linux-kernel - [PATCH] cpu/bugs: Disable CPU mitigations at compilation time

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Message-Id: <20230203120615.1121272-1-leitao@debian.org>
Date:   Fri,  3 Feb 2023 04:06:15 -0800
From:   Breno Leitao <leitao@...ian.org>
To:     tglx@...utronix.de, bp@...en8.de,
        pawan.kumar.gupta@...ux.intel.com, paul@...l-moore.com
Cc:     leit@...a.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] cpu/bugs: Disable CPU mitigations at compilation time

Right now it is not possible to disable CPU vulnerabilities mitigations
at build time. Mitigation needs to be disabled passing kernel
parameters, such as 'mitigations=off'.

Create a new config option (CONFIG_CPU_MITIGATIONS_DEFAULT_OFF) that
sets the global variable `cpu_mitigations` to OFF, instead of AUTO. This
allows the creation of kernel binaries that boots with the CPU
mitigations turned off by default, and does not require dealing kernel
parameters.

Signed-off-by: Breno Leitao <leitao@...ian.org>
---
 kernel/cpu.c     |  7 +++++--
 security/Kconfig | 11 +++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6c0a92ca6bb5..90afb29eb62f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2727,8 +2727,11 @@ enum cpu_mitigations {
 	CPU_MITIGATIONS_AUTO_NOSMT,
 };
 
-static enum cpu_mitigations cpu_mitigations __ro_after_init =
-	CPU_MITIGATIONS_AUTO;
+#ifdef CONFIG_CPU_MITIGATIONS_DEFAULT_OFF
+static enum cpu_mitigations cpu_mitigations __ro_after_init = CPU_MITIGATIONS_OFF;
+#else
+static enum cpu_mitigations cpu_mitigations __ro_after_init = CPU_MITIGATIONS_AUTO;
+#endif
 
 static int __init mitigations_parse_cmdline(char *arg)
 {
diff --git a/security/Kconfig b/security/Kconfig
index e6db09a779b7..644f91b6c26a 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -258,6 +258,17 @@ config LSM
 
 	  If unsure, leave this as the default.
 
+config CPU_MITIGATIONS_DEFAULT_OFF
+	bool "Disable mitigations for CPU vulnerabilities by default"
+	default n
+	help
+	  This option disables mitigations for CPU vulnerabilities by default.
+	  Disabling CPU mitigations improves system performance,
+	  but it may also expose users to several CPU vulnerabilities.
+	  This option has the same effect of passing `mitigations=off` kernel
+	  parameter. The CPU mitigations could be enabled back using the
+	  'mitigations' parameter.
+
 source "security/Kconfig.hardening"
 
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-- 
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