linux-kernel - [RFC PATCH 30/34] x86/bugs: Add attack vector controls for bhi

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Message-ID: <20240912190857.235849-31-david.kaplan@amd.com>
Date: Thu, 12 Sep 2024 14:08:53 -0500
From: David Kaplan <david.kaplan@....com>
To: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.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>
CC: <linux-kernel@...r.kernel.org>
Subject: [RFC PATCH 30/34] x86/bugs: Add attack vector controls for bhi

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, 26 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9859f650f25f..cc26f5680523 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1858,8 +1858,9 @@ static bool __init spec_ctrl_bhi_dis(void)
 enum bhi_mitigations {
 	BHI_MITIGATION_OFF,
 	BHI_MITIGATION_AUTO,
-	BHI_MITIGATION_ON,
-	BHI_MITIGATION_VMEXIT_ONLY,
+	BHI_MITIGATION_FULL,
+	BHI_MITIGATION_VMEXIT,
+	BHI_MITIGATION_SYSCALL
 };
 
 static enum bhi_mitigations bhi_mitigation __ro_after_init =
@@ -1873,9 +1874,9 @@ 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;
+		bhi_mitigation = BHI_MITIGATION_VMEXIT;
 	else
 		pr_err("Ignoring unknown spectre_bhi option (%s)", str);
 
@@ -1891,8 +1892,17 @@ static void __init bhi_select_mitigation(void)
 	if (bhi_mitigation == BHI_MITIGATION_OFF)
 		return;
 
-	if (bhi_mitigation == BHI_MITIGATION_AUTO)
-		bhi_mitigation = BHI_MITIGATION_ON;
+	if (bhi_mitigation == BHI_MITIGATION_AUTO) {
+		if (cpu_mitigate_attack_vector(CPU_MITIGATE_USER_KERNEL)) {
+			if (cpu_mitigate_attack_vector(CPU_MITIGATE_GUEST_HOST))
+				bhi_mitigation = BHI_MITIGATION_FULL;
+			else
+				bhi_mitigation = BHI_MITIGATION_SYSCALL;
+		} else if (cpu_mitigate_attack_vector(CPU_MITIGATE_GUEST_HOST))
+			bhi_mitigation = BHI_MITIGATION_VMEXIT;
+		else
+			bhi_mitigation = BHI_MITIGATION_OFF;
+	}
 }
 
 static void __init bhi_apply_mitigation(void)
@@ -1915,15 +1925,19 @@ static void __init bhi_apply_mitigation(void)
 	if (!IS_ENABLED(CONFIG_X86_64))
 		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) {
 		setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_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_LOOP_ON_VMEXIT);
+	/* Mitigate syscalls if user->kernel protection is desired */
+	if (bhi_mitigation == BHI_MITIGATION_FULL ||
+	    bhi_mitigation == BHI_MITIGATION_SYSCALL) {
+		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)
-- 
2.34.1