x86/bugs: Add a separate config for RETBLEED

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 RETBLEED CPU mitigation under
CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
it at compilation time.
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20240729164105.554296-6-leitao@debian.org

arch/x86/Kconfig

@@ -2692,6 +2692,19 @@ config MITIGATION_L1TF
 	  hardware vulnerability which allows unprivileged speculative access to data
 	  available in the Level 1 Data Cache.
 	  See <file:Documentation/admin-guide/hw-vuln/l1tf.rst
+
+config MITIGATION_RETBLEED
+	bool "Mitigate RETBleed hardware bug"
+	depends on (CPU_SUP_INTEL && MITIGATION_SPECTRE_V2) || MITIGATION_UNRET_ENTRY || MITIGATION_IBPB_ENTRY
+	default y
+	help
+	  Enable mitigation for RETBleed (Arbitrary Speculative Code Execution
+	  with Return Instructions) vulnerability.  RETBleed is a speculative
+	  execution attack which takes advantage of microarchitectural behavior
+	  in many modern microprocessors, similar to Spectre v2. An
+	  unprivileged attacker can use these flaws to bypass conventional
+	  memory security restrictions to gain read access to privileged memory
+	  that would otherwise be inaccessible.
 endif
 
 config ARCH_HAS_ADD_PAGES

arch/x86/kernel/cpu/bugs.c

@@ -989,7 +989,7 @@ static const char * const retbleed_strings[] = {
 static enum retbleed_mitigation retbleed_mitigation __ro_after_init =
 	RETBLEED_MITIGATION_NONE;
 static enum retbleed_mitigation_cmd retbleed_cmd __ro_after_init =
-	RETBLEED_CMD_AUTO;
+	IS_ENABLED(CONFIG_MITIGATION_RETBLEED) ? RETBLEED_CMD_AUTO : RETBLEED_CMD_OFF;
 
 static int __ro_after_init retbleed_nosmt = false;