x86/speculation: Add Kconfig option for GDS

commit 53cf5797f114ba2bd86d23a862302119848eff19 upstream

Gather Data Sampling (GDS) is mitigated in microcode. However, on
systems that haven't received the updated microcode, disabling AVX
can act as a mitigation. Add a Kconfig option that uses the microcode
mitigation if available and disables AVX otherwise. Setting this
option has no effect on systems not affected by GDS. This is the
equivalent of setting gather_data_sampling=force.

Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e9e1ccaf4ff228c5f425ca457c790c18e3c74b91..c62c2b9c37738cfb50c46d22b973c258a0de549c 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2466,6 +2466,25 @@ config SLS
          against straight line speculation. The kernel image might be slightly
          larger.
 
+config GDS_FORCE_MITIGATION
+       bool "Force GDS Mitigation"
+       depends on CPU_SUP_INTEL
+       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
+         the microcode any userspace code that unconditionally uses AVX will
+         break with this option set.
+
+         Setting this option on systems not vulnerable to GDS has no effect.
+
+         If in doubt, say N.
+
 endif
 
 config ARCH_HAS_ADD_PAGES

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9b047051fb1158b75fb61ba590d44c6761d154e4..f9a1293e99205f23db470e1bc257b3b8cd0989f0 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -658,7 +658,11 @@ enum gds_mitigations {
        GDS_MITIGATION_HYPERVISOR,
 };
 
+#if IS_ENABLED(CONFIG_GDS_FORCE_MITIGATION)
+static enum gds_mitigations gds_mitigation __ro_after_init = GDS_MITIGATION_FORCE;
+#else
 static enum gds_mitigations gds_mitigation __ro_after_init = GDS_MITIGATION_FULL;
+#endif
 
 static const char * const gds_strings[] = {
        [GDS_MITIGATION_OFF]            = "Vulnerable",