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Date:   Wed, 31 May 2023 17:53:55 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     Jon Kohler <jon@nutanix.com>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Daniel Sneddon <daniel.sneddon@linux.intel.com>,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Milburn, Alyssa" <alyssa.milburn@intel.com>
Subject: Re: [PATCH] KVM: VMX: remove LFENCE in vmx_spec_ctrl_restore_host()
Message-ID: <20230601005355.7bvxufmyx27rbvyb@treble>
References: <20230531150112.76156-1-jon@nutanix.com>
 <3b48fae8-1bae-c30a-e6ef-25901314ee39@citrix.com>
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On Thu, Jun 01, 2023 at 01:29:12AM +0100, Andrew Cooper wrote:
> On 31/05/2023 4:01 pm, Jon Kohler wrote:
> > Remove barrier_nospec(), which translates to LFENCE, in
> > vmx_spec_ctrl_restore_host() as RSB-barriers (as defined by [1])
> > already exist prior to this point.
> >
> > This LFENCE was added on commit fc02735b14ff ("KVM: VMX: Prevent guest
> > RSB poisoning attacks with eIBRS") in the 5.19 timeframe; however,
> > commit 2b1299322016 ("x86/speculation: Add RSB VM Exit protections") in
> > 6.0 timeframe added a LFENCE for X86_FEATURE_RSB_VMEXIT_LITE was added
> > directly in vmx_vmexit, prior to CALL vmx_spec_ctrl_restore_host.
> >
> > For posterity, vmx_spec_ctrl_restore_host also will execute WRMSR to
> > IA32_SPEC_CTRL for X86_FEATURE_KERNEL_IBRS or when guest/host MSR value
> > does not match, which serves as an additional RSB-barrier.
> >
> > [1] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/post-barrier-return-stack-buffer-predictions.html
> 
> Yeah, unfortunately PBRSB is insidious.
> 
> From memory (please correct me if I'm wrong), the required safety
> property is this:  After a VMExit (if IBRS was set prior to exit) or the
> write to MSR_SPEC_CTRL setting IBRS (if IBRS was not set prior to exit),
> one single CALL instruction must architecturally retire before any RET
> instructions execute (speculatively).
> 
> There are several ways to arrange this, but they all basically boil down
> to having some serialising instruction between the first CALL and first
> RET on any reachable path from VMExit.

The document says the problem is *unbalanced* RET, i.e. RSB underflow.

So the mitigation needs a single RSB stuff (i.e., unbalanced CALL) and
then an LFENCE anytime before the next unbalanced RET.

-- 
Josh