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

[Jim Mattson]

On Wed, Oct 5, 2022 at 5:26 PM Daniel Sneddon
<daniel.sneddon@xxxxxxxxxxxxxxx> wrote:
>
> 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.

The lfence could be buried in an ALTERNATIVE keyed to
X86_FEATURE_RSB_VMEXIT_LITE.

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

For a mere 18 cycles, it doesn't really seem worth the obfuscation.