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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Message-ID: <bb85a9bb-4c00-f8f7-13f6-3f1bed997e9c@intel.com>
Date:   Fri, 26 Jan 2018 07:27:38 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yves-Alexis Perez <corsac@...ian.org>,
        David Woodhouse <dwmw@...zon.co.uk>, arjan@...ux.intel.com,
        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 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?

I haven't found anything compelling enough to justify the downsides,
especially since things without SMEP tend not to have PCIDs as well.