[PATCH v4 11/11] x86/bugs: Add a separate config for GDS

[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 a new kernel config that allows GDS to be completely disabled,
similarly to the "gather_data_sampling=off" or "mitigations=off" kernel
command-line.

Now, there are two options for GDS mitigation:

* CONFIG_MITIGATION_GDS=n -> Mitigation disabled (New)
* CONFIG_MITIGATION_GDS=y -> Mitigation enabled (GDS_MITIGATION_FULL)

Suggested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
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 ab5b210c8315..475bc538615e 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2610,6 +2610,16 @@ config MITIGATION_SLS
 	  against straight line speculation. The kernel image might be slightly
 	  larger.
 
+config MITIGATION_GDS
+	bool "Mitigate Gather Data Sampling"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for Gather Data Sampling (GDS). GDS is a hardware
+	  vulnerability which allows unprivileged speculative access to data
+	  which was previously stored in vector registers. The attacker uses gather
+	  instructions to infer the stale vector register data.
+
 config MITIGATION_RFDS
 	bool "RFDS Mitigation"
 	depends on CPU_SUP_INTEL
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index b2e752eeb098..189840db2f8d 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -735,7 +735,8 @@ enum gds_mitigations {
 	GDS_MITIGATION_HYPERVISOR,
 };
 
-static enum gds_mitigations gds_mitigation __ro_after_init = GDS_MITIGATION_FULL;
+static enum gds_mitigations gds_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_GDS) ? GDS_MITIGATION_FULL : GDS_MITIGATION_OFF;
 
 static const char * const gds_strings[] = {
 	[GDS_MITIGATION_OFF]		= "Vulnerable",
-- 
2.43.0