KVM: VMX: Handle MMIO Stale Data in VM-Enter assembly via ALTERNATIVES_2

Rework the handling of the MMIO Stale Data mitigation to clear CPU buffers
immediately prior to VM-Enter, i.e. in the same location that KVM emits a
VERW for unconditional (at runtime) clearing.  Co-locating the code and
using a single ALTERNATIVES_2 makes it more obvious how VMX mitigates the
various vulnerabilities.

Deliberately order the alternatives as:

 0. Do nothing
 1. Clear if vCPU can access MMIO
 2. Clear always

since the last alternative wins in ALTERNATIVES_2(), i.e. so that KVM will
honor the strictest mitigation (always clear CPU buffers) if multiple
mitigations are selected.  E.g. even if the kernel chooses to mitigate
MMIO Stale Data via X86_FEATURE_CLEAR_CPU_BUF_VM_MMIO, another mitigation
may enable X86_FEATURE_CLEAR_CPU_BUF_VM, and that other thing needs to win.

Note, decoupling the MMIO mitigation from the L1TF mitigation also fixes
a mostly-benign flaw where KVM wouldn't do any clearing/flushing if the
L1TF mitigation is configured to conditionally flush the L1D, and the MMIO
mitigation but not any other "clear CPU buffers" mitigation is enabled.
For that specific scenario, KVM would skip clearing CPU buffers for the
MMIO mitigation even though the kernel requested a clear on every VM-Enter.

Note #2, the flaw goes back to the introduction of the MDS mitigation.  The
MDS mitigation was inadvertently fixed by commit 43fb862 ("KVM/VMX:
Move VERW closer to VMentry for MDS mitigation"), but previous kernels
that flush CPU buffers in vmx_vcpu_enter_exit() are affected (though it's
unlikely the flaw is meaningfully exploitable even older kernels).

Fixes: 650b68a ("x86/kvm/vmx: Add MDS protection when L1D Flush is not active")
Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Link: https://patch.msgid.link/20251113233746.1703361-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/vmenter.S

@@ -71,6 +71,7 @@
  * @regs:	unsigned long * (to guest registers)
  * @flags:	VMX_RUN_VMRESUME:	use VMRESUME instead of VMLAUNCH
  *		VMX_RUN_SAVE_SPEC_CTRL: save guest SPEC_CTRL into vmx->spec_ctrl
+ *		VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO: vCPU can access host MMIO
  *
  * Returns:
  *	0 on VM-Exit, 1 on VM-Fail
@@ -154,8 +155,19 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	/* Load guest RAX.  This kills the @regs pointer! */
 	mov VCPU_RAX(%_ASM_AX), %_ASM_AX
 
-	/* Clobbers EFLAGS.ZF */
-	VM_CLEAR_CPU_BUFFERS
+	/*
+	 * Note, ALTERNATIVE_2 works in reverse order.  If CLEAR_CPU_BUF_VM is
+	 * enabled, do VERW unconditionally.  If CPU_BUF_VM_MMIO is enabled,
+	 * check @flags to see if the vCPU has access to host MMIO, and if so,
+	 * do VERW.  Else, do nothing (no mitigations needed/enabled).
+	 */
+	ALTERNATIVE_2 "",									  \
+		      __stringify(testl $VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO, WORD_SIZE(%_ASM_SP); \
+				  jz .Lskip_mmio_verw;						  \
+				  VERW;								  \
+				  .Lskip_mmio_verw:),					  	  \
+		      X86_FEATURE_CLEAR_CPU_BUF_VM_MMIO,					  \
+		      __stringify(VERW), X86_FEATURE_CLEAR_CPU_BUF_VM
 
 	/* Check @flags to see if VMLAUNCH or VMRESUME is needed. */
 	testl $VMX_RUN_VMRESUME, WORD_SIZE(%_ASM_SP)

arch/x86/kvm/vmx/vmx.c

@@ -7313,21 +7313,8 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 
 	guest_state_enter_irqoff();
 
-	/*
-	 * 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.
-	 *
-	 * cpu_buf_vm_clear is used when system is not vulnerable to MDS/TAA,
-	 * and is affected by MMIO Stale Data. In such cases mitigation in only
-	 * needed against an MMIO capable guest.
-	 */
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
-	else if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF_VM_MMIO) &&
-		 (flags & VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO))
-		x86_clear_cpu_buffers();
 
 	vmx_disable_fb_clear(vmx);