KVM: Add GDS_NO support to KVM

commit 81ac7e5d741742d650b4ed6186c4826c1a0631a7 upstream

Gather Data Sampling (GDS) is a transient execution attack using
gather instructions from the AVX2 and AVX512 extensions. This attack
allows malicious code to infer data that was previously stored in
vector registers. Systems that are not vulnerable to GDS will set the
GDS_NO bit of the IA32_ARCH_CAPABILITIES MSR. This is useful for VM
guests that may think they are on vulnerable systems that are, in
fact, not affected. Guests that are running on affected hosts where
the mitigation is enabled are protected as if they were running
on an unaffected system.

On all hosts that are not affected or that are mitigated, set the
GDS_NO bit.

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>

arch/x86/kernel/cpu/bugs.c

@@ -628,6 +628,13 @@ static const char * const gds_strings[] = {
 	[GDS_MITIGATION_HYPERVISOR]	= "Unknown: Dependent on hypervisor status",
 };
 
+bool gds_ucode_mitigated(void)
+{
+	return (gds_mitigation == GDS_MITIGATION_FULL ||
+		gds_mitigation == GDS_MITIGATION_FULL_LOCKED);
+}
+EXPORT_SYMBOL_GPL(gds_ucode_mitigated);
+
 void update_gds_msr(void)
 {
 	u64 mcu_ctrl_after;

arch/x86/kvm/x86.c

@@ -226,6 +226,8 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
 
 u64 __read_mostly host_xcr0;
 
+extern bool gds_ucode_mitigated(void);
+
 struct kmem_cache *x86_fpu_cache;
 EXPORT_SYMBOL_GPL(x86_fpu_cache);
 
@@ -1409,6 +1411,9 @@ static u64 kvm_get_arch_capabilities(void)
 	/* Guests don't need to know "Fill buffer clear control" exists */
 	data &= ~ARCH_CAP_FB_CLEAR_CTRL;
 
+	if (!boot_cpu_has_bug(X86_BUG_GDS) || gds_ucode_mitigated())
+		data |= ARCH_CAP_GDS_NO;
+
 	return data;
 }