x86/bugs: Add a separate config for GDS

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@kernel.org>
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/20240729164105.554296-12-leitao@debian.org

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index ab5b210c8315b75cde615a3e328919b291e1f5ef..475bc538615ef072c272e867efb5580ae98ca74a 100644 (file)
--- 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 b2e752eeb098e5f78a9bfffff49ac35f64eb50d4..189840db2f8dc195c6b6e32a7bfcee4167a8dbda 100644 (file)
--- 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",