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.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
arch/x86/kvm/vmx/nested.c | 6 ++++--
arch/x86/kvm/vmx/vmx.c | 7 +++++++
2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 238c26155c2a..7e8a646e2851 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/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);
/*
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index f18c2d8c7476..63d656032384 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4219,6 +4219,13 @@ 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) {
/*
--
2.46.0.469.g59c65b2a67-goog
