[RFC PATCH 1/2] KVM: arm64: vgic-v3: Roll back failed redistributor region setup

[Karl Mehltretter <kmehltretter@gmail.com>]

A later REDIST_REGION attribute can be inserted successfully and then
fail while registering redistributor iodevs. For example, a region that
overlaps the distributor is caught by vgic_v3_check_base() when the first
unassigned vCPU is processed.

The existing rollback unregisters all previously registered
redistributor iodevs. It leaves their rd_iodev.base_addr values set, so a
corrected retry skips re-registering those stale vCPUs and succeeds with
holes in the MMIO bus.

A failure from kvm_io_bus_register_dev() can leave another hole. The
failing vCPU already has its base address and region assigned, but the
old i < c rollback does not include it.

Preserve devices assigned by earlier successful setters. On failure,
unregister only vCPUs associated with the newly inserted region, clear
their cached base addresses, and free that region. This also includes the
current vCPU when iodev registration itself fails.

Fixes: c011f4ea106b ("KVM: arm/arm64: Check vcpu redist base before registering an iodev")
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 arch/arm64/kvm/vgic/vgic-mmio-v3.c | 42 ++++++++++++++++++------------
 1 file changed, 26 insertions(+), 16 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v3.c b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
index 5913a20d83019..832a7fe65984c 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
@@ -841,6 +841,31 @@ void vgic_unregister_redist_iodev(struct kvm_vcpu *vcpu)
 	kvm_io_bus_unregister_dev(vcpu->kvm, KVM_MMIO_BUS, &rd_dev->dev);
 }
 
+static void vgic_v3_rollback_redist_region(struct kvm *kvm, u32 index)
+{
+	struct vgic_redist_region *rdreg;
+	struct kvm_vcpu *vcpu;
+	unsigned long c;
+
+	lockdep_assert_held(&kvm->slots_lock);
+
+	rdreg = vgic_v3_rdist_region_from_index(kvm, index);
+
+	kvm_for_each_vcpu(c, vcpu, kvm) {
+		if (vcpu->arch.vgic_cpu.rdreg == rdreg)
+			vgic_unregister_redist_iodev(vcpu);
+	}
+
+	guard(mutex)(&kvm->arch.config_lock);
+
+	kvm_for_each_vcpu(c, vcpu, kvm) {
+		if (vcpu->arch.vgic_cpu.rdreg == rdreg)
+			vcpu->arch.vgic_cpu.rd_iodev.base_addr = VGIC_ADDR_UNDEF;
+	}
+
+	vgic_v3_free_redist_region(kvm, rdreg);
+}
+
 static int vgic_register_all_redist_iodevs(struct kvm *kvm)
 {
 	struct kvm_vcpu *vcpu;
@@ -855,16 +880,6 @@ static int vgic_register_all_redist_iodevs(struct kvm *kvm)
 			break;
 	}
 
-	if (ret) {
-		/* The current c failed, so iterate over the previous ones. */
-		int i;
-
-		for (i = 0; i < c; i++) {
-			vcpu = kvm_get_vcpu(kvm, i);
-			vgic_unregister_redist_iodev(vcpu);
-		}
-	}
-
 	return ret;
 }
 
@@ -984,12 +999,7 @@ int vgic_v3_set_redist_base(struct kvm *kvm, u32 index, u64 addr, u32 count)
 	 */
 	ret = vgic_register_all_redist_iodevs(kvm);
 	if (ret) {
-		struct vgic_redist_region *rdreg;
-
-		mutex_lock(&kvm->arch.config_lock);
-		rdreg = vgic_v3_rdist_region_from_index(kvm, index);
-		vgic_v3_free_redist_region(kvm, rdreg);
-		mutex_unlock(&kvm->arch.config_lock);
+		vgic_v3_rollback_redist_region(kvm, index);
 		return ret;
 	}
 
-- 
2.53.0