dm ioctl: add flag to wipe buffers for secure data

Add DM_SECURE_DATA_FLAG which userspace can use to ensure
that all buffers allocated for dm-ioctl are wiped
immediately after use.

The user buffer is wiped as well (we do not want to keep
and return sensitive data back to userspace if the flag is set).

Wiping is useful for cryptsetup to ensure that the key
is present in memory only in defined places and only
for the time needed.

(For crypt, key can be present in table during load or table
status, wait and message commands).

Signed-off-by: Milan Broz <[e-mail]>
Signed-off-by: Alasdair G Kergon <[e-mail]>

drivers/md/dm-ioctl.c

···
 static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl **param)
 {
 	struct dm_ioctl tmp, *dmi;
+	int secure_data;
 
 	if (copy_from_user(&tmp, user, sizeof(tmp) - sizeof(tmp.data)))
 		return -EFAULT;
···
 	if (tmp.data_size < (sizeof(tmp) - sizeof(tmp.data)))
 		return -EINVAL;
 
+	secure_data = tmp.flags & DM_SECURE_DATA_FLAG;
+
 	dmi = vmalloc(tmp.data_size);
-	if (!dmi)
+	if (!dmi) {
+		if (secure_data && clear_user(user, tmp.data_size))
+			return -EFAULT;
 		return -ENOMEM;
+	}
 
 	if (copy_from_user(dmi, user, tmp.data_size))
+		goto bad;
+
+	/* Wipe the user buffer so we do not return it to userspace */
+	if (secure_data && clear_user(user, tmp.data_size))
 		goto bad;
 
 	*param = dmi;
 	return 0;
 
 bad:
+	if (secure_data)
+		memset(dmi, 0, tmp.data_size);
 	vfree(dmi);
 	return -EFAULT;
 }
···
 	/* Always clear this flag */
 	param->flags &= ~DM_BUFFER_FULL_FLAG;
 	param->flags &= ~DM_UEVENT_GENERATED_FLAG;
+	param->flags &= ~DM_SECURE_DATA_FLAG;
 
 	/* Ignores parameters */
 	if (cmd == DM_REMOVE_ALL_CMD ||
···
 static int ctl_ioctl(uint command, struct dm_ioctl __user *user)
 {
 	int r = 0;
+	int wipe_buffer;
 	unsigned int cmd;
 	struct dm_ioctl *uninitialized_var(param);
 	ioctl_fn fn = NULL;
···
 	 * Copy the parameters into kernel space.
 	 */
 	r = copy_params(user, &param);
-	input_param_size = param->data_size;
 
 	current->flags &= ~PF_MEMALLOC;
 
 	if (r)
 		return r;
+
+	input_param_size = param->data_size;
+	wipe_buffer = param->flags & DM_SECURE_DATA_FLAG;
 
 	r = validate_params(cmd, param);
 	if (r)
···
 		r = -EFAULT;
 
 out:
+	if (wipe_buffer)
+		memset(param, 0, input_param_size);
+
 	vfree(param);
 	return r;
 }

include/linux/dm-ioctl.h

···
 #define DM_DEV_SET_GEOMETRY	_IOWR(DM_IOCTL, DM_DEV_SET_GEOMETRY_CMD, struct dm_ioctl)
 
 #define DM_VERSION_MAJOR	4
-#define DM_VERSION_MINOR	19
-#define DM_VERSION_PATCHLEVEL	1
-#define DM_VERSION_EXTRA	"-ioctl (2011-01-07)"
+#define DM_VERSION_MINOR	20
+#define DM_VERSION_PATCHLEVEL	0
+#define DM_VERSION_EXTRA	"-ioctl (2011-02-02)"
 
 /* Status bits */
 #define DM_READONLY_FLAG	(1 << 0) /* In/Out */
···
  * if no uuid was previously supplied: an existing uuid cannot be changed.
  */
 #define DM_UUID_FLAG			(1 << 14) /* In */
+
+/*
+ * If set, all buffers are wiped after use. Use when sending
+ * or requesting sensitive data such as an encryption key.
+ */
+#define DM_SECURE_DATA_FLAG		(1 << 15) /* In */
 
 #endif				/* _LINUX_DM_IOCTL_H */