Re: [PATCH v5 3/3] x86/bugs: Add 'spectre_bhi=vmexit' cmdline option

[Daniel Sneddon]

On 5/6/24 22:30, Josh Poimboeuf wrote:
> In cloud environments it can be useful to *only* enable the vmexit
> mitigation and leave syscalls vulnerable.  Add that as an option.
> 
> This is similar to the old spectre_bhi=auto option which was removed
> with the following commit:
> 
>   36d4fe147c87 ("x86/bugs: Remove CONFIG_BHI_MITIGATION_AUTO and spectre_bhi=auto")
> 
> with the main difference being that this has a more descriptive name and
> is disabled by default.
> 
> Requested-by: Maksim Davydov <davydov-max@xxxxxxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---

Does the KConfig option need to be updated to support this as well? Other than
that,
Reviewed-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>

>  Documentation/admin-guide/kernel-parameters.txt | 12 +++++++++---
>  arch/x86/kernel/cpu/bugs.c                      | 16 +++++++++++-----
>  2 files changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 213d0719e2b7..9c1f63f04502 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6072,9 +6072,15 @@
>  			deployment of the HW BHI control and the SW BHB
>  			clearing sequence.
>  
> -			on   - (default) Enable the HW or SW mitigation
> -			       as needed.
> -			off  - Disable the mitigation.
> +			on     - (default) Enable the HW or SW mitigation as
> +				 needed.  This protects the kernel from
> +				 both syscalls and VMs.
> +			vmexit - On systems which don't have the HW mitigation
> +				 available, enable the SW mitigation on vmexit
> +				 ONLY.  On such systems, the host kernel is
> +				 protected from VM-originated BHI attacks, but
> +				 may still be vulnerable to syscall attacks.
> +			off    - Disable the mitigation.
>  
>  	spectre_v2=	[X86,EARLY] Control mitigation of Spectre variant 2
>  			(indirect branch speculation) vulnerability.
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index ab18185894df..6974c8c9792d 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1625,6 +1625,7 @@ static bool __init spec_ctrl_bhi_dis(void)
>  enum bhi_mitigations {
>  	BHI_MITIGATION_OFF,
>  	BHI_MITIGATION_ON,
> +	BHI_MITIGATION_VMEXIT_ONLY,
>  };
>  
>  static enum bhi_mitigations bhi_mitigation __ro_after_init =
> @@ -1639,6 +1640,8 @@ static int __init spectre_bhi_parse_cmdline(char *str)
>  		bhi_mitigation = BHI_MITIGATION_OFF;
>  	else if (!strcmp(str, "on"))
>  		bhi_mitigation = BHI_MITIGATION_ON;
> +	else if (!strcmp(str, "vmexit"))
> +		bhi_mitigation = BHI_MITIGATION_VMEXIT_ONLY;
>  	else
>  		pr_err("Ignoring unknown spectre_bhi option (%s)", str);
>  
> @@ -1659,19 +1662,22 @@ static void __init bhi_select_mitigation(void)
>  			return;
>  	}
>  
> +	/* Mitigate in hardware if supported */
>  	if (spec_ctrl_bhi_dis())
>  		return;
>  
>  	if (!IS_ENABLED(CONFIG_X86_64))
>  		return;
>  
> -	/* Mitigate KVM by default */
> -	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
> -	pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit\n");
> +	if (bhi_mitigation == BHI_MITIGATION_VMEXIT_ONLY) {
> +		pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit only\n");
> +		setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
> +		return;
> +	}
>  
> -	/* Mitigate syscalls when the mitigation is forced =on */
> +	pr_info("Spectre BHI mitigation: SW BHB clearing on syscall and vm exit\n");
>  	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
> -	pr_info("Spectre BHI mitigation: SW BHB clearing on syscall\n");
> +	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
>  }
>  
>  static void __init spectre_v2_select_mitigation(void)