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

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Message-ID: <877fftiblc.fsf@x220.int.ebiederm.org>
Date:	Tue, 19 Apr 2016 15:32:15 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security\@kernel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security\@ubuntu.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?

"H. Peter Anvin" <hpa@...or.com> writes:

> We could add another ioctl for that purpose of we need to.  Perhaps an
> ioctl which returns a file descriptor to the slave device?
>
> However, since we are now defining ptmx to explicitly look up pts/ by
> name it seems like /dev/ptmx -> /dev/pts/# is true by definition.  If
> what you worry about is namespace reshuffling then ptsname() is the
> wrong interface in the first place since it returns a pathname.

Good point.  Yes if we are not using devpts_mnt (hooray!) there should
be no complications, and the largest check we would need is to verify
that /dev/ptmx is in the current namespace.

> Fwiw, in klibc ptsname() is basically just an sprintf().

The challenge came in operations such as granpt.  Where you are passed
in a ptmx file descriptor from who knows where, and you pass it on
to applications such as pt_chown which run with elevatated privileged.

As the information is available of where devpts is mounted in
relationship to /dev/ptmx I have no more concerns about implementing
ptsname.  Path pased is also sufficiently backwards compatible it would
not usually be wrong even on existing kernels.

Good enough.

Eric