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

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Message-ID: <20180109131059.GI6176@hirez.programming.kicks-ass.net>
Date:   Tue, 9 Jan 2018 14:10:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
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>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.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 09, 2018 at 01:04:20PM +0000, David Woodhouse 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?

That doesn't really matter for upstream though; but see here:

  https://lkml.kernel.org/r/1515502580-12261-5-git-send-email-w@1wt.eu