dm ioctl: harden copy_params()'s copy_from_user() from malicious users

In copy_params(), the struct 'dm_ioctl' is first copied from the user
space buffer 'user' to 'param_kernel' and the field 'data_size' is
checked against 'minimum_data_size' (size of 'struct dm_ioctl' payload
up to its 'data' member).  If the check fails, an error code EINVAL will be
returned.  Otherwise, param_kernel->data_size is used to do a second copy,
which copies from the same user-space buffer to 'dmi'.  After the second
copy, only 'dmi->data_size' is checked against 'param_kernel->data_size'.
Given that the buffer 'user' resides in the user space, a malicious
user-space process can race to change the content in the buffer between
the two copies.  This way, the attacker can inject inconsistent data
into 'dmi' (versus previously validated 'param_kernel').

Fix redundant copying of 'minimum_data_size' from user-space buffer by
using the first copy stored in 'param_kernel'.  Also remove the
'data_size' check after the second copy because it is now unnecessary.

Cc: stable@vger.kernel.org
Signed-off-by: Wenwen Wang <wang6495@umn.edu>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-ioctl.c

@@ -1720,8 +1720,7 @@ static void free_params(struct dm_ioctl *param, size_t param_size, int param_fla
 }
 
 static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl *param_kernel,
-		       int ioctl_flags,
-		       struct dm_ioctl **param, int *param_flags)
+		       int ioctl_flags, struct dm_ioctl **param, int *param_flags)
 {
 	struct dm_ioctl *dmi;
 	int secure_data;
@@ -1762,18 +1761,13 @@ static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl *param_kern
 
 	*param_flags |= DM_PARAMS_MALLOC;
 
-	if (copy_from_user(dmi, user, param_kernel->data_size))
-		goto bad;
+	/* Copy from param_kernel (which was already copied from user) */
+	memcpy(dmi, param_kernel, minimum_data_size);
 
-data_copied:
-	/*
-	 * Abort if something changed the ioctl data while it was being copied.
-	 */
-	if (dmi->data_size != param_kernel->data_size) {
-		DMERR("rejecting ioctl: data size modified while processing parameters");
+	if (copy_from_user(&dmi->data, (char __user *)user + minimum_data_size,
+			   param_kernel->data_size - minimum_data_size))
 		goto bad;
-	}
-
+data_copied:
 	/* Wipe the user buffer so we do not return it to userspace */
 	if (secure_data && clear_user(user, param_kernel->data_size))
 		goto bad;