _known_ implicit supervisor-mode
accesses.  I think it can only get triggered in the case of hardware or
software bugs.