x86/mmio: Disable KVM mitigation when X86_FEATURE_CLEAR_CPU_BUF is set

Currently MMIO Stale Data mitigation for CPUs not affected by MDS/TAA is
to only deploy VERW at VMentry by enabling mmio_stale_data_clear static
branch. No mitigation is needed for kernel->user transitions. If such
CPUs are also affected by RFDS, its mitigation may set
X86_FEATURE_CLEAR_CPU_BUF to deploy VERW at kernel->user and VMentry.
This could result in duplicate VERW at VMentry.

Fix this by disabling mmio_stale_data_clear static branch when
X86_FEATURE_CLEAR_CPU_BUF is enabled.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 48d049cd74e7123a178564ba6fc8ef1dc0212e2d..cd6ac89c1a0df32d7bcf5830ead5ee6db2538e9c 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -422,6 +422,13 @@ static void __init mmio_select_mitigation(void)
        if (boot_cpu_has_bug(X86_BUG_MDS) || (boot_cpu_has_bug(X86_BUG_TAA) &&
                                              boot_cpu_has(X86_FEATURE_RTM)))
                setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+
+       /*
+        * X86_FEATURE_CLEAR_CPU_BUF could be enabled by other VERW based
+        * mitigations, disable KVM-only mitigation in that case.
+        */
+       if (boot_cpu_has(X86_FEATURE_CLEAR_CPU_BUF))
+               static_branch_disable(&mmio_stale_data_clear);
        else
                static_branch_enable(&mmio_stale_data_clear);
 
@@ -498,8 +505,11 @@ static void __init md_clear_update_mitigation(void)
                taa_mitigation = TAA_MITIGATION_VERW;
                taa_select_mitigation();
        }
-       if (mmio_mitigation == MMIO_MITIGATION_OFF &&
-           boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
+       /*
+        * MMIO_MITIGATION_OFF is not checked here so that mmio_stale_data_clear
+        * gets updated correctly as per X86_FEATURE_CLEAR_CPU_BUF state.
+        */
+       if (boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
                mmio_mitigation = MMIO_MITIGATION_VERW;
                mmio_select_mitigation();
        }