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Message-ID: <20250630124038.GFaGKFxtAiYEBUAFRy@fat_crate.local>
Date: Mon, 30 Jun 2025 14:40:38 +0200
From: Borislav Petkov <bp@...en8.de>
To: David Kaplan <david.kaplan@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
"H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 14/20] x86/bugs: Add attack vector controls for BHI
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@....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.
https://people.kernel.org/tglx/notes-about-netiquette
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