linux-kernel - [tip: x86/urgent] x86/bugs: Clarify that syscall hardening isn't a BHI mitigation

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Message-ID: <171282482058.10875.1352246687357623520.tip-bot2@tip-bot2>
Date: Thu, 11 Apr 2024 08:40:20 -0000
From: "tip-bot2 for Josh Poimboeuf" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Sean Christopherson <seanjc@...gle.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/bugs: Clarify that syscall hardening isn't a
 BHI mitigation

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     5f882f3b0a8bf0788d5a0ee44b1191de5319bb8a
Gitweb:        https://git.kernel.org/tip/5f882f3b0a8bf0788d5a0ee44b1191de5319bb8a
Author:        Josh Poimboeuf <jpoimboe@...nel.org>
AuthorDate:    Wed, 10 Apr 2024 22:40:48 -07:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Thu, 11 Apr 2024 10:30:33 +02:00

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>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Sean Christopherson <seanjc@...gle.com>
Link: https://lore.kernel.org/r/b5951dae3fdee7f1520d5136a27be3bdfe95f88b.1712813475.git.jpoimboe@kernel.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 3cf18e4..5a39acf 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 a029ad6..a3874cc 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 08dfb94..9eeb60f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2818,10 +2818,10 @@ static const char *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)