x86/bugs: Add a separate config for TAA

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 TAA 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/20240729164105.554296-3-leitao@debian.org

arch/x86/Kconfig

@@ -2659,6 +2659,17 @@ config MITIGATION_MDS
 	  a hardware vulnerability which allows unprivileged speculative access
 	  to data which is available in various CPU internal buffers.
 	  See also <file:Documentation/admin-guide/hw-vuln/mds.rst>
+
+config MITIGATION_TAA
+	bool "Mitigate TSX Asynchronous Abort (TAA) hardware bug"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for TSX Asynchronous Abort (TAA). TAA is a hardware
+	  vulnerability that allows unprivileged speculative access to data
+	  which is available in various CPU internal buffers by using
+	  asynchronous aborts within an Intel TSX transactional region.
+	  See also <file:Documentation/admin-guide/hw-vuln/tsx_async_abort.rst>
 endif
 
 config ARCH_HAS_ADD_PAGES

arch/x86/kernel/cpu/bugs.c

@@ -294,7 +294,8 @@ enum taa_mitigations {
 };
 
 /* Default mitigation for TAA-affected CPUs */
-static enum taa_mitigations taa_mitigation __ro_after_init = TAA_MITIGATION_VERW;
+static enum taa_mitigations taa_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_TAA) ? TAA_MITIGATION_VERW : TAA_MITIGATION_OFF;
 static bool taa_nosmt __ro_after_init;
 
 static const char * const taa_strings[] = {