[PATCH v3 1/5] KVM: arm64: vgic-v3: Undo assignment on iodev registration failure

[Karl Mehltretter <kmehltretter@gmail.com>]

vgic_register_redist_iodev() assigns a redistributor region and base
address to the vCPU before adding its iodev to the MMIO bus. However, the
region's free_index is advanced only after registration succeeds.

If kvm_io_bus_register_dev() fails, the vCPU retains the assignment while
free_index still identifies the same slot as free. A later registration can
therefore reuse a slot that remains assigned to the vCPU.

Reserve the slot before registering its iodev. The caller holds slots_lock,
so a registration failure cannot race with a later assignment. Undo the
reservation and clear the cached assignment on failure.

Fixes: dbd9733ab674 ("KVM: arm/arm64: Replace the single rdist region by a list")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 arch/arm64/kvm/vgic/vgic-mmio-v3.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v3.c b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
index 5913a20d8301..22897ce64dbf 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
@@ -766,6 +766,19 @@ unsigned int vgic_v3_init_dist_iodev(struct vgic_io_device *dev)
 	return SZ_64K;
 }
 
+static void vgic_undo_redist_assignment(struct kvm_vcpu *vcpu)
+{
+	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
+
+	lockdep_assert_held(&vcpu->kvm->slots_lock);
+
+	guard(mutex)(&vcpu->kvm->arch.config_lock);
+
+	vgic_cpu->rdreg->free_index--;
+	vgic_cpu->rdreg = NULL;
+	vgic_cpu->rd_iodev.base_addr = VGIC_ADDR_UNDEF;
+}
+
 /**
  * vgic_register_redist_iodev - register a single redist iodev
  * @vcpu:    The VCPU to which the redistributor belongs
@@ -818,16 +831,17 @@ int vgic_register_redist_iodev(struct kvm_vcpu *vcpu)
 	rd_dev->nr_regions = ARRAY_SIZE(vgic_v3_rd_registers);
 	rd_dev->redist_vcpu = vcpu;
 
+	/* Protected by slots_lock */
+	rdreg->free_index++;
+
 	mutex_unlock(&kvm->arch.config_lock);
 
 	ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, rd_base,
 				      2 * SZ_64K, &rd_dev->dev);
 	if (ret)
-		return ret;
+		vgic_undo_redist_assignment(vcpu);
 
-	/* Protected by slots_lock */
-	rdreg->free_index++;
-	return 0;
+	return ret;
 
 out_unlock:
 	mutex_unlock(&kvm->arch.config_lock);
-- 
2.39.5 (Apple Git-154)