[PATCH v3 2/5] KVM: arm64: vgic-v3: Reset redistributors after failed region setup

[Karl Mehltretter]

A REDIST_REGION write can fail after redistributor iodevs have been
registered. The existing rollback unregisters only vCPUs processed before
the failure, leaving their cached assignments and free_index values intact.
A retry then skips the unregistered iodevs.

Userspace has no guarantee that redistributor assignments survive a failed
region update. On failure, unregister every redistributor iodev and clear
every cached vCPU assignment. Reset all region free_index values and free
the newly inserted region. The next successful region update rebuilds all
possible assignments in region-index order.

While a vCPU is being created, its redistributor iodev may be registered
before the vCPU is visible to kvm_for_each_vcpu(). Reject REDIST and
REDIST_REGION writes while creation is in flight, so rollback can reset every
assignment.

The registration failure for the current vCPU is already undone by
vgic_register_redist_iodev().

Fixes: c011f4ea106b ("KVM: arm/arm64: Check vcpu redist base before registering an iodev")
Suggested-by: Marc Zyngier <maz at kernel.org>
Cc: stable at vger.kernel.org
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Karl Mehltretter <kmehltretter at gmail.com>
---
 arch/arm64/kvm/vgic/vgic-kvm-device.c | 20 ++++++++++
 arch/arm64/kvm/vgic/vgic-mmio-v3.c    | 53 +++++++++++++++++++--------
 2 files changed, 57 insertions(+), 16 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-kvm-device.c b/arch/arm64/kvm/vgic/vgic-kvm-device.c
index 90be99443df3..48c3b2a48c20 100644
--- a/arch/arm64/kvm/vgic/vgic-kvm-device.c
+++ b/arch/arm64/kvm/vgic/vgic-kvm-device.c
@@ -97,6 +97,9 @@ static int kvm_vgic_addr(struct kvm *kvm, struct kvm_device_attr *attr, bool wri
 	phys_addr_t *addr_ptr, alignment, size;
 	u64 undef_value = VGIC_ADDR_UNDEF;
 	u64 addr;
+	bool redist_write = write &&
+		(attr->attr == KVM_VGIC_V3_ADDR_TYPE_REDIST ||
+		 attr->attr == KVM_VGIC_V3_ADDR_TYPE_REDIST_REGION);
 	int r;
 
 	/* Reading a redistributor region addr implies getting the index */
@@ -104,6 +107,19 @@ static int kvm_vgic_addr(struct kvm *kvm, struct kvm_device_attr *attr, bool wri
 		if (get_user(addr, uaddr))
 			return -EFAULT;
 
+	/*
+	 * A vCPU can have an RD assignment before it is visible to
+	 * kvm_for_each_vcpu(). Reject redistributor updates while vCPU creation
+	 * is in progress, so rollback can reset every assignment.
+	 */
+	if (redist_write) {
+		mutex_lock(&kvm->lock);
+		if (kvm->created_vcpus != atomic_read(&kvm->online_vcpus)) {
+			r = -EBUSY;
+			goto out_unlock_kvm;
+		}
+	}
+
 	/*
 	 * Since we can't hold config_lock while registering the redistributor
 	 * iodevs, take the slots_lock immediately.
@@ -201,6 +217,10 @@ static int kvm_vgic_addr(struct kvm *kvm, struct kvm_device_attr *attr, bool wri
 out:
 	mutex_unlock(&kvm->slots_lock);
 
+out_unlock_kvm:
+	if (redist_write)
+		mutex_unlock(&kvm->lock);
+
 	if (!r && !write)
 		r =  put_user(addr, uaddr);
 
diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v3.c b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
index 22897ce64dbf..6c009deb11d4 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
@@ -855,6 +855,40 @@ 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_reset_redist_iodev(struct kvm_vcpu *vcpu)
+{
+	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
+
+	lockdep_assert_held(&vcpu->kvm->arch.config_lock);
+
+	vgic_cpu->rdreg = NULL;
+	vgic_cpu->rd_iodev.base_addr = VGIC_ADDR_UNDEF;
+}
+
+static void vgic_v3_rollback_redist_region(struct kvm *kvm, u32 index)
+{
+	struct vgic_redist_region *rdreg, *iter;
+	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)
+		vgic_unregister_redist_iodev(vcpu);
+
+	guard(mutex)(&kvm->arch.config_lock);
+
+	kvm_for_each_vcpu(c, vcpu, kvm)
+		vgic_reset_redist_iodev(vcpu);
+
+	list_for_each_entry(iter, &kvm->arch.vgic.rd_regions, list)
+		iter->free_index = 0;
+
+	vgic_v3_free_redist_region(kvm, rdreg);
+}
+
 static int vgic_register_all_redist_iodevs(struct kvm *kvm)
 {
 	struct kvm_vcpu *vcpu;
@@ -869,16 +903,6 @@ 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;
 }
 
@@ -986,6 +1010,8 @@ int vgic_v3_set_redist_base(struct kvm *kvm, u32 index, u64 addr, u32 count)
 {
 	int ret;
 
+	lockdep_assert_held(&kvm->lock);
+
 	mutex_lock(&kvm->arch.config_lock);
 	ret = vgic_v3_alloc_redist_region(kvm, index, addr, count);
 	mutex_unlock(&kvm->arch.config_lock);
@@ -998,12 +1024,7 @@ 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;
 	}
 
-- 
2.39.5 (Apple Git-154)