linux-kernel - Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

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Date:	Fri, 08 Apr 2016 15:05:56 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.com>,
	Aurelien Jarno <aurelien@...el32.net>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security\@kernel.org" <security@...nel.org>,
	"security\@ubuntu.com \>\> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Fri, Apr 8, 2016 at 11:51 AM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>>
>> Given that concern under the rule we don't break userspace we have to
>> check the permissions of /dev/pts/ptmx when we are creating a new pty,
>> on a instance of devpts that was created with newinstance.
>
> The rule is that we don't break existing installations.
>
> If somebody has root and installs a "ptmx" node in an existing mount
> space next to a pts subdirectory, that's not a security issue, nor is
> it going to break any existing installation.

Anyone can do that with "mount --bind".  All it takes is root in a user
namespace.  I can get root in a user namespace as no one special.

So someone may have set such a thing up, and it may now be possible
to defeat such a regime as anyone.

In practice I suspect all such cases are handled by actually hiding the
mount of devpts in another mount namespace.

> The whole point of the patch is that yes, we change semantics. A
> change of semantics means that people will see situations where the
> behavior is different. But that's not "breaking user space", that's
> just "ok, you can see a difference".

If we don't want to care about this case, and if someone complains about
a security regression readd my permission checks I am fine with that.

But I don't want to let a possibility of breaking someone (that I don't
know how to test for, and would be silent breakage) slip through.

Eric