From patchwork Mon Jan 11 22:57:50 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Kees Cook X-Patchwork-Id: 8012651 Return-Path: X-Original-To: patchwork-linux-fsdevel@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork2.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.136]) by patchwork2.web.kernel.org (Postfix) with ESMTP id 21AE3BEEE5 for ; Mon, 11 Jan 2016 22:58:28 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 3B53620165 for ; Mon, 11 Jan 2016 22:58:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 36522201EC for ; Mon, 11 Jan 2016 22:58:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760317AbcAKW6Y (ORCPT ); Mon, 11 Jan 2016 17:58:24 -0500 Received: from mail-pa0-f53.google.com ([209.85.220.53]:35616 "EHLO mail-pa0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759982AbcAKW6W (ORCPT ); Mon, 11 Jan 2016 17:58:22 -0500 Received: by mail-pa0-f53.google.com with SMTP id ho8so67518419pac.2 for ; Mon, 11 Jan 2016 14:58:22 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=date:from:to:cc:subject:message-id:mime-version:content-type :content-disposition; bh=7zC3g3xqcSgVdHPmfs9WRDR8Grfdb5Cv+kkkn6QMNoY=; b=mAcGS1WkAgFyIrG1cI+1vBEPiLBnFzeHauAGTK2TJFECP5V6VYaBbtQSmWIcgGWKmE Z9HoHv2Sd/AiI+PiSOETVey/8yKZbk/w9Gp83gzESI5TRv1FLH99DTiE6AUsnt/8ujf8 c6MYn5QY5jv/8lvJbx9eeh8E84t4Zf8CGKJcw= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:cc:subject:message-id:mime-version :content-type:content-disposition; bh=7zC3g3xqcSgVdHPmfs9WRDR8Grfdb5Cv+kkkn6QMNoY=; b=Csk1IQuJxzSonqx6EtlR5dsHze3D1J9n5Z9QpMyfYQpcLZ9OYDyu8FKMyjz6zJv1uj /S05lSEcnaRAUGIKm5qmlwW3c3fkebIpibgt9wAroHa9+JEN316kAlj4KOnVfe5C/SpH CE278n9dXIO/wlnuJDMYJtAZL6eQwtg5NHK/ErP1nOJ6fHmeb+XpglnuI8tOUhr+eWR+ RVS3aErA3m4ItyryKAOJQr8HcRHfdN4LjlzEoo8I4arGg0qxwuxYTs7eXCeDW5LGeYEp pbHhJkRKsSVeUPpgn6tby3+o1ig9rW0ffLXxddfVWYtfqpM//BM5mgflEXarWqsJf+OE NneQ== X-Gm-Message-State: ALoCoQl8yzRDxhVRLoFF86DTbpOyQ9N7VkytYhHa/R6Py1dPbygxhdG1RBo2My/mn2/l6NSATGz+Jc2huWBrXeYHQdAeSr1dZA== X-Received: by 10.66.142.168 with SMTP id rx8mr182993143pab.16.1452553102323; Mon, 11 Jan 2016 14:58:22 -0800 (PST) Received: from www.outflux.net (smtp.outflux.net. [198.145.64.163]) by smtp.gmail.com with ESMTPSA id g26sm25548665pfg.35.2016.01.11.14.58.21 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Jan 2016 14:58:21 -0800 (PST) Date: Mon, 11 Jan 2016 14:57:50 -0800 From: Kees Cook To: Alexander Viro Cc: Andy Lutomirski , Jan Kara , yalin wang , Willy Tarreau , Konstantin Khlebnikov , Andrew Morton , linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v7] fs: clear file privilege bits when mmap writing Message-ID: <20160111225750.GA4256@www.outflux.net> MIME-Version: 1.0 Content-Disposition: inline Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Spam-Status: No, score=-6.8 required=5.0 tests=BAYES_00,DKIM_SIGNED, RCVD_IN_DNSWL_HI,RP_MATCHES_RCVD,T_DKIM_INVALID,UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Normally, when a user can modify a file that has setuid or setgid bits, those bits are cleared when they are not the file owner or a member of the group. This is enforced when using write and truncate but not when writing to a shared mmap on the file. This could allow the file writer to gain privileges by changing a binary without losing the setuid/setgid/caps bits. Changing the bits requires holding inode->i_mutex, so it cannot be done during the page fault (due to mmap_sem being held during the fault). We could do this during vm_mmap_pgoff, but that would need coverage in mprotect as well, but to check for MAP_SHARED, we'd need to hold mmap_sem again. We could clear at open() time, but it's possible things are accidentally opening with O_RDWR and only reading. Better to clear on close and error failures (i.e. an improvement over now, which is not clearing at all). Instead, detect the need to clear the bits during the page fault, and actually remove the bits during final fput. Since the file was open for writing, it wouldn't have been possible to execute it yet (ETXTBSY). Signed-off-by: Kees Cook --- v7: - document and avoid arch-specific O_* values, viro v6: - clarify ETXTBSY situation in comments, luto v5: - add to f_flags instead, viro - add i_mutex during __fput, jack v4: - delay removal instead of still needing mmap_sem for mprotect, yalin v3: - move outside of mmap_sem for real now, fengguang - check return code of file_remove_privs, akpm v2: - move to mmap from fault handler, jack --- fs/file_table.c | 15 +++++++++++++++ fs/open.c | 2 +- include/uapi/asm-generic/fcntl.h | 11 +++++++++++ mm/memory.c | 5 +++++ 4 files changed, 32 insertions(+), 1 deletion(-) diff --git a/fs/file_table.c b/fs/file_table.c index ad17e05ebf95..ca11b86613cf 100644 --- a/fs/file_table.c +++ b/fs/file_table.c @@ -191,6 +191,21 @@ static void __fput(struct file *file) might_sleep(); + /* + * XXX: This is a delayed removal of privs (we've already been + * written to), since we must avoid mmap_sem. But a race shouldn't + * be possible since when open for writing, execve() will fail + * with ETXTBSY (via deny_write_access()). A remaining problem + * is that since we've already been written to, we must ignore the + * return value of file_remove_privs(), since we can't reject the + * writes of the past. + */ + if (unlikely(file->f_flags & O_REMOVEPRIV)) { + mutex_lock(&inode->i_mutex); + file_remove_privs(file); + mutex_unlock(&inode->i_mutex); + } + fsnotify_close(file); /* * The function eventpoll_release() should be the first called diff --git a/fs/open.c b/fs/open.c index b6f1e96a7c0b..89069d16ca80 100644 --- a/fs/open.c +++ b/fs/open.c @@ -895,7 +895,7 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o op->mode = 0; /* Must never be set by userspace */ - flags &= ~FMODE_NONOTIFY & ~O_CLOEXEC; + flags &= ~FMODE_NONOTIFY & ~O_CLOEXEC & ~O_REMOVEPRIV; /* * O_SYNC is implemented as __O_SYNC|O_DSYNC. As many places only diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h index e063effe0cc1..1b6c49edb393 100644 --- a/include/uapi/asm-generic/fcntl.h +++ b/include/uapi/asm-generic/fcntl.h @@ -88,6 +88,17 @@ #define __O_TMPFILE 020000000 #endif +/* + * Reserved arch-specific values for __O_TMPFILE: + * 0040000000 (parisc) + * 0100000000 (alpha) + * 0200000000 (sparc) + */ + +#ifndef O_REMOVEPRIV +#define O_REMOVEPRIV 0400000000 +#endif + /* a horrid kludge trying to make sure that this will fail on old kernels */ #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY) #define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT) diff --git a/mm/memory.c b/mm/memory.c index c387430f06c3..ad4188a8f279 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2036,6 +2036,11 @@ static inline int wp_page_reuse(struct mm_struct *mm, if (!page_mkwrite) file_update_time(vma->vm_file); + if (unlikely((vma->vm_file->f_flags & O_REMOVEPRIV) == 0)) { + spin_lock(&vma->vm_file->f_lock); + vma->vm_file->f_flags |= O_REMOVEPRIV; + spin_unlock(&vma->vm_file->f_lock); + } } return VM_FAULT_WRITE;