linux-kernel - Re: [PATCH RFC] x86/speculation: Clarify Spectre-v2 mitigation when STIBP/IBPB features are unsupported

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Message-ID: <20200121161412.GL7808@zn.tnic>
Date:   Tue, 21 Jan 2020 17:14:12 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RFC] x86/speculation: Clarify Spectre-v2 mitigation when
 STIBP/IBPB features are unsupported

On Tue, Jan 21, 2020 at 05:02:57PM +0100, Vitaly Kuznetsov wrote:
> When STIBP/IBPB features are not supported (no microcode update,
> AWS/Azure/... instances deliberately hiding SPEC_CTRL for performance
> reasons,...) /sys/devices/system/cpu/vulnerabilities/spectre_v2 looks like
> 
>   Mitigation: Full generic retpoline, STIBP: disabled, RSB filling
> 
> and this looks imperfect. In particular, STIBP is 'disabled' and 'IBPB'
> is not mentioned while both features are just not supported. Also, for
> STIBP the 'disabled' state (SPECTRE_V2_USER_NONE) can represent both
> the absence of hardware support and deliberate user's choice
> (spectre_v2_user=off)
> 
> Make the following adjustments:
> - Output 'unsupported' for both STIBP/IBPB when there's no support in
>   hardware.
> - Output 'unneeded' for STIBP when SMT is disabled/missing (and this
>   switch_to_cond_stibp is off).
> 
> RFC. Some tools out there may be looking at this information so by
> changing the output we're breaking them. Also, it may make sense to
> separate kernel and userspace protections and switch to something like
> 
>   Mitigation: Kernel: Full generic retpoline, RSB filling; Userspace:
>    Vulnerable
> 
> for the above mentioned case.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  Documentation/admin-guide/hw-vuln/spectre.rst | 3 +++
>  arch/x86/kernel/cpu/bugs.c                    | 9 +++++++--
>  2 files changed, 10 insertions(+), 2 deletions(-)

There's another attempt to fix similar aspects of this whole deal going
on ATM:

https://lkml.kernel.org/r/20191229164830.62144-1-asteinhauser@google.com

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette