linux-kernel - Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch

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Message-ID: <CAGXu5jKiHvqEXM3H-Euij-acPF4BoDFJR-WinwXaeWOguf1+Hg@mail.gmail.com>
Date:   Tue, 9 Jan 2018 09:53:00 -0800
From:   Kees Cook <keescook@...gle.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "riel@...hat.com" <riel@...hat.com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on
 context switch

On Tue, Jan 9, 2018 at 5:04 AM, David Woodhouse <dwmw2@...radead.org> wrote:
> On Mon, 2018-01-08 at 19:27 -0800, Andy Lutomirski wrote:
>> >
>> > If SMEP is not active, speculation can go anywhere, including to a user
>> > controlled gadget which can reload any registers it needs, including
>> > with immediate constants.
>>
>> I thought that, even on pre-SMEP hardware, the CPU wouldn't
>> speculatively execute from NX pages.  And PTI marks user memory NX
>> in kernel mode.
>
> Hm, now that could be useful.
>
> Do *all* the KPTI backports (some of which are reimplementations rather
> than strictly backports) mark user memory NX?

Yup. The KAISERish ports (4.9 and 4.4) have the same feature.

-Kees

-- 
Kees Cook
Pixel Security