linux-kernel - Re: Does anyone care about a race free ptsname?

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Message-ID: <20160419204213.GA32593@mail.hallyn.com>
Date:	Tue, 19 Apr 2016 15:42:13 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Willy Tarreau <w@....eu>,
	Aurelien Jarno <aurelien@...el32.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jann Horn <jann@...jh.net>, Greg KH <greg@...ah.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>, Florian Weimer <fw@...eb.enyo.de>
Subject: Re: Does anyone care about a race free ptsname?

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> Linus Torvalds <torvalds@...ux-foundation.org> writes:
> 
> > What this does is get rid of the horrible notion of having that
> >
> >     struct inode *ptmx_inode
> >
> > be the interface between the pty code and devpts. By de-emphasizing the 
> > ptmx inode, a lot of things actually get cleaner, and we will have a much 
> > saner way forward.
> 
> I will take a look in a minute.   Before I do that I want to mention
> why I care about /dev/pts/ptmx.
> 
> There is a posix function that is widely used called ptsname.  It's
> function is to take a master file descriptor and returns the path to the
> slave.
> 
> All we have in the kernel to support ptsname is an ioctl TIOCGPTN that
> returns the pty number in the appropriate instance of devpts.
> 
> The only way we have today to query which instance of devpts the pty is
> on is through fstat and look st_dev to see if the file is on the correct
> filesystem.  This works when /dev/pts/ptmx is used and fails when
> /dev/ptmx is used.
> 
> Does anyone else care?
> 
> If no one cares I will stop worrying about it and just get on with
> fixing the rest of this mess which there definitely seems to be the will
> to do.

There seem to be quite a few users of ptsname (as found by
codesearch.debian.net).  I'm going to look through those results
a bit more tonight.  One common idiom is

	blah = ptsname(fd);
	slavefd = open(blah, ...);

That's more easily solved with a new helper.

The scariest thing of course is any callers of ptsname which are setuid-root.

short answer: i'm going to do some research to try and answer "who cares".

-serge