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

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Message-ID: <20180111190551.kefaiyemiljuolyd@treble>
Date:   Thu, 11 Jan 2018 13:05:51 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>, aarcange@...hat.com
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set

On Thu, Jan 11, 2018 at 10:57:51AM -0800, Dave Hansen wrote:
> 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.

Instead of the meltdown attack, why not just run the KASLR attack, with
prefetches + cache timing?

Something like this (I haven't tested it though):

  https://github.com/IAIK/prefetch/blob/master/addrspace/addrspace.c

Andrea also created such a tool, but IIRC, it requires knowing a kernel
address, so it wouldn't work with KASLR.  It could probably be extended
to scan kernel space until it finds something.

We could maybe even add such a tool to the kernel tree.

-- 
Josh