From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DCC9FC433E6 for ; Sun, 30 Aug 2020 16:37:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C6D0C20714 for ; Sun, 30 Aug 2020 16:37:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726434AbgH3QhD (ORCPT ); Sun, 30 Aug 2020 12:37:03 -0400 Received: from brightrain.aerifal.cx ([216.12.86.13]:48218 "EHLO brightrain.aerifal.cx" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726134AbgH3QhA (ORCPT ); Sun, 30 Aug 2020 12:37:00 -0400 Date: Sun, 30 Aug 2020 12:36:58 -0400 From: Rich Felker To: Jann Horn Cc: linux-fsdevel , kernel list , Linux API , Alexander Viro Subject: Re: [RESEND PATCH] vfs: add RWF_NOAPPEND flag for pwritev2 Message-ID: <20200830163657.GD3265@brightrain.aerifal.cx> References: <20200829020002.GC3265@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-api-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Sun, Aug 30, 2020 at 05:05:45PM +0200, Jann Horn wrote: > On Sat, Aug 29, 2020 at 4:00 AM Rich Felker wrote: > > The pwrite function, originally defined by POSIX (thus the "p"), is > > defined to ignore O_APPEND and write at the offset passed as its > > argument. However, historically Linux honored O_APPEND if set and > > ignored the offset. This cannot be changed due to stability policy, > > but is documented in the man page as a bug. > > > > Now that there's a pwritev2 syscall providing a superset of the pwrite > > functionality that has a flags argument, the conforming behavior can > > be offered to userspace via a new flag. > [...] > > diff --git a/include/linux/fs.h b/include/linux/fs.h > [...] > > @@ -3411,6 +3413,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags) > > ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC); > > if (flags & RWF_APPEND) > > ki->ki_flags |= IOCB_APPEND; > > + if (flags & RWF_NOAPPEND) > > + ki->ki_flags &= ~IOCB_APPEND; > > return 0; > > } > > Linux enforces the S_APPEND flag (set by "chattr +a") only at open() > time, not at write() time: > > # touch testfile > # exec 100>testfile > # echo foo > testfile > # cat testfile > foo > # chattr +a testfile > # echo bar > testfile > bash: testfile: Operation not permitted > # echo bar >&100 > # cat testfile > bar > # > > At open() time, the kernel enforces that you can't use O_WRONLY/O_RDWR > without also setting O_APPEND if the file is marked as append-only: > > static int may_open(const struct path *path, int acc_mode, int flag) > { > [...] > /* > * An append-only file must be opened in append mode for writing. > */ > if (IS_APPEND(inode)) { > if ((flag & O_ACCMODE) != O_RDONLY && !(flag & O_APPEND)) > return -EPERM; > if (flag & O_TRUNC) > return -EPERM; > } > [...] > } > > It seems to me like your patch will permit bypassing S_APPEND by > opening an append-only file with O_WRONLY|O_APPEND, then calling > pwritev2() with RWF_NOAPPEND? I think you'll have to add an extra > check for IS_APPEND() somewhere. > > > One could also argue that if an O_APPEND file descriptor is handed > across privilege boundaries, a programmer might reasonably expect that > the recipient will not be able to use the file descriptor for > non-append writes; if that is not actually true, that should probably > be noted in the open.2 manpage, at the end of the description of > O_APPEND. fcntl F_SETFL can remove O_APPEND, so it is not a security boundary. I'm not sure how this interacts with S_APPEND; presumably fcntl rechecks it. So just checking IS_APPEND in the code paths used by pwritev2 (and erroring out rather than silently writing output at the wrong place) should suffice to preserve all existing security invariants. Rich