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Date:   Wed, 5 Oct 2022 16:45:33 -0700
From:   Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To:     Jim Mattson <jmattson@google.com>
Cc:     Suraj Jitindar Singh <surajjs@amazon.com>, 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, benh@kernel.crashing.org,
        stable@vger.kernel.org
Subject: Re: [PATCH] x86/speculation: Mitigate eIBRS PBRSB predictions with
 WRMSR
Message-ID: <20221005234533.cr4jpzsb3cprlc2c@desk>
References: <20221005220227.1959-1-surajjs@amazon.com>
 <CALMp9eTU9s+2fZ809bfOWYoGXsiziQOxCM-5Ly0JF2HeSEkwhA@mail.gmail.com>
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On Wed, Oct 05, 2022 at 04:24:54PM -0700, Jim Mattson wrote:
>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.
>>
>> == 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.
>>
>> 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
>>
>> Signed-off-by: Suraj Jitindar Singh <surajjs@amazon.com>
>> Cc: stable@vger.kernel.org
>> ---
>>  arch/x86/include/asm/nospec-branch.h | 7 +++----
>>  arch/x86/kvm/vmx/vmenter.S           | 3 +--
>>  arch/x86/kvm/vmx/vmx.c               | 5 +++++
>>  3 files changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
>> index c936ce9f0c47..e5723e024b47 100644
>> --- a/arch/x86/include/asm/nospec-branch.h
>> +++ b/arch/x86/include/asm/nospec-branch.h
>> @@ -159,10 +159,9 @@
>>    * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
>>    * monstrosity above, manually.
>>    */
>> -.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req ftr2=ALT_NOT(X86_FEATURE_ALWAYS)
>> -       ALTERNATIVE_2 "jmp .Lskip_rsb_\@", \
>> -               __stringify(__FILL_RETURN_BUFFER(\reg,\nr)), \ftr, \
>> -               __stringify(__FILL_ONE_RETURN), \ftr2
>> +.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
>> +       ALTERNATIVE "jmp .Lskip_rsb_\@", \
>> +               __stringify(__FILL_RETURN_BUFFER(\reg,\nr)), \ftr
>>
>>  .Lskip_rsb_\@:
>>  .endm
>> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
>> index 6de96b943804..eb82797bd7bf 100644
>> --- a/arch/x86/kvm/vmx/vmenter.S
>> +++ b/arch/x86/kvm/vmx/vmenter.S
>> @@ -231,8 +231,7 @@ SYM_INNER_LABEL(vmx_vmexit, SYM_L_GLOBAL)
>>          * single call to retire, before the first unbalanced RET.
>>           */
>>
>> -       FILL_RETURN_BUFFER %_ASM_CX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT,\
>> -                          X86_FEATURE_RSB_VMEXIT_LITE
>> +       FILL_RETURN_BUFFER %_ASM_CX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT
>>
>>
>>         pop %_ASM_ARG2  /* @flags */
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index c9b49a09e6b5..fdcd8e10c2ab 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -7049,8 +7049,13 @@ void noinstr vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx,
>>          * For legacy IBRS, the IBRS bit always needs to be written after
>>          * transitioning from a less privileged predictor mode, regardless of
>>          * whether the guest/host values differ.
>> +        *
>> +        * For eIBRS affected by Post Barrier RSB Predictions a serialising
>> +        * instruction (wrmsr) must be executed to ensure a call instruction has
>> +        * retired before the prediction of a following unbalanced ret.
>>          */
>>         if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) ||
>> +           cpu_feature_enabled(X86_FEATURE_RSB_VMEXIT_LITE) ||
>>             vmx->spec_ctrl != hostval)
>>                 native_wrmsrl(MSR_IA32_SPEC_CTRL, hostval);
>
>Okay. I see how this almost meets the requirements. But this WRMSR is
>conditional, which means that there's a speculative path through this
>code that ends up at the unbalanced RET without executing the WRMSR.

Agree. I was just about to post this.