linux-kernel - Re: [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which are not vulnerable to Meltdown

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Date:   Fri, 26 Jan 2018 07:30:15 -0800
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Yves-Alexis Perez <corsac@...ian.org>,
        David Woodhouse <dwmw@...zon.co.uk>, tglx@...utronix.de,
        karahmed@...zon.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, bp@...en8.de, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which are
 not vulnerable to Meltdown

On 1/26/2018 7:27 AM, Dave Hansen wrote:
> On 01/26/2018 04:14 AM, Yves-Alexis Perez wrote:
>> I know we'll still be able to manually enable PTI with a command line option,
>> but it's also a hardening feature which has the nice side effect of emulating
>> SMEP on CPU which don't support it (e.g the Atom boxes above).
> 
> For Meltdown-vulnerable systems, it's a no brainer: pti=on.  The
> vulnerability there is just too much.
> 
> But, if we are going to change the default, IMNHO, we need a clear list
> of what SMEP emulation mitigates and where.  RSB-related Variant 2 stuff
> on Atom where the kernel speculatively 'ret's back to userspace is
> certainly a concern.  But, there's a lot of other RSB stuffing that's
> going on that will mitigate that too.
> 
> Were you thinking of anything concrete?

not Atom though. Atom has has SMEP for a very long time, at least the ones
that do speculation do afaict.

SMEP is for other bugs (dud kernel function pointer) and for that,
emulating SMEP is an interesting opt-in for sure.