Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755277AbbLKXIX (ORCPT ); Fri, 11 Dec 2015 18:08:23 -0500 Received: from terminus.zytor.com ([198.137.202.10]:52040 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753508AbbLKXIW (ORCPT ); Fri, 11 Dec 2015 18:08:22 -0500 User-Agent: K-9 Mail for Android In-Reply-To: References: <20151209013859.GA12442@kroah.com> <20151209083225.GA30452@1wt.eu> <87wpskyds7.fsf_-_@x220.int.ebiederm.org> <20151211210400.GL20997@ZenIV.linux.org.uk> <87h9jovgf7.fsf@x220.int.ebiederm.org> <566B4931.5080806@zytor.com> <87y4d0sjff.fsf@x220.int.ebiederm.org> <20151211225825.GA24676@pc.thejh.net> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance From: "H. Peter Anvin" Date: Fri, 11 Dec 2015 15:07:09 -0800 To: Andy Lutomirski , Jann Horn CC: "Eric W. Biederman" , Al Viro , Greg KH , Jiri Slaby , Linus Torvalds , Aurelien Jarno , Florian Weimer , Serge Hallyn , "security@kernel.org" , "security@ubuntu.com >> security" , security@debian.org, Willy Tarreau , "linux-kernel@vger.kernel.org" Message-ID: <5B921124-286C-4D01-A8A5-4BBE1E145D3B@zytor.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3675 Lines: 100 On December 11, 2015 3:00:49 PM PST, Andy Lutomirski wrote: >On Fri, Dec 11, 2015 at 2:58 PM, Jann Horn wrote: >> On Fri, Dec 11, 2015 at 02:52:01PM -0800, Andy Lutomirski wrote: >>> On Fri, Dec 11, 2015 at 2:35 PM, Eric W. Biederman >>> wrote: >>> > Andy Lutomirski writes: >>> > >>> >> On Fri, Dec 11, 2015 at 2:07 PM, H. Peter Anvin >wrote: >>> >>> On 12/11/15 13:48, Andy Lutomirski wrote: >>> >>>> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman >>> >>>> wrote: >>> >>>>> Al Viro writes: >>> >>>>> >>> >>>>>> On Fri, Dec 11, 2015 at 01:40:40PM -0600, Eric W. Biederman >wrote: >>> >>>>>> >>> >>>>>>> + inode = path.dentry->d_inode; >>> >>>>>>> + filp->f_path = path; >>> >>>>>>> + filp->f_inode = inode; >>> >>>>>>> + filp->f_mapping = inode->i_mapping; >>> >>>>>>> + path_put(&old); >>> >>>>>> >>> >>>>>> Don't. You are creating a fairly subtle constraint on what >the code in >>> >>>>>> fs/open.c and fs/namei.c can do, for no good reason. You can >bloody >>> >>>>>> well maintain the information you need without that. >>> >>>>> >>> >>>>> There is a good reason. We can not write a race free version >of ptsname >>> >>>>> without it. >>> >>>> >>> >>>> As long as this is for new userspace code, would it make sense >to just >>> >>>> add a new ioctl to ask "does this ptmx fd match this /dev/pts >fd?" >>> >>>> >>> >>> >>> >>> For the newinstance case st_dev should match between the master >and the >>> >>> slave. Unfortunately this is not the case for a legacy ptmx, as >a >>> >>> stat() on the master descriptor still returns the st_dev, >st_rdev, and >>> >>> st_ino for the ptmx device node. >>> >> >>> >> Sure, but I'm not talking about stat. I'm saying that we could >add a >>> >> new ioctl that works on any ptmx fd (/dev/ptmx or /dev/pts/ptmx) >that >>> >> answers the question "does this ptmx logically belong to the >given >>> >> devpts filesystem". >>> >> >>> >> Since it's not stat, we can make it do whatever we want, >including >>> >> following a link to the devpts instance that isn't f_path or >f_inode. >>> > >>> > The useful ioctl to add in my opinion would be one that actually >opens >>> > the slave, at which point ptsname could become ttyname, and that >closes >>> > races in grantpt. >>> >>> Unfortunately, ptsname is POSIX, so we can't get rid of it. It's a >>> bad idea, but it's in the standard. >> >> But then ptsname could become "open the slave, call ttyname() on it, >close >> the slave". Unless opening the slave would have side effects? > >Hmm, fair enough. So maybe that does make sense after all. > >Anyway, I still think there are two pieces here: > >1. Fix /dev/ptmx so that we can banish newinstance=0. > >2. Fix libc. If that needs kernel help, then so be it. > >ISTM we could still implement the "open the slave" operation for (2) >as an ioctl that does the appropriate magic the fd is /dev/ptmx as >opposed to /dev/pts/ptmx. > > >--Andy I want to be clear: If /dev/ptmx -> pts/ptmx and devpts is mounted with the proper options, I believe ask the remaining parts of userspace should be fine, and pt_chown can be removed even with glibc. The magic ptmx we are talking about is all about dealing with a mismanaged /dev. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/