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/gfs2/acl.c

@@ -92,17 +92,11 @@ int __gfs2_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 	if (type == ACL_TYPE_ACCESS) {
 		umode_t mode = inode->i_mode;
 
-		error = posix_acl_equiv_mode(acl, &mode);
-		if (error < 0)
+		error = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+		if (error)
 			return error;
-
-		if (error == 0)
-			acl = NULL;
-
-		if (mode != inode->i_mode) {
-			inode->i_mode = mode;
+		if (mode != inode->i_mode)
 			mark_inode_dirty(inode);
-		}
 	}
 
 	if (acl) {