From patchwork Wed Jan 25 21:06:52 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andy Lutomirski X-Patchwork-Id: 9537947 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 077C96046A for ; Wed, 25 Jan 2017 21:07:32 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id EC46627FBC for ; Wed, 25 Jan 2017 21:07:31 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id E0BFE27FC0; Wed, 25 Jan 2017 21:07:31 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 902B61FF60 for ; Wed, 25 Jan 2017 21:07:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752050AbdAYVHN (ORCPT ); Wed, 25 Jan 2017 16:07:13 -0500 Received: from mail.kernel.org ([198.145.29.136]:58468 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751254AbdAYVHL (ORCPT ); Wed, 25 Jan 2017 16:07:11 -0500 Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 4FB43203F4; Wed, 25 Jan 2017 21:07:05 +0000 (UTC) Received: from localhost (c-71-202-137-17.hsd1.ca.comcast.net [71.202.137.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 44E51203AD; Wed, 25 Jan 2017 21:07:04 +0000 (UTC) From: Andy Lutomirski To: security@kernel.org Cc: Konstantin Khlebnikov , Alexander Viro , Kees Cook , Willy Tarreau , "linux-mm@kvack.org" , Andrew Morton , yalin wang , Linux Kernel Mailing List , Jan Kara , Linux FS Devel , Andy Lutomirski Subject: [PATCH 2/2] fs: Harden against open(..., O_CREAT, 02777) in a setgid directory Date: Wed, 25 Jan 2017 13:06:52 -0800 Message-Id: <826ec4aab64ec304944098d15209f8c1ae65bb29.1485377903.git.luto@kernel.org> X-Mailer: git-send-email 2.9.3 In-Reply-To: References: In-Reply-To: References: X-Virus-Scanned: ClamAV using ClamSMTP Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Currently, if you open("foo", O_WRONLY | O_CREAT | ..., 02777) in a directory that is setgid and owned by a different gid than current's fsgid, you end up with an SGID executable that is owned by the directory's GID. This is a Bad Thing (tm). Exploiting this is nontrivial because most ways of creating a new file create an empty file and empty executables aren't particularly interesting, but this is nevertheless quite dangerous. Harden against this type of attack by detecting this particular corner case (unprivileged program creates SGID executable inode in SGID directory owned by a different GID) and clearing the new inode's SGID bit. Signed-off-by: Andy Lutomirski --- fs/inode.c | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/fs/inode.c b/fs/inode.c index f7029c40cfbd..d7e4b80470dd 100644 --- a/fs/inode.c +++ b/fs/inode.c @@ -2007,11 +2007,28 @@ void inode_init_owner(struct inode *inode, const struct inode *dir, { inode->i_uid = current_fsuid(); if (dir && dir->i_mode & S_ISGID) { + bool changing_gid = !gid_eq(inode->i_gid, dir->i_gid); + inode->i_gid = dir->i_gid; - if (S_ISDIR(mode)) + if (S_ISDIR(mode)) { mode |= S_ISGID; - } else + } else if (((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) + && S_ISREG(mode) && changing_gid + && !capable(CAP_FSETID)) { + /* + * Whoa there! An unprivileged program just + * tried to create a new executable with SGID + * set in a directory with SGID set that belongs + * to a different group. Don't let this program + * create a SGID executable that ends up owned + * by the wrong group. + */ + mode &= ~S_ISGID; + } + + } else { inode->i_gid = current_fsgid(); + } inode->i_mode = mode; } EXPORT_SYMBOL(inode_init_owner);