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

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Message-ID: <f6d1d28f-cd79-16b6-ab2f-aa113b26bf3f@linux.intel.com>
Date:   Thu, 11 Jan 2018 10:30:46 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Willy Tarreau <w@....eu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "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 10:21 AM, Alexei Starovoitov wrote:
>> I was thinking that maybe we should add a new field or two to pt_regs.
>> They could store CR2 and maybe CR3 as well.  I'd also like to expose
>> the error code of exceptions in stack traces.  We should get this
>> integrated right into the unwinder.
> 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 also gives userspace a pretty valuable physical address to go after.
That, plus a KASLR defeat gives you a known-valuable virtual address to
target.  That's no good.

I think CR3 is a non-starter.