KVM: lock slots_lock around device assignment

As pointed out by Jason Baron, when assigning a device to a guest
we first set the iommu domain pointer, which enables mapping
and unmapping of memory slots to the iommu.  This leaves a window
where this path is enabled, but we haven't synchronized the iommu
mappings to the existing memory slots.  Thus a slot being removed
at that point could send us down unexpected code paths removing
non-existent pinnings and iommu mappings.  Take the slots_lock
around creating the iommu domain and initial mappings as well as
around iommu teardown to avoid this race.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 15 insertions, 8 deletions

diff --git a/virt/kvm/iommu.c b/virt/kvm/iommu.c
index fec1723de9b4..e9fff9830bf0 100644
--- a/virt/kvm/iommu.c
+++ b/virt/kvm/iommu.c
@@ -240,9 +240,13 @@ int kvm_iommu_map_guest(struct kvm *kvm)
                 return -ENODEV;
         }
 
+        mutex_lock(&kvm->slots_lock);
+
         kvm->arch.iommu_domain = iommu_domain_alloc(&pci_bus_type);
-        if (!kvm->arch.iommu_domain)
-                return -ENOMEM;
+        if (!kvm->arch.iommu_domain) {
+                r = -ENOMEM;
+                goto out_unlock;
+        }
 
         if (!allow_unsafe_assigned_interrupts &&
             !iommu_domain_has_cap(kvm->arch.iommu_domain,
@@ -253,17 +257,16 @@ int kvm_iommu_map_guest(struct kvm *kvm)
                        " module option.\n", __func__);
                 iommu_domain_free(kvm->arch.iommu_domain);
                 kvm->arch.iommu_domain = NULL;
-                return -EPERM;
+                r = -EPERM;
+                goto out_unlock;
         }
 
         r = kvm_iommu_map_memslots(kvm);
         if (r)
-                goto out_unmap;
-
-        return 0;
+                kvm_iommu_unmap_memslots(kvm);
 
-out_unmap:
-        kvm_iommu_unmap_memslots(kvm);
+out_unlock:
+        mutex_unlock(&kvm->slots_lock);
         return r;
 }
 
@@ -340,7 +343,11 @@ int kvm_iommu_unmap_guest(struct kvm *kvm)
         if (!domain)
                 return 0;
 
+        mutex_lock(&kvm->slots_lock);
         kvm_iommu_unmap_memslots(kvm);
+        kvm->arch.iommu_domain = NULL;
+        mutex_unlock(&kvm->slots_lock);
+
         iommu_domain_free(domain);
         return 0;
 }