[PATCH v2 1/2] KVM: arm64: vgic-its: Don't map an LPI from more than one event

[Fuad Tabba <fuad.tabba@linux.dev>]

KVM keeps one vgic_irq per LPI. A MAPTI of an LPI whose vgic_irq
already exists reuses it for the new ITE and leaves its target vCPU as
it was. Mapping one LPI from two events is UNPREDICTABLE (IHI 0069H.b,
5.2.10), but nothing stops a guest from doing it. If the first MAPTI
names a collection the guest never maps and the second a mapped one,
the LPI translates through the second event with a NULL target vCPU.

When the VMM routes an assigned device's MSI to that event on a host
with GICv4 enabled, kvm_vgic_v4_set_forwarding() takes the vPE from the
NULL target vCPU and the host oopses:

  Unable to handle kernel paging request at virtual address 0000000000001980
  [...]
  Call trace:
   its_build_vmapti_cmd (drivers/irqchip/irq-gic-v3-its.c:943) (P)
   its_irq_set_vcpu_affinity (drivers/irqchip/irq-gic-v3-its.c:1380)
   irq_set_vcpu_affinity (kernel/irq/manage.c:670)
   its_map_vlpi (drivers/irqchip/irq-gic-v4.c:326)
   kvm_vgic_v4_set_forwarding (arch/arm64/kvm/vgic/vgic-v4.c:485)
   kvm_arch_irq_bypass_add_producer (arch/arm64/kvm/arm.c:2977)
   irq_bypass_register_consumer (virt/lib/irqbypass.c:44)
   kvm_irqfd (arch/arm64/kvm/../../../virt/kvm/eventfd.c:496)
   kvm_vm_ioctl (arch/arm64/kvm/../../../virt/kvm/kvm_main.c:5270)

Track in each vgic_irq whether an ITE maps it. Drop a MAPTI of an LPI
that is already mapped, as a failed MAPTI already is. Restoring the ITS
tables keeps the first ITE it reads for each LPI and drops any later
one, so a guest saved with a double mapping still restores. Key the
check on the mapping, not on the vgic_irq. A vgic_irq outlives its ITE
while anything holds a reference to it, such as the vCPU it was last
queued on. A later MAPTI of such an LPI succeeds as before, and now
retargets it to its collection.

Fixes: 196b136498b35 ("KVM: arm/arm64: GICv4: Wire mapping/unmapping of VLPIs in VFIO irq bypass")
Suggested-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/vgic/vgic-its.c | 26 ++++++++++++++++++++++++--
 include/kvm/arm_vgic.h         |  1 +
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 9e782a4fea7e5..44b92c01bc727 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -772,6 +772,7 @@ static void its_free_ite(struct kvm *kvm, struct its_ite *ite)
 				its_unmap_vlpi(ite->irq->host_irq);
 
 			irq->hw = false;
+			irq->mapped = false;
 		}
 
 		vgic_put_irq(kvm, ite->irq);
@@ -780,6 +781,23 @@ static void its_free_ite(struct kvm *kvm, struct its_ite *ite)
 	kfree(ite);
 }
 
+/* Mapping one LPI from two events is UNPREDICTABLE: the first ITE keeps it. */
+static bool vgic_its_map_lpi(struct kvm *kvm, struct its_ite *ite,
+			     struct vgic_irq *irq, struct kvm_vcpu *vcpu)
+{
+	scoped_guard(raw_spinlock_irqsave, &irq->irq_lock) {
+		if (!irq->mapped) {
+			irq->mapped = true;
+			irq->target_vcpu = vcpu;
+			ite->irq = irq;
+			return true;
+		}
+	}
+
+	vgic_put_irq(kvm, irq);
+	return false;
+}
+
 static u64 its_cmd_mask_field(u64 *its_cmd, int word, int shift, int size)
 {
 	return (le64_to_cpu(its_cmd[word]) >> shift) & (BIT_ULL(size) - 1);
@@ -1089,13 +1107,14 @@ static int vgic_its_cmd_handle_mapi(struct kvm *kvm, struct vgic_its *its,
 		vcpu = collection_to_vcpu(kvm, collection);
 
 	irq = vgic_add_lpi(kvm, lpi_nr, vcpu);
+	if (!IS_ERR(irq) && !vgic_its_map_lpi(kvm, ite, irq, vcpu))
+		irq = ERR_PTR(-EEXIST);
 	if (IS_ERR(irq)) {
 		if (new_coll)
 			vgic_its_free_collection(its, coll_id);
 		its_free_ite(kvm, ite);
 		return PTR_ERR(irq);
 	}
-	ite->irq = irq;
 
 	return 0;
 }
@@ -2195,7 +2214,10 @@ static int vgic_its_restore_ite(struct vgic_its *its, u32 event_id,
 		its_free_ite(kvm, ite);
 		return PTR_ERR(irq);
 	}
-	ite->irq = irq;
+
+	/* Drop an ITE whose LPI another ITE maps, as MAPTI does. */
+	if (!vgic_its_map_lpi(kvm, ite, irq, vcpu))
+		its_free_ite(kvm, ite);
 
 	return offset;
 }
diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
index 086b7578e5f33..5c3a0154becc4 100644
--- a/include/kvm/arm_vgic.h
+++ b/include/kvm/arm_vgic.h
@@ -256,6 +256,7 @@ struct vgic_irq {
 	bool active:1;
 	bool hw:1;			/* Tied to HW IRQ */
 	bool on_lr:1;			/* Present in a CPU LR */
+	bool mapped:1;			/* LPIs: mapped by an ITE */
 	refcount_t refcount;		/* Used for LPIs */
 	u32 hwintid;			/* HW INTID number */
 	unsigned int host_irq;		/* linux irq corresponding to hwintid */
-- 
2.39.5