From: Jim Mattson <jmattson@google.com>
To: Suraj Jitindar Singh <surajjs@amazon.com>
Cc: kvm@vger.kernel.org, sjitindarsingh@gmail.com,
linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
mingo@redhat.com, bp@suse.de, dave.hansen@linux.intel.com,
seanjc@google.com, pbonzini@redhat.com, peterz@infradead.org,
jpoimboe@kernel.org, daniel.sneddon@linux.intel.com,
pawan.kumar.gupta@linux.intel.com, benh@kernel.crashing.org,
stable@vger.kernel.org
Subject: Re: [PATCH] x86/speculation: Mitigate eIBRS PBRSB predictions with WRMSR
Date: Wed, 5 Oct 2022 15:29:13 -0700 [thread overview]
Message-ID: <CALMp9eTy2w_ZbkVSVvTwOW3wYH6vnn5waEWc0BesXL-kYRFy4g@mail.gmail.com> (raw)
In-Reply-To: <20221005220227.1959-1-surajjs@amazon.com>
On Wed, Oct 5, 2022 at 3:03 PM Suraj Jitindar Singh <surajjs@amazon.com> wrote:
>
> tl;dr: The existing mitigation for eIBRS PBRSB predictions uses an INT3 to
> ensure a call instruction retires before a following unbalanced RET. Replace
> this with a WRMSR serialising instruction which has a lower performance
> penalty.
The INT3 is only on a speculative path and should not impact performance.
> == Background ==
>
> eIBRS (enhanced indirect branch restricted speculation) is used to prevent
> predictor addresses from one privilege domain from being used for prediction
> in a higher privilege domain.
>
> == Problem ==
>
> On processors with eIBRS protections there can be a case where upon VM exit
> a guest address may be used as an RSB prediction for an unbalanced RET if a
> CALL instruction hasn't yet been retired. This is termed PBRSB (Post-Barrier
> Return Stack Buffer).
>
> A mitigation for this was introduced in:
> (2b1299322016731d56807aa49254a5ea3080b6b3 x86/speculation: Add RSB VM Exit protections)
>
> This mitigation [1] has a ~1% performance impact on VM exit compared to without
> it [2].
>
> == Solution ==
>
> The WRMSR instruction can be used as a speculation barrier and a serialising
> instruction. Use this on the VM exit path instead to ensure that a CALL
> instruction (in this case the call to vmx_spec_ctrl_restore_host) has retired
> before the prediction of a following unbalanced RET.
I don't buy this solution. According to
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/post-barrier-return-stack-buffer-predictions.html:
"Note that a WRMSR instruction (used to set IBRS, for example), could
also serve as a speculation barrier for such a sequence in place of
LFENCE."
This says only that you can replace the LFENCE with a WRMSR. It
doesn't say that you can drop the rest of the sequence.
> This mitigation [3] has a negligible performance impact.
>
> == Testing ==
>
> Run the outl_to_kernel kvm-unit-tests test 200 times per configuration which
> counts the cycles for an exit to kernel mode.
>
> [1] With existing mitigation:
> Average: 2026 cycles
> [2] With no mitigation:
> Average: 2008 cycles
> [3] With proposed mitigation:
> Average: 2008 cycles
What testing did you do to see that this is an effective mitigation?
Improved timings are irrelevant if it doesn't work.
next prev parent reply other threads:[~2022-10-05 22:29 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-05 22:02 [PATCH] x86/speculation: Mitigate eIBRS PBRSB predictions with WRMSR Suraj Jitindar Singh
2022-10-05 22:29 ` Jim Mattson [this message]
2022-10-06 8:25 ` David Laight
2022-10-06 20:27 ` pawan.kumar.gupta
2022-10-05 23:24 ` Jim Mattson
2022-10-05 23:45 ` Pawan Gupta
2022-10-05 23:46 ` Jim Mattson
2022-10-06 0:26 ` Daniel Sneddon
2022-10-06 1:28 ` Jim Mattson
2022-10-06 8:18 ` Peter Zijlstra
2022-10-06 2:42 ` Andrew Cooper
2022-10-07 1:44 ` pawan.kumar.gupta
2022-10-07 1:54 ` Pawan Gupta
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