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From: Dave Hansen <dave.hansen@intel.com>
To: Yves-Alexis Perez <corsac@debian.org>,
	David Woodhouse <dwmw@amazon.co.uk>,
	arjan@linux.intel.com, tglx@linutronix.de, karahmed@amazon.de,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	tim.c.chen@linux.intel.com, bp@alien8.de, peterz@infradead.org,
	pbonzini@redhat.com, ak@linux.intel.com,
	torvalds@linux-foundation.org, gregkh@linux-foundation.org,
	gnomes@lxorguk.ukuu.org.uk
Subject: Re: [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which are not vulnerable to Meltdown
Date: Fri, 26 Jan 2018 07:27:38 -0800	[thread overview]
Message-ID: <bb85a9bb-4c00-f8f7-13f6-3f1bed997e9c@intel.com> (raw)
In-Reply-To: <1516968886.19619.7.camel@debian.org>

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.

  parent reply	other threads:[~2018-01-26 15:27 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-24 16:56 [PATCH v3 0/6] Basic Speculation Control feature support David Woodhouse
2018-01-24 16:57 ` [PATCH v3 1/6] x86/cpufeatures: Add CPUID_7_EDX CPUID leaf David Woodhouse
2018-01-24 17:18   ` Greg KH
2018-01-24 16:57 ` [PATCH v3 2/6] x86/cpufeatures: Add Intel feature bits for Speculation Control David Woodhouse
2018-01-24 17:18   ` Greg KH
2018-01-24 16:57 ` [PATCH v3 3/6] x86/cpufeatures: Add AMD " David Woodhouse
2018-01-24 17:20   ` Greg KH
2018-01-24 17:57     ` David Woodhouse
2018-01-24 22:52       ` Tom Lendacky
2018-02-20 15:22         ` Tom Lendacky
2018-01-24 16:57 ` [PATCH v3 4/6] x86/msr: Add definitions for new speculation control MSRs David Woodhouse
2018-01-24 17:16   ` Greg KH
2018-01-24 16:57 ` [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which are not vulnerable to Meltdown David Woodhouse
2018-01-24 17:26   ` Greg KH
2018-01-26 12:14   ` Yves-Alexis Perez
2018-01-26 12:27     ` David Woodhouse
2018-01-26 15:27     ` Dave Hansen [this message]
2018-01-26 15:30       ` Arjan van de Ven
2018-01-26 16:47     ` Alan Cox
2018-01-24 16:57 ` [PATCH v3 6/6] x86/cpufeature: Blacklist SPEC_CTRL on early Spectre v2 microcodes David Woodhouse
2018-01-24 17:29   ` Greg KH
2018-01-24 17:58     ` David Woodhouse
2018-01-24 21:29     ` Ingo Molnar
2018-01-26 12:16   ` Yves-Alexis Perez
2018-01-26 12:27     ` David Woodhouse

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