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

[Jim Mattson]

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.

This is still more convoluted than having the mitigation in one place.