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

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Date:   Thu,  2 Feb 2023 10:08:58 -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: [RFC 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'.

This patch creates an easy way to disable mitigation during compilation
time (CONFIG_DEFAULT_CPU_MITIGATIONS_OFF), so, insecure kernel users don't
need to deal with kernel parameters when booting insecure kernels.

Signed-off-by: Breno Leitao <leitao@...ian.org>
---
 kernel/cpu.c     |  5 +++++
 security/Kconfig | 11 +++++++++++
 2 files changed, 16 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6c0a92ca6bb5..497e9a3d3d77 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2727,8 +2727,13 @@ enum cpu_mitigations {
 	CPU_MITIGATIONS_AUTO_NOSMT,
 };
 
+#ifdef CONFIG_DEFAULT_CPU_MITIGATIONS_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..a70427dc6ace 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -258,6 +258,17 @@ config LSM
 
 	  If unsure, leave this as the default.
 
+config CONFIG_DEFAULT_CPU_MITIGATIONS_OFF
+	bool "Disable mitigations for CPU vulnerabilities by default"
+	default n
+	help
+	  This option disable mitigations for CPU vulnerabilities by default.
+	  This improves system performance, but it may also expose users
+	  to several CPU vulnerabilities.
+	  This has the same effect as passing `mitigations=off` kernel
+	  parameter. The mitigations could be enabled back passing the
+	  'mitigations' parameter.
+
 source "security/Kconfig.hardening"
 
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-- 
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