Re: [PATCH v5 14/20] x86/bugs: Add attack vector controls for BHI

[Borislav Petkov <bp@alien8.de>]

On Fri, May 09, 2025 at 11:28:33AM -0500, David Kaplan wrote:
> There are two BHI mitigations, one for SYSCALL and one for VMEXIT.
> Split these up so they can be selected individually based on attack
> vector.
> 
> Signed-off-by: David Kaplan <david.kaplan@amd.com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 38 +++++++++++++++++++++++++++-----------
>  1 file changed, 27 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 305a11fa9521..667385808400 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1905,8 +1905,9 @@ static bool __init spec_ctrl_bhi_dis(void)
>  enum bhi_mitigations {
>  	BHI_MITIGATION_OFF,
>  	BHI_MITIGATION_AUTO,
> -	BHI_MITIGATION_ON,
> +	BHI_MITIGATION_FULL,
>  	BHI_MITIGATION_VMEXIT_ONLY,
> +	BHI_MITIGATION_SYSCALL_ONLY

We usually call those USER_KERNEL or so...

But I don't think you need it. The rename to _FULL is ok but the rest could
look like this:

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 32edf0b8a495..624d8d766dca 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2096,7 +2096,7 @@ static bool __init spec_ctrl_bhi_dis(void)
 enum bhi_mitigations {
 	BHI_MITIGATION_OFF,
 	BHI_MITIGATION_AUTO,
-	BHI_MITIGATION_ON,
+	BHI_MITIGATION_FULL,
 	BHI_MITIGATION_VMEXIT_ONLY,
 };
 
@@ -2111,7 +2111,7 @@ static int __init spectre_bhi_parse_cmdline(char *str)
 	if (!strcmp(str, "off"))
 		bhi_mitigation = BHI_MITIGATION_OFF;
 	else if (!strcmp(str, "on"))
-		bhi_mitigation = BHI_MITIGATION_ON;
+		bhi_mitigation = BHI_MITIGATION_FULL;
 	else if (!strcmp(str, "vmexit"))
 		bhi_mitigation = BHI_MITIGATION_VMEXIT_ONLY;
 	else
@@ -2123,11 +2123,11 @@ early_param("spectre_bhi", spectre_bhi_parse_cmdline);
 
 static void __init bhi_select_mitigation(void)
 {
-	if (!boot_cpu_has(X86_BUG_BHI) || cpu_mitigations_off())
+	if (!boot_cpu_has(X86_BUG_BHI))
 		bhi_mitigation = BHI_MITIGATION_OFF;
 
 	if (bhi_mitigation == BHI_MITIGATION_AUTO)
-		bhi_mitigation = BHI_MITIGATION_ON;
+		bhi_mitigation = BHI_MITIGATION_FULL;
 }
 
 static void __init bhi_update_mitigation(void)
@@ -2160,15 +2160,19 @@ static void __init bhi_apply_mitigation(void)
 	if (spec_ctrl_bhi_dis())
 		return;
 
-	if (bhi_mitigation == BHI_MITIGATION_VMEXIT_ONLY) {
-		pr_info("Spectre BHI mitigation: SW BHB clearing on VM exit only\n");
+	/* Mitigate KVM if guest->host protection is desired */
+	if (bhi_mitigation == BHI_MITIGATION_FULL ||
+	    bhi_mitigation == BHI_MITIGATION_VMEXIT_ONLY) {
 		setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_VMEXIT);
-		return;
+		pr_info("Spectre BHI mitigation: SW BHB clearing on VM exit\n");
 	}
 
-	pr_info("Spectre BHI mitigation: SW BHB clearing on syscall and VM exit\n");
-	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
-	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_VMEXIT);
+	/* Mitigate syscalls if user->kernel protection is desired */
+	if (bhi_mitigation == BHI_MITIGATION_FULL ||
+	    cpu_attack_vector_mitigated(CPU_MITIGATE_USER_KERNEL)) {
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
+		pr_info("Spectre BHI mitigation: SW BHB clearing on syscall\n");
+	}
 }
 
 static void __init spectre_v2_select_mitigation(void)

-- 
Regards/Gruss,
    Boris.

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