staging: lustre: llite: fix potential missing-check bug when copying lumv

In ll_dir_ioctl(), the object lumv3 is firstly copied from the user space
using Its address, i.e., lumv1 = &lumv3. If the lmm_magic field of lumv3 is
LOV_USER_MAGIC_V3, lumv3 will be modified by the second copy from the user
space. The second copy is necessary, because the two versions (i.e.,
lov_user_md_v1 and lov_user_md_v3) have different data formats and lengths.
However, given that the user data resides in the user space, a malicious
user-space process can race to change the data between the two copies. By
doing so, the attacker can provide a data with an inconsistent version,
e.g., v1 version + v3 data. This can lead to logical errors in the
following execution in ll_dir_setstripe(), which performs different actions
according to the version specified by the field lmm_magic.

This patch rechecks the version field lmm_magic in the second copy.  If the
version is not as expected, i.e., LOV_USER_MAGIC_V3, an error code will be
returned: -EINVAL.
Signed-off-by: Wenwen Wang <wang6495@umn.edu>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/llite/dir.c

@@ -1185,6 +1185,8 @@ static long ll_dir_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		if (lumv1->lmm_magic == LOV_USER_MAGIC_V3) {
 			if (copy_from_user(&lumv3, lumv3p, sizeof(lumv3)))
 				return -EFAULT;
+			if (lumv3.lmm_magic != LOV_USER_MAGIC_V3)
+				return -EINVAL;
 		}
 
 		if (is_root_inode(inode))