[PATCH v3 01/10] x86/bugs: Add a separate config for GDS

[Breno Leitao]

Currently there is no way to disable GDS mitigation at build time.
Disabling the current config option (GDS_MITIGATION_FORCE) does not
disable the mitigation, but set it to GDS_MITIGATION_FULL, which does
not disable it.

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. Move the GDS_MITIGATION_FORCE under this new mitigation.

Now, there are three options for GDS mitigation:

* CONFIG_MITIGATION_GDS=n -> Mitigation disabled (New)
* CONFIG_MITIGATION_GDS=y -> Mitigation enabled (GDS_MITIGATION_FULL)
* CONFIG_GDS_MITIGATION_FORCE=y -> Forceful mitigation (disable AVX)

Suggested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/Kconfig           | 16 +++++++++++-----
 arch/x86/kernel/cpu/bugs.c |  7 ++++---
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index a902680b6537..d99b758c8d35 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2607,15 +2607,21 @@ 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_GDS_FORCE
 	bool "Force GDS Mitigation"
-	depends on CPU_SUP_INTEL
+	depends on MITIGATION_GDS
 	default n
 	help
-	  Gather Data Sampling (GDS) is a hardware vulnerability which allows
-	  unprivileged speculative access to data which was previously stored in
-	  vector registers.
-
 	  This option is equivalent to setting gather_data_sampling=force on the
 	  command line. The microcode mitigation is used if present, otherwise
 	  AVX is disabled as a mitigation. On affected systems that are missing
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9a9685c9244b..f2bdfb359f6b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -731,10 +731,11 @@ enum gds_mitigations {
 	GDS_MITIGATION_HYPERVISOR,
 };
 
-#if IS_ENABLED(CONFIG_MITIGATION_GDS_FORCE)
-static enum gds_mitigations gds_mitigation __ro_after_init = GDS_MITIGATION_FORCE;
+#if IS_ENABLED(CONFIG_MITIGATION_GDS)
+static enum gds_mitigations gds_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_GDS_FORCE) ? GDS_MITIGATION_FORCE : GDS_MITIGATION_FULL;
 #else
-static enum gds_mitigations gds_mitigation __ro_after_init = GDS_MITIGATION_FULL;
+static enum gds_mitigations gds_mitigation __ro_after_init = GDS_MITIGATION_OFF;
 #endif
 
 static const char * const gds_strings[] = {
-- 
2.43.0