From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Tyler Hicks <tyhicks@canonical.com>,
Waiman Long <longman@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Anthony Steinhauser <asteinhauser@google.com>
Subject: Re: [PATCH RFC] x86/speculation: Clarify Spectre-v2 mitigation when STIBP/IBPB features are unsupported
Date: Tue, 21 Jan 2020 17:24:06 +0100 [thread overview]
Message-ID: <87v9p4eq1l.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20200121161412.GL7808@zn.tnic>
Borislav Petkov <bp@alien8.de> writes:
> 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@redhat.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
Missed that, thanks! (Cc: Anthony)
This patch seem to address my STIBP: disabled/unsupported concern but
not 'unneeded'. And not IBPB.
--
Vitaly
next prev parent reply other threads:[~2020-01-21 16:24 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-21 16:02 [PATCH RFC] x86/speculation: Clarify Spectre-v2 mitigation when STIBP/IBPB features are unsupported Vitaly Kuznetsov
2020-01-21 16:14 ` Borislav Petkov
2020-01-21 16:24 ` Vitaly Kuznetsov [this message]
2020-01-21 16:22 ` Waiman Long
2020-01-21 16:37 ` Vitaly Kuznetsov
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