Re: [PATCH] KVM: x86: use X86_FEATURE_RSB_CTXSW for RSB stuffing in vmexit

[Venkatesh Srinivas]

On Fri, May 7, 2021 at 8:08 AM Jon Kohler <jon@xxxxxxxxxxx> wrote:
>
> cpufeatures.h defines X86_FEATURE_RSB_CTXSW as "Fill RSB on context
> switches" which seems more accurate than using X86_FEATURE_RETPOLINE
> in the vmxexit path for RSB stuffing.
>
> X86_FEATURE_RSB_CTXSW is used for FILL_RETURN_BUFFER in
> arch/x86/entry/entry_{32|64}.S. This change makes KVM vmx and svm
> follow that same pattern. This pairs up nicely with the language in
> bugs.c, where this cpu_cap is enabled, which indicates that RSB
> stuffing should be unconditional with spectrev2 enabled.
>         /*
>          * If spectre v2 protection has been enabled, unconditionally fill
>          * RSB during a context switch; this protects against two independent
>          * issues:
>          *
>          *      - RSB underflow (and switch to BTB) on Skylake+
>          *      - SpectreRSB variant of spectre v2 on X86_BUG_SPECTRE_V2 CPUs
>          */
>         setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
>
> Furthermore, on X86_FEATURE_IBRS_ENHANCED CPUs && SPECTRE_V2_CMD_AUTO,
> we're bypassing setting X86_FEATURE_RETPOLINE, where as far as I could
> find, we should still be doing RSB stuffing no matter what when
> CONFIG_RETPOLINE is enabled and spectrev2 is set to auto.

If I'm reading https://software.intel.com/security-software-guidance/deep-dives/deep-dive-indirect-branch-restricted-speculation
correctly, I don't think an RSB fill sequence is required on VMExit on
processors w/ Enhanced IBRS. Specifically:
"""
On processors with enhanced IBRS, an RSB overwrite sequence may not
suffice to prevent the predicted target of a near return from using an
RSB entry created in a less privileged predictor mode.  Software can
prevent this by enabling SMEP (for transitions from user mode to
supervisor mode) and by having IA32_SPEC_CTRL.IBRS set during VM exits
"""
On Enhanced IBRS processors, it looks like SPEC_CTRL.IBRS is set
across all #VMExits via x86_virt_spec_ctrl in kvm.

So is this patch needed?

Thanks,
-- vs;