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

[Breno Leitao]

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@xxxxxxxxxx>
---
 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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2.30.2