linux-kernel - Re: [PATCH v3] Documentation: Add section about CPU vulnerabilities for Spectre

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Date:   Tue, 18 Jun 2019 22:33:10 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
cc:     Jon Masters <jcm@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ben Greear <greearb@...delatech.com>, stable@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Jiri Kosina <jikos@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Waiman Long <longman9394@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Mark Gross <mgross@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: Re: [PATCH v3] Documentation: Add section about CPU vulnerabilities
 for Spectre

On Tue, 18 Jun 2019, Tim Chen wrote:
> On 6/17/19 1:30 PM, Jon Masters wrote:
> > On 6/17/19 4:22 PM, Jon Masters wrote:
> > 
> >>> +   For kernel code that has been identified where data pointers could
> >>> +   potentially be influenced for Spectre attacks, new "nospec" accessor
> >>> +   macros are used to prevent speculative loading of data.
> >>
> >> Maybe explain that nospec (speculative clamping) relies on the absence
> >> of value prediction in the masking (in current hardware). It may NOT
> >> always be a safe approach in future hardware, where Spectre-v1 attacks
> >> are likely to persist but hardware may speculate about the mask value.
> > 
> > Something like the Arm CSDB barrier would seem to be potentially useful
> > for $FUTURE_X86 as a fence with lighter-weight semantics than an *fence.
> > 
> 
> Is it necessary to go into such level of implementation details on nospec?
> These seem to be appropriate as code comments in nospec for kernel developer.
> But for an admin-guide doc, it may confuse sys admin to think that nospec
> could not be ineffective.
> 
> When new hardware appears that need new implementations of nospec, we should
> tweak nospec and not need the admin to worry about such implementation details.

Correct. Those details are architecture level details. See the split of the
MDS documentation...

Thanks,

	tglx