From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/449 Path: news.gmane.org!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: fd 0-2 on SUID/SGID program startup Date: Mon, 22 Aug 2011 14:32:04 -0400 Message-ID: <20110822183204.GB132@brightrain.aerifal.cx> References: <20110822170754.GA16515@openwall.com> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: dough.gmane.org 1314038035 18794 80.91.229.12 (22 Aug 2011 18:33:55 GMT) X-Complaints-To: usenet@dough.gmane.org NNTP-Posting-Date: Mon, 22 Aug 2011 18:33:55 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-450-gllmg-musl=m.gmane.org@lists.openwall.com Mon Aug 22 20:33:51 2011 Return-path: Envelope-to: gllmg-musl@lo.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by lo.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1QvZJk-00008H-B8 for gllmg-musl@lo.gmane.org; Mon, 22 Aug 2011 20:33:48 +0200 Original-Received: (qmail 18332 invoked by uid 550); 22 Aug 2011 18:33:47 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Original-Received: (qmail 18324 invoked from network); 22 Aug 2011 18:33:47 -0000 Content-Disposition: inline In-Reply-To: <20110822170754.GA16515@openwall.com> User-Agent: Mutt/1.5.21 (2010-09-15) Xref: news.gmane.org gmane.linux.lib.musl.general:449 Archived-At: On Mon, Aug 22, 2011 at 09:07:54PM +0400, Solar Designer wrote: > Rich, > > As you're probably aware, glibc makes sure that fd 0-2 are open on > SUID/SGID program startup (opening them to /dev/null / /dev/full if > they're not already open). This is needed to prevent misdirected > reads/writes by programs that use those well-known fd's (in fact, even > libc itself does) yet also open other files/sockets/whatever (so it may > get opened on one of these special fd's if they're not already taken). > > I think musl must have the same countermeasure. I think it lacks it > currently. > > Do you agree? Indeed, this is useful, and POSIX explicitly allows that fd 0-2 might be automatically opened for suid programs. I have an efficient test using a single syscall: struct pollfd pfd[3] = { { .fd = 0 }, { .fd = 1 }, { .fd = 2 } }; poll(pfd, 3, 0); Then check each of pfd[0..2].revents for POLLNVAL: for (i=0; i<3; i++) if ((pfd[i].revents&POLLNVAL) && open("/dev/null", O_RDWR)<0) *(volatile char *)0=0; I assume crashing is the best action on failure to open, but I'd welcome other ideas... perhaps raising SIGKILL? Plain _exit seems like a really bad idea as it could be misinterpreted by the parent as a normal exit. Rich