[PATCH v3 04/10] x86/bugs: Add a separate config for MMIO Stable Data

[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 MMIO Stale data CPU mitigation under
CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
it at compilation time.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/Kconfig           | 12 ++++++++++++
 arch/x86/kernel/cpu/bugs.c |  3 ++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c7ce800fcdb2..bba5b65034dc 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2673,6 +2673,18 @@ config MITIGATION_TAA
 	  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>
+
+config MITIGATION_MMIO_STALE_DATA
+	bool "Mitigate MMIO Stale Data hardware bug"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for MMIO Stale Data hardware bugs.  Processor MMIO
+	  Stale Data Vulnerabilities are a class of memory-mapped I/O (MMIO)
+	  vulnerabilities that can expose data. The vulnerabilities require the
+	  attacker to have access to MMIO.
+	  See also
+	  <file:Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst>
 endif
 
 config ARCH_HAS_ADD_PAGES
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 87f3cc6c438d..21daaf202b7f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -393,7 +393,8 @@ enum mmio_mitigations {
 };
 
 /* Default mitigation for Processor MMIO Stale Data vulnerabilities */
-static enum mmio_mitigations mmio_mitigation __ro_after_init = MMIO_MITIGATION_VERW;
+static enum mmio_mitigations mmio_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_MMIO_STALE_DATA) ? MMIO_MITIGATION_VERW : MMIO_MITIGATION_OFF;
 static bool mmio_nosmt __ro_after_init = false;
 
 static const char * const mmio_strings[] = {
-- 
2.43.0