Re: [PATCH 3/3] x86/spec-ctrl: Fix NMI race condition with VT-x MSR_SPEC_CTRL handling

[Jan Beulich <jbeulich@suse.com>]

On 13.01.2022 17:38, Andrew Cooper wrote:
> Second, update vmx_msr_{read,write}_intercept() to use the load/save lists
> rather than vcpu_msrs, and update the comment to describe the new state
> location.

Nit: Assuming "the comment" refers to something in the named function,
I'm afraid I can't spot any comment being updated there. Perhaps you
mean the comment in the declaration of struct vcpu_msrs?

> --- a/xen/arch/x86/hvm/vmx/entry.S
> +++ b/xen/arch/x86/hvm/vmx/entry.S
> @@ -35,7 +35,14 @@ ENTRY(vmx_asm_vmexit_handler)
>  
>          /* SPEC_CTRL_ENTRY_FROM_VMX    Req: b=curr %rsp=regs/cpuinfo, Clob: acd */
>          ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
> -        ALTERNATIVE "", DO_SPEC_CTRL_ENTRY_FROM_HVM, X86_FEATURE_SC_MSR_HVM
> +
> +        .macro restore_spec_ctrl
> +            mov    $MSR_SPEC_CTRL, %ecx
> +            movzbl CPUINFO_xen_spec_ctrl(%rsp), %eax
> +            xor    %edx, %edx
> +            wrmsr
> +        .endm
> +        ALTERNATIVE "", restore_spec_ctrl, X86_FEATURE_SC_MSR_HVM
>          /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
>  
>          /* Hardware clears MSR_DEBUGCTL on VMExit.  Reinstate it if debugging Xen. */
> @@ -83,7 +90,6 @@ UNLIKELY_END(realmode)
>  
>          /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
>          /* SPEC_CTRL_EXIT_TO_VMX   Req: a=spec_ctrl %rsp=regs/cpuinfo, Clob: cd */
> -        ALTERNATIVE "", DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_SC_MSR_HVM
>          ALTERNATIVE "", __stringify(verw CPUINFO_verw_sel(%rsp)), X86_FEATURE_SC_VERW_HVM

Shouldn't you update the "Clob:" remarks here as well?

> @@ -119,12 +125,11 @@ UNLIKELY_END(realmode)
>          SAVE_ALL
>  
>          /*
> -         * PV variant needed here as no guest code has executed (so
> -         * MSR_SPEC_CTRL can't have changed value), and NMIs/MCEs are liable
> -         * to hit (in which case the HVM variant might corrupt things).
> +         * SPEC_CTRL_ENTRY notes
> +         *
> +         * If we end up here, no guest code has executed.  We still have Xen's
> +         * choice of MSR_SPEC_CTRL in context, and the RSB is safe.
>           */
> -        SPEC_CTRL_ENTRY_FROM_PV /* Req: %rsp=regs/cpuinfo Clob: acd */
> -        /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */

Is "no guest code has executed" actually enough here? If VM entry failed
due to a later MSR-load-list entry, SPEC_CTRL could have changed value
already, couldn't it?

> @@ -601,17 +602,27 @@ static void vmx_cpuid_policy_changed(struct vcpu *v)
>  
>      vmx_vmcs_enter(v);
>      vmx_update_exception_bitmap(v);
> -    vmx_vmcs_exit(v);
>  
>      /*
>       * We can safely pass MSR_SPEC_CTRL through to the guest, even if STIBP
>       * isn't enumerated in hardware, as SPEC_CTRL_STIBP is ignored.
>       */
>      if ( cp->feat.ibrsb )
> +    {
>          vmx_clear_msr_intercept(v, MSR_SPEC_CTRL, VMX_MSR_RW);
> +
> +        rc = vmx_add_guest_msr(v, MSR_SPEC_CTRL, 0);
> +        if ( rc )
> +            goto err;
> +    }
>      else
> +    {
>          vmx_set_msr_intercept(v, MSR_SPEC_CTRL, VMX_MSR_RW);
>  
> +        /* Can only fail with -ESRCH, and we don't care. */
> +        vmx_del_msr(v, MSR_SPEC_CTRL, VMX_MSR_GUEST);

To be forward-compatible with possible future changes to the
function (or load list handling as a whole), how about having an
assertion proving what the comment says:

        rc = vmx_del_msr(v, MSR_SPEC_CTRL, VMX_MSR_GUEST);
        if ( rc )
        {
            ASSERT(rc == -ESRCH);
            rc = 0;
        }

?

Jan