dm: fix a race condition in retrieve_deps

There's a race condition in the multipath target when retrieve_deps
races with multipath_message calling dm_get_device and dm_put_device.
retrieve_deps walks the list of open devices without holding any lock
but multipath may add or remove devices to the list while it is
running. The end result may be memory corruption or use-after-free
memory access.

See this description of a UAF with multipath_message():
https://listman.redhat.com/archives/dm-devel/2022-October/052373.html

Fix this bug by introducing a new rw semaphore "devices_lock". We grab
devices_lock for read in retrieve_deps and we grab it for write in
dm_get_device and dm_put_device.

Reported-by: Luo Meng <[e-mail]>
Signed-off-by: Mikulas Patocka <[e-mail]>
Cc: [e-mail]
Tested-by: Li Lingfeng <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-core.h

···
 
 	/* a list of devices used by this table */
 	struct list_head devices;
+	struct rw_semaphore devices_lock;
 
 	/* events get handed up using this callback */
 	void (*event_fn)(void *data);

drivers/md/dm-ioctl.c

···
 	struct dm_dev_internal *dd;
 	struct dm_target_deps *deps;
 
+	down_read(&table->devices_lock);
+
 	deps = get_result_buffer(param, param_size, &len);
 
 	/*
···
 	needed = struct_size(deps, dev, count);
 	if (len < needed) {
 		param->flags |= DM_BUFFER_FULL_FLAG;
-		return;
+		goto out;
 	}
 
 	/*
···
 		deps->dev[count++] = huge_encode_dev(dd->dm_dev->bdev->bd_dev);
 
 	param->data_size = param->data_start + needed;
+
+out:
+	up_read(&table->devices_lock);
 }
 
 static int table_deps(struct file *filp, struct dm_ioctl *param, size_t param_size)

drivers/md/dm-table.c

···
 		return -ENOMEM;
 
 	INIT_LIST_HEAD(&t->devices);
+	init_rwsem(&t->devices_lock);
 
 	if (!num_targets)
 		num_targets = KEYS_PER_NODE;
···
 	if (dev == disk_devt(t->md->disk))
 		return -EINVAL;
 
+	down_write(&t->devices_lock);
+
 	dd = find_device(&t->devices, dev);
 	if (!dd) {
 		dd = kmalloc(sizeof(*dd), GFP_KERNEL);
-		if (!dd)
-			return -ENOMEM;
+		if (!dd) {
+			r = -ENOMEM;
+			goto unlock_ret_r;
+		}
 
 		r = dm_get_table_device(t->md, dev, mode, &dd->dm_dev);
 		if (r) {
 			kfree(dd);
-			return r;
+			goto unlock_ret_r;
 		}
 
 		refcount_set(&dd->count, 1);
···
 	} else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
 		r = upgrade_mode(dd, mode, t->md);
 		if (r)
-			return r;
+			goto unlock_ret_r;
 	}
 	refcount_inc(&dd->count);
 out:
+	up_write(&t->devices_lock);
 	*result = dd->dm_dev;
 	return 0;
+
+unlock_ret_r:
+	up_write(&t->devices_lock);
+	return r;
 }
 EXPORT_SYMBOL(dm_get_device);
 
···
 void dm_put_device(struct dm_target *ti, struct dm_dev *d)
 {
 	int found = 0;
-	struct list_head *devices = &ti->table->devices;
+	struct dm_table *t = ti->table;
+	struct list_head *devices = &t->devices;
 	struct dm_dev_internal *dd;
+
+	down_write(&t->devices_lock);
 
 	list_for_each_entry(dd, devices, list) {
 		if (dd->dm_dev == d) {
···
 	}
 	if (!found) {
 		DMERR("%s: device %s not in table devices list",
-		      dm_device_name(ti->table->md), d->name);
-		return;
+		      dm_device_name(t->md), d->name);
+		goto unlock_ret;
 	}
 	if (refcount_dec_and_test(&dd->count)) {
-		dm_put_table_device(ti->table->md, d);
+		dm_put_table_device(t->md, d);
 		list_del(&dd->list);
 		kfree(dd);
 	}
+
+unlock_ret:
+	up_write(&t->devices_lock);
 }
 EXPORT_SYMBOL(dm_put_device);