posix_acl: Clear SGID bit when setting file permissions

When file permissions are modified via chmod(2) and the user is not in
the owning group or capable of CAP_FSETID, the setgid bit is cleared in
inode_change_ok().  Setting a POSIX ACL via setxattr(2) sets the file
permissions as well as the new ACL, but doesn't clear the setgid bit in
a similar way; this allows to bypass the check in chmod(2).  Fix that.

References: CVE-2016-7097
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/reiserfs/xattr_acl.c

@@ -242,13 +242,9 @@ __reiserfs_set_acl(struct reiserfs_transaction_handle *th, struct inode *inode,
 	case ACL_TYPE_ACCESS:
 		name = XATTR_NAME_POSIX_ACL_ACCESS;
 		if (acl) {
-			error = posix_acl_equiv_mode(acl, &inode->i_mode);
-			if (error < 0)
+			error = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+			if (error)
 				return error;
-			else {
-				if (error == 0)
-					acl = NULL;
-			}
 		}
 		break;
 	case ACL_TYPE_DEFAULT: