[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'.

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@xxxxxxxxxx>
---
 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"
 
 endmenu
-- 
2.30.2