KVM: PPC: Book3S HV: Protect updates to spapr_tce_tables list

Al Viro pointed out that while one thread of a process is executing
in kvm_vm_ioctl_create_spapr_tce(), another thread could guess the
file descriptor returned by anon_inode_getfd() and close() it before
the first thread has added it to the kvm->arch.spapr_tce_tables list.
That highlights a more general problem: there is no mutual exclusion
between writers to the spapr_tce_tables list, leading to the
possibility of the list becoming corrupted, which could cause a
host kernel crash.

To fix the mutual exclusion problem, we add a mutex_lock/unlock
pair around the list_del_rce in kvm_spapr_tce_release().  Also,
this moves the call to anon_inode_getfd() inside the region
protected by the kvm->lock mutex, after we have done the check for
a duplicate LIOBN.  This means that if another thread does guess the
file descriptor and closes it, its call to kvm_spapr_tce_release()
will not do any harm because it will have to wait until the first
thread has released kvm->lock.  With this, there are no failure
points in kvm_vm_ioctl_create_spapr_tce() after the call to
anon_inode_getfd().

The other things that the second thread could do with the guessed
file descriptor are to mmap it or to pass it as a parameter to a
KVM_DEV_VFIO_GROUP_SET_SPAPR_TCE ioctl on a KVM device fd.  An mmap
call won't cause any harm because kvm_spapr_tce_mmap() and
kvm_spapr_tce_fault() don't access the spapr_tce_tables list or
the kvmppc_spapr_tce_table.list field, and the fields that they do use
have been properly initialized by the time of the anon_inode_getfd()
call.

The KVM_DEV_VFIO_GROUP_SET_SPAPR_TCE ioctl calls
kvm_spapr_tce_attach_iommu_group(), which scans the spapr_tce_tables
list looking for the kvmppc_spapr_tce_table struct corresponding to
the fd given as the parameter.  Either it will find the new entry
or it won't; if it doesn't, it just returns an error, and if it
does, it will function normally.  So, in each case there is no
harmful effect.
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_64_vio.c

@@ -265,8 +265,11 @@ static int kvm_spapr_tce_release(struct inode *inode, struct file *filp)
 {
 	struct kvmppc_spapr_tce_table *stt = filp->private_data;
 	struct kvmppc_spapr_tce_iommu_table *stit, *tmp;
+	struct kvm *kvm = stt->kvm;
 
+	mutex_lock(&kvm->lock);
 	list_del_rcu(&stt->list);
+	mutex_unlock(&kvm->lock);
 
 	list_for_each_entry_safe(stit, tmp, &stt->iommu_tables, next) {
 		WARN_ON(!kref_read(&stit->kref));
@@ -298,7 +301,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
 	unsigned long npages, size;
 	int ret = -ENOMEM;
 	int i;
-	int fd = -1;
 
 	if (!args->size)
 		return -EINVAL;
@@ -328,11 +330,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
 			goto fail;
 	}
 
-	ret = fd = anon_inode_getfd("kvm-spapr-tce", &kvm_spapr_tce_fops,
-				    stt, O_RDWR | O_CLOEXEC);
-	if (ret < 0)
-		goto fail;
-
 	mutex_lock(&kvm->lock);
 
 	/* Check this LIOBN hasn't been previously allocated */
@@ -344,17 +341,19 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
 		}
 	}
 
-	if (!ret) {
+	if (!ret)
+		ret = anon_inode_getfd("kvm-spapr-tce", &kvm_spapr_tce_fops,
+				       stt, O_RDWR | O_CLOEXEC);
+
+	if (ret >= 0) {
 		list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables);
 		kvm_get_kvm(kvm);
 	}
 
 	mutex_unlock(&kvm->lock);
 
-	if (!ret)
-		return fd;
-
-	put_unused_fd(fd);
+	if (ret >= 0)
+		return ret;
 
  fail:
 	for (i = 0; i < npages; i++)