KVM: nVMX: Explicitly invalidate posted_intr_nv if PI is disabled at VM-Enter

Explicitly invalidate posted_intr_nv when emulating nested VM-Enter and
posted interrupts are disabled to make it clear that posted_intr_nv is
valid if and only if nested posted interrupts are enabled, and as a cheap
way to harden against KVM bugs.

KVM initializes posted_intr_nv to -1 at vCPU creation and resets it to -1
when unloading vmcs12 and/or leaving nested mode, i.e. this is not a bug
fix (or at least, it's not intended to be a bug fix).

Note, tracking nested.posted_intr_nv as a u16 subtly adds a measure of
safety, as it prevents unintentionally matching KVM's informal "no IRQ"
vector of -1, stored as a signed int.  Because a u16 can be always be
represented as a signed int, the effective "invalid" value of
posted_intr_nv, 65535, will be preserved as-is when comparing against an
int, i.e. will be zero-extended, not sign-extended, and thus won't get a
false positive if KVM is buggy and compares posted_intr_nv against -1.

Opportunistically add a comment in vmx_deliver_nested_posted_interrupt()
to call out that it must check vmx->nested.posted_intr_nv, not the vector
in vmcs12, which is presumably the _entire_ reason nested.posted_intr_nv
exists.  E.g. vmcs12 is a KVM-controlled snapshot, so there are no TOCTOU
races to worry about, the only potential badness is if the vCPU leaves
nested and frees vmcs12 between the sender checking is_guest_mode() and
dereferencing the vmcs12 pointer.

Link: https://lore.kernel.org/r/20240906043413.1049633-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/nested.c

@@ -2317,10 +2317,12 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
 
 	/* Posted interrupts setting is only taken from vmcs12.  */
 	vmx->nested.pi_pending = false;
-	if (nested_cpu_has_posted_intr(vmcs12))
+	if (nested_cpu_has_posted_intr(vmcs12)) {
 		vmx->nested.posted_intr_nv = vmcs12->posted_intr_nv;
-	else
+	} else {
+		vmx->nested.posted_intr_nv = -1;
 		exec_control &= ~PIN_BASED_POSTED_INTR;
+	}
 	pin_controls_set(vmx, exec_control);
 
 	/*

arch/x86/kvm/vmx/vmx.c

@@ -4215,6 +4215,13 @@ static int vmx_deliver_nested_posted_interrupt(struct kvm_vcpu *vcpu,
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
+	/*
+	 * DO NOT query the vCPU's vmcs12, as vmcs12 is dynamically allocated
+	 * and freed, and must not be accessed outside of vcpu->mutex.  The
+	 * vCPU's cached PI NV is valid if and only if posted interrupts
+	 * enabled in its vmcs12, i.e. checking the vector also checks that
+	 * L1 has enabled posted interrupts for L2.
+	 */
 	if (is_guest_mode(vcpu) &&
 	    vector == vmx->nested.posted_intr_nv) {
 		/*