device-dax: don't leak kernel memory to user space after unloading kmem

Assume we have kmem configured and loaded:

  [root@localhost ~]# cat /proc/iomem
  ...
  140000000-33fffffff : Persistent Memory$
    140000000-1481fffff : namespace0.0
    150000000-33fffffff : dax0.0
      150000000-33fffffff : System RAM

Assume we try to unload kmem. This force-unloading will work, even if
memory cannot get removed from the system.

  [root@localhost ~]# rmmod kmem
  [   86.380228] removing memory fails, because memory [0x0000000150000000-0x0000000157ffffff] is onlined
  ...
  [   86.431225] kmem dax0.0: DAX region [mem 0x150000000-0x33fffffff] cannot be hotremoved until the next reboot

Now, we can reconfigure the namespace:

  [root@localhost ~]# ndctl create-namespace --force --reconfig=namespace0.0 --mode=devdax
  [  131.409351] nd_pmem namespace0.0: could not reserve region [mem 0x140000000-0x33fffffff]dax
  [  131.410147] nd_pmem: probe of namespace0.0 failed with error -16namespace0.0 --mode=devdax
  ...

This fails as expected due to the busy memory resource, and the memory
cannot be used.  However, the dax0.0 device is removed, and along its
name.

The name of the memory resource now points at freed memory (name of the
device):

  [root@localhost ~]# cat /proc/iomem
  ...
  140000000-33fffffff : Persistent Memory
    140000000-1481fffff : namespace0.0
    150000000-33fffffff : �_�^7_��/_��wR��WQ���^��� ...
    150000000-33fffffff : System RAM

We have to make sure to duplicate the string.  While at it, remove the
superfluous setting of the name and fixup a stale comment.

Fixes: 9f960da ("device-dax: "Hotremove" persistent memory that is used like normal RAM")
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org>	[5.3]
Link: http://lkml.kernel.org/r/20200508084217.9160-2-david@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/dax/kmem.c

@@ -22,6 +22,7 @@ int dev_dax_kmem_probe(struct device *dev)
 	resource_size_t kmem_size;
 	resource_size_t kmem_end;
 	struct resource *new_res;
+	const char *new_res_name;
 	int numa_node;
 	int rc;
 
@@ -48,11 +49,16 @@ int dev_dax_kmem_probe(struct device *dev)
 	kmem_size &= ~(memory_block_size_bytes() - 1);
 	kmem_end = kmem_start + kmem_size;
 
-	/* Region is permanently reserved.  Hot-remove not yet implemented. */
-	new_res = request_mem_region(kmem_start, kmem_size, dev_name(dev));
+	new_res_name = kstrdup(dev_name(dev), GFP_KERNEL);
+	if (!new_res_name)
+		return -ENOMEM;
+
+	/* Region is permanently reserved if hotremove fails. */
+	new_res = request_mem_region(kmem_start, kmem_size, new_res_name);
 	if (!new_res) {
 		dev_warn(dev, "could not reserve region [%pa-%pa]\n",
 			 &kmem_start, &kmem_end);
+		kfree(new_res_name);
 		return -EBUSY;
 	}
 
@@ -63,12 +69,12 @@ int dev_dax_kmem_probe(struct device *dev)
 	 * unknown to us that will break add_memory() below.
 	 */
 	new_res->flags = IORESOURCE_SYSTEM_RAM;
-	new_res->name = dev_name(dev);
 
 	rc = add_memory(numa_node, new_res->start, resource_size(new_res));
 	if (rc) {
 		release_resource(new_res);
 		kfree(new_res);
+		kfree(new_res_name);
 		return rc;
 	}
 	dev_dax->dax_kmem_res = new_res;
@@ -83,6 +89,7 @@ static int dev_dax_kmem_remove(struct device *dev)
 	struct resource *res = dev_dax->dax_kmem_res;
 	resource_size_t kmem_start = res->start;
 	resource_size_t kmem_size = resource_size(res);
+	const char *res_name = res->name;
 	int rc;
 
 	/*
@@ -102,6 +109,7 @@ static int dev_dax_kmem_remove(struct device *dev)
 	/* Release and free dax resources */
 	release_resource(res);
 	kfree(res);
+	kfree(res_name);
 	dev_dax->dax_kmem_res = NULL;
 
 	return 0;