linux-kernel - Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti

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Message-ID: <4e32af93-f632-ae17-eed4-c7023c1b9cc5@linux.intel.com>
Date:   Thu, 11 Jan 2018 10:57:51 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andy Lutomirski <luto@...nel.org>, Willy Tarreau <w@....eu>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set

On 01/11/2018 10:51 AM, Linus Torvalds wrote:
> On Thu, Jan 11, 2018 at 10:38 AM, Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
>> On 01/11/2018 10:32 AM, Josh Poimboeuf wrote:
>>>> hmm. Exposing cr3 to user space will make it trivial for user process
>>>> to know whether kpti is active. Not sure how exploitable such
>>>> information leak.
>>> It's already trivial to detect PTI from user space.
>> Do tell.
> One way to do it is to just run the attack, and see if you get something.

Not judging how trivial (or not) the attack is, I was hoping for
something that was *not* the attack itself. :)

I'd love to have a tool that tells you for sure "KPTI enabled or not",
but I'd also love to have it be something I can easily distribute
without it being handled like a WMD.