Re: [PATCH] x86/speculation: Mitigate eIBRS PBRSB predictions with WRMSR

[Daniel Sneddon]

On 10/5/22 16:46, Jim Mattson wrote:
> On Wed, Oct 5, 2022 at 3:03 PM Suraj Jitindar Singh <surajjs@xxxxxxxxxx> 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@xxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>> ---
>>  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);
> 
> Better, I think, would be to leave the condition alone and put an
> LFENCE on the 'else' path:
> 
>          if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) ||
>              vmx->spec_ctrl != hostval)
>                  native_wrmsrl(MSR_IA32_SPEC_CTRL, hostval);
>         else
>                 rmb();
> 
> When the guest and host have different IA32_SPEC_CTRL values, you get
> the serialization from the WRMSR. Otherwise, you get it from the
> cheaper LFENCE.
In this case systems that don't suffer from PBRSB (i.e. don've have
X86_FEATURE_RSB_VMEXIT_LITE set) would be doing a barrier for no reason.  We're
just trading performance on vulnerable systems for a performance hit on systems
that aren't vulnerable.
> 
> This is still more convoluted than having the mitigation in one place.
Agreed.