linux-kernel - Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 11 Dec 2015 14:52:01 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Al Viro <viro@...iv.linux.org.uk>, Greg KH <greg@...ah.com>,
	Jiri Slaby <jslaby@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Aurelien Jarno <aurelien@...el32.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

On Fri, Dec 11, 2015 at 2:35 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Andy Lutomirski <luto@...capital.net> writes:
>
>> On Fri, Dec 11, 2015 at 2:07 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>> On 12/11/15 13:48, Andy Lutomirski wrote:
>>>> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman
>>>> <ebiederm@...ssion.com> wrote:
>>>>> Al Viro <viro@...IV.linux.org.uk> 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.

>
> I even posted an implementation earlier in the discussion and no one was
> interested.
>
> Honestly the more weird special cases we add to devpts the less likely
> userspace will be to get things right.  We have been trying since 1998
> and devpts is still a poor enough design we have not been able to get
> rid of /usr/lib/pt_chown.  Adding another case where we have to sand on
> one foot and touch our nose does not seem to likely to achieve
> widespread adoption.  How many version of libc are there now?

Old libc can stay buggy, I think.  Given that this mess is partially a
libc mis-design, I don't see why it needs to be fixed entirely in the
kernel.

For new systems, it would be really nice if we can make a /dev/ptmx
symlink be 100% functional.

In any event, this is semi-moot.  If we actually want to retire the
newinstance=0 thing from the kernel, we apparently need a magic
/dev/ptmx node.  That doesn't mean that new userspace needs to *use*
that magic node.  So why not implement the magic node without fiddling
with f_path?

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/