[PATCH v4 06/11] x86/bugs: Add a separate config for Spectre v1

[Breno Leitao]

Currently, the CONFIG_SPECULATION_MITIGATIONS is halfway populated,
where some mitigations have entries in Kconfig, and they could be
modified, while others mitigations do not have Kconfig entries, and
could not be controlled at build time.

Create an entry for the Spectre v1 CPU mitigation under
CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
it at compilation time.

Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 arch/x86/Kconfig           | 10 ++++++++++
 arch/x86/kernel/cpu/bugs.c |  3 ++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c9a9f928dddf..e3c63e5208ab 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2705,6 +2705,16 @@ config MITIGATION_RETBLEED
 	  unprivileged attacker can use these flaws to bypass conventional
 	  memory security restrictions to gain read access to privileged memory
 	  that would otherwise be inaccessible.
+
+config MITIGATION_SPECTRE_V1
+	bool "Mitigate SPECTRE V1 hardware bug"
+	default y
+	help
+	  Enable mitigation for Spectre V1 (Bounds Check Bypass). Spectre V1 is a
+	  class of side channel attacks that takes advantage of speculative
+	  execution that bypasses conditional branch instructions used for
+	  memory access bounds check.
+	  See also <file:Documentation/admin-guide/hw-vuln/spectre.rst>
 endif
 
 config ARCH_HAS_ADD_PAGES
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 08edca8c2c1f..ebb6a2f578d1 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -874,7 +874,8 @@ enum spectre_v1_mitigation {
 };
 
 static enum spectre_v1_mitigation spectre_v1_mitigation __ro_after_init =
-	SPECTRE_V1_MITIGATION_AUTO;
+	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V1) ?
+		SPECTRE_V1_MITIGATION_AUTO : SPECTRE_V1_MITIGATION_NONE;
 
 static const char * const spectre_v1_strings[] = {
 	[SPECTRE_V1_MITIGATION_NONE] = "Vulnerable: __user pointer sanitization and usercopy barriers only; no swapgs barriers",
-- 
2.43.0