[PATCH 2/3] x86/bugs: Create an option to disable TAA

[Breno Leitao]

There is no way to disable TAA mitigation today at compilation time. TAA
is enabled even if CONFIG_SPECULATION_MITIGATIONS is unset.

Create a new KCONFIG option that allow TAA mitigation to be disabled in
compilation time.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 arch/x86/Kconfig           | 10 ++++++++++
 arch/x86/kernel/cpu/bugs.c |  4 +++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d25132b2d54f..140af3b30c45 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2660,6 +2660,16 @@ config MITIGATE_MDS
 	  technical information is available in the MDS specific x86 architecture
 	  section: Documentation/arch/x86/mds.rst.
 
+config MITIGATE_TAA
+	bool "Mitigate TSX Asynchronous Abort (TAA) hardware bug"
+	depends on CPU_SUP_INTEL && X86_64
+	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.
+
 endif
 
 config ARCH_HAS_ADD_PAGES
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 50f12829dce9..3615bda9573f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -336,6 +336,7 @@ enum taa_mitigations {
 /* Default mitigation for TAA-affected CPUs */
 static enum taa_mitigations taa_mitigation __ro_after_init = TAA_MITIGATION_VERW;
 static bool taa_nosmt __ro_after_init;
+#define TAA_WARN_MSG "WARNING: TSX Asynchronous Abort (TAA) speculative mitigation disabled!\n"
 
 static const char * const taa_strings[] = {
 	[TAA_MITIGATION_OFF]		= "Vulnerable",
@@ -359,8 +360,9 @@ static void __init taa_select_mitigation(void)
 		return;
 	}
 
-	if (cpu_mitigations_off()) {
+	if (cpu_mitigations_off() || !IS_ENABLED(CONFIG_MITIGATE_TAA)) {
 		taa_mitigation = TAA_MITIGATION_OFF;
+		pr_err(TAA_WARN_MSG);
 		return;
 	}
 
-- 
2.34.1