linux-kernel - [PATCH 4/7] x86/bugs: Clarify that syscall hardening isn't a BHI mitigation

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Message-ID: <b5951dae3fdee7f1520d5136a27be3bdfe95f88b.1712813475.git.jpoimboe@kernel.org>
Date: Wed, 10 Apr 2024 22:40:48 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alexandre Chartre <alexandre.chartre@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	KP Singh <kpsingh@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Borislav Petkov <bp@...en8.de>
Subject: [PATCH 4/7] x86/bugs: Clarify that syscall hardening isn't a BHI mitigation

While syscall hardening helps prevent some BHI attacks, there's still
other low-hanging fruit remaining.  Don't classify it as a mitigation
and make it clear that the system may still be vulnerable if it doesn't
have a HW or SW mitigation enabled.

Fixes: ec9404e40e8f ("x86/bhi: Add BHI mitigation knob")
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 Documentation/admin-guide/hw-vuln/spectre.rst   | 11 +++++------
 Documentation/admin-guide/kernel-parameters.txt |  3 +--
 arch/x86/kernel/cpu/bugs.c                      |  6 +++---
 3 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index 3cf18e4a1d9a..5a39acf82483 100644
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -441,10 +441,10 @@ The possible values in this file are:
    - System is protected by BHI_DIS_S
  * - BHI: SW loop, KVM SW loop
    - System is protected by software clearing sequence
- * - BHI: Syscall hardening
-   - Syscalls are hardened against BHI
- * - BHI: Syscall hardening, KVM: SW loop
-   - System is protected from userspace attacks by syscall hardening; KVM is protected by software clearing sequence
+ * - BHI: Vulnerable
+   - System is vulnerable to BHI
+ * - BHI: Vulnerable, KVM: SW loop
+   - System is vulnerable; KVM is protected by software clearing sequence
 
 Full mitigation might require a microcode update from the CPU
 vendor. When the necessary microcode is not available, the kernel will
@@ -661,8 +661,7 @@ kernel command line.
 	spectre_bhi=
 
 		[X86] Control mitigation of Branch History Injection
-		(BHI) vulnerability. Syscalls are hardened against BHI
-		regardless of this setting. This setting affects the deployment
+		(BHI) vulnerability.  This setting affects the deployment
 		of the HW BHI control and the SW BHB clearing sequence.
 
 		on
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a029ad6c4963..a3874cc97892 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6065,8 +6065,7 @@
 			See Documentation/admin-guide/laptops/sonypi.rst
 
 	spectre_bhi=	[X86] Control mitigation of Branch History Injection
-			(BHI) vulnerability. Syscalls are hardened against BHI
-			reglardless of this setting. This setting affects the
+			(BHI) vulnerability.  This setting affects the
 			deployment of the HW BHI control and the SW BHB
 			clearing sequence.
 
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 0755600d5d18..a65c70709bb5 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2818,10 +2818,10 @@ static const char * const spectre_bhi_state(void)
 		return "; BHI: SW loop, KVM: SW loop";
 	else if (boot_cpu_has(X86_FEATURE_RETPOLINE) && rrsba_disabled)
 		return "; BHI: Retpoline";
-	else if  (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT))
-		return "; BHI: Syscall hardening, KVM: SW loop";
+	else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT))
+		return "; BHI: Vulnerable, KVM: SW loop";
 
-	return "; BHI: Vulnerable (Syscall hardening enabled)";
+	return "; BHI: Vulnerable";
 }
 
 static ssize_t spectre_v2_show_state(char *buf)
-- 
2.44.0