Re: [PATCH v4 09/11] x86/vmscape: Deploy BHB clearing mitigation

[Nikolay Borisov]

On 11/20/25 08:19, Pawan Gupta wrote:
> IBPB mitigation for VMSCAPE is an overkill on CPUs that are only affected
> by the BHI variant of VMSCAPE. On such CPUs, eIBRS already provides
> indirect branch isolation between guest and host userspace. However, branch
> history from guest may also influence the indirect branches in host
> userspace.
>
> To mitigate the BHI aspect, use clear_bhb_loop().
>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> ---
>   Documentation/admin-guide/hw-vuln/vmscape.rst |  4 ++++
>   arch/x86/include/asm/nospec-branch.h          |  2 ++
>   arch/x86/kernel/cpu/bugs.c                    | 30 ++++++++++++++++++++-------
>   3 files changed, 29 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/admin-guide/hw-vuln/vmscape.rst b/Documentation/admin-guide/hw-vuln/vmscape.rst
> index d9b9a2b6c114c05a7325e5f3c9d42129339b870b..dc63a0bac03d43d1e295de0791dd6497d101f986 100644
> --- a/Documentation/admin-guide/hw-vuln/vmscape.rst
> +++ b/Documentation/admin-guide/hw-vuln/vmscape.rst
> @@ -86,6 +86,10 @@ The possible values in this file are:
>      run a potentially malicious guest and issues an IBPB before the first
>      exit to userspace after VM-exit.
>   
> + * 'Mitigation: Clear BHB before exit to userspace':
> +
> +   As above, conditional BHB clearing mitigation is enabled.
> +
>    * 'Mitigation: IBPB on VMEXIT':
>   
>      IBPB is issued on every VM-exit. This occurs when other mitigations like
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 15a2fa8f2f48a066e102263513eff9537ac1d25f..1e8c26c37dbed4256b35101fb41c0e1eb6ef9272 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -388,6 +388,8 @@ extern void write_ibpb(void);
>   
>   #ifdef CONFIG_X86_64
>   extern void clear_bhb_loop(void);
> +#else
> +static inline void clear_bhb_loop(void) {}
>   #endif
>   
>   extern void (*x86_return_thunk)(void);
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index cbb3341b9a19f835738eda7226323d88b7e41e52..d12c07ccf59479ecf590935607394492c988b2ff 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -109,9 +109,8 @@ DEFINE_PER_CPU(u64, x86_spec_ctrl_current);
>   EXPORT_PER_CPU_SYMBOL_GPL(x86_spec_ctrl_current);
>   
>   /*
> - * Set when the CPU has run a potentially malicious guest. An IBPB will
> - * be needed to before running userspace. That IBPB will flush the branch
> - * predictor content.
> + * Set when the CPU has run a potentially malicious guest. Indicates that a
> + * branch predictor flush is needed before running userspace.
>    */
>   DEFINE_PER_CPU(bool, x86_predictor_flush_exit_to_user);
>   EXPORT_PER_CPU_SYMBOL_GPL(x86_predictor_flush_exit_to_user);
> @@ -3200,13 +3199,15 @@ enum vmscape_mitigations {
>   	VMSCAPE_MITIGATION_AUTO,
>   	VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER,
>   	VMSCAPE_MITIGATION_IBPB_ON_VMEXIT,
> +	VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER,
>   };
>   
>   static const char * const vmscape_strings[] = {
> -	[VMSCAPE_MITIGATION_NONE]		= "Vulnerable",
> +	[VMSCAPE_MITIGATION_NONE]			= "Vulnerable",
>   	/* [VMSCAPE_MITIGATION_AUTO] */
> -	[VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER]	= "Mitigation: IBPB before exit to userspace",
> -	[VMSCAPE_MITIGATION_IBPB_ON_VMEXIT]	= "Mitigation: IBPB on VMEXIT",
> +	[VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER]		= "Mitigation: IBPB before exit to userspace",
> +	[VMSCAPE_MITIGATION_IBPB_ON_VMEXIT]		= "Mitigation: IBPB on VMEXIT",
> +	[VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER]	= "Mitigation: Clear BHB before exit to userspace",
>   };
>   
>   static enum vmscape_mitigations vmscape_mitigation __ro_after_init =
> @@ -3253,8 +3254,19 @@ static void __init vmscape_select_mitigation(void)
>   			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
>   		break;
>   
> +	case VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER:
> +		if (!boot_cpu_has(X86_FEATURE_BHI_CTRL))
> +			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> +		break;

Am I missing something or this case can never execute because 
VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER is only ever set if mitigation 
is VMSCAPE_MITIGATION_AUTO in the below branch? Perhaps just remove it? 
This just shows how confusing the logic for choosing the mitigations has 
become....



>   	case VMSCAPE_MITIGATION_AUTO:
> -		if (boot_cpu_has(X86_FEATURE_IBPB))
> +		/*
> +		 * CPUs with BHI_CTRL(ADL and newer) can avoid the IBPB and use BHB
> +		 * clear sequence. These CPUs are only vulnerable to the BHI variant
> +		 * of the VMSCAPE attack and does not require an IBPB flush.
> +		 */
> +		if (boot_cpu_has(X86_FEATURE_BHI_CTRL))
> +			vmscape_mitigation = VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER;
> +		else if (boot_cpu_has(X86_FEATURE_IBPB))
>   			vmscape_mitigation = VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER;
>   		else
>   			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;


<snip>