dm ioctl: Check dm_target_spec is sufficiently aligned

Otherwise subsequent code, if given malformed input, could dereference
a misaligned 'struct dm_target_spec *'.
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au> # use %zu
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-ioctl.c

@@ -1394,6 +1394,15 @@ static inline blk_mode_t get_mode(struct dm_ioctl *param)
 static int next_target(struct dm_target_spec *last, uint32_t next, void *end,
 		       struct dm_target_spec **spec, char **target_params)
 {
+	static_assert(__alignof__(struct dm_target_spec) <= 8,
+		"struct dm_target_spec must not require more than 8-byte alignment");
+
+	if (next % __alignof__(struct dm_target_spec)) {
+		DMERR("Next dm_target_spec (offset %u) is not %zu-byte aligned",
+		      next, __alignof__(struct dm_target_spec));
+		return -EINVAL;
+	}
+
 	*spec = (struct dm_target_spec *) ((unsigned char *) last + next);
 	*target_params = (char *) (*spec + 1);