[PATCH 7/7] KVM/VMX: Move VERW closer to VMentry for MDS mitigation

[Nikolay Borisov <nik.borisov@suse.com>]

From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

During VMentry VERW is executed to mitigate MDS. After VERW, any memory
access like register push onto stack may put host data in MDS affected
CPU buffers. A guest can then use MDS to sample host data.

Although likelihood of secrets surviving in registers at current VERW
callsite is less, but it can't be ruled out. Harden the MDS mitigation
by moving the VERW mitigation late in VMentry path.

Note that VERW for MMIO Stale Data mitigation is unchanged because of
the complexity of per-guest conditional VERW which is not easy to handle
that late in asm with no GPRs available. If the CPU is also affected by
MDS, VERW is unconditionally executed late in asm regardless of guest
having MMIO access.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/all/20240213-delay-verw-v8-6-a6216d83edb7%40linux.intel.com
Signed-off-by: Nikolay Borisov <nik.borisov@suse.com>
---
 arch/x86/kvm/vmx/vmenter.S |  3 +++
 arch/x86/kvm/vmx/vmx.c     | 12 ++++++++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 04517546e3dc..1ca759f74bb5 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -98,6 +98,9 @@ ENTRY(__vmx_vcpu_run)
 	/* Load guest RAX.  This kills the @regs pointer! */
 	mov VCPU_RAX(%_ASM_AX), %_ASM_AX
 
+	/* Clobbers EFLAGS.ZF */
+	CLEAR_CPU_BUFFERS
+
 	/* Check EFLAGS.CF from the VMX_RUN_VMRESUME bit test above. */
 	jnc .Lvmlaunch
 
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 4bf0c6221ec8..56f044854c29 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -377,7 +377,8 @@ static __always_inline void vmx_enable_fb_clear(struct vcpu_vmx *vmx)
 
 static void vmx_update_fb_clear_dis(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx)
 {
-	vmx->disable_fb_clear = vmx_fb_clear_ctrl_available;
+	vmx->disable_fb_clear = !cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF) &&
+		vmx_fb_clear_ctrl_available;
 
 	/*
 	 * If guest will not execute VERW, there is no need to set FB_CLEAR_DIS
@@ -6659,11 +6660,14 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
 	 */
 	x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
 
-	/* L1D Flush includes CPU buffer clear to mitigate MDS */
+       /*
+        * L1D Flush includes CPU buffer clear to mitigate MDS, but VERW
+        * mitigation for MDS is done late in VMentry and is still
+        * executed in spite of L1D Flush. This is because an extra VERW
+        * should not matter much after the big hammer L1D Flush.
+        */
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
-	else if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF))
-		mds_clear_cpu_buffers();
 	else if (static_branch_unlikely(&mmio_stale_data_clear) &&
 		 kvm_arch_has_assigned_device(vcpu->kvm))
 		mds_clear_cpu_buffers();
-- 
2.34.1