iommu/virtio: Fix freeing of incomplete domains

Calling viommu_domain_free() on a domain that hasn't been finalised (not
attached to any device, for example) can currently cause an Oops,
because we attempt to call ida_free() on ID 0, which may either be
unallocated or used by another domain.

Only initialise the vdomain->viommu pointer, which denotes a finalised
domain, at the end of a successful viommu_domain_finalise().

Fixes: edcd69a ("iommu: Add virtio-iommu driver")
Reported-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20200326093558.2641019-3-jean-philippe@linaro.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/virtio-iommu.c

@@ -613,18 +613,20 @@ static int viommu_domain_finalise(struct viommu_dev *viommu,
 	int ret;
 	struct viommu_domain *vdomain = to_viommu_domain(domain);
 
-	vdomain->viommu		= viommu;
-	vdomain->map_flags	= viommu->map_flags;
+	ret = ida_alloc_range(&viommu->domain_ids, viommu->first_domain,
+			      viommu->last_domain, GFP_KERNEL);
+	if (ret < 0)
+		return ret;
+
+	vdomain->id		= (unsigned int)ret;
 
 	domain->pgsize_bitmap	= viommu->pgsize_bitmap;
 	domain->geometry	= viommu->geometry;
 
-	ret = ida_alloc_range(&viommu->domain_ids, viommu->first_domain,
-			      viommu->last_domain, GFP_KERNEL);
-	if (ret >= 0)
-		vdomain->id = (unsigned int)ret;
+	vdomain->map_flags	= viommu->map_flags;
+	vdomain->viommu		= viommu;
 
-	return ret > 0 ? 0 : ret;
+	return 0;
 }
 
 static void viommu_domain_free(struct iommu_domain *domain)