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

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Date:	Fri, 11 Dec 2015 15:16:48 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jann Horn <jann@...jh.net>,
	"Eric W. Biederman" <ebiederm@...ssion.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>,
	"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 3:07 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On December 11, 2015 3:00:49 PM PST, Andy Lutomirski <luto@...capital.net> wrote:
>>On Fri, Dec 11, 2015 at 2:58 PM, Jann Horn <jann@...jh.net> 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
>>>> <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.
>>>
>>> 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.

I think you're right, modulo the one stupidity that a configuration
like that is prone to breakage with container apps running on the same
system.

Hmm.  Could userspace be changed to set newinstance=1 on its /dev/pts
mount to work around that?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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