From: ebiederm@xmission.com (Eric W. Biederman)
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Peter Hurley <peter@hurleysoftware.com>,
        Greg KH <greg@kroah.com>, Jiri Slaby <jslaby@suse.com>,
        Aurelien Jarno <aurelien@aurel32.net>,
        Andy Lutomirski <luto@amacapital.net>,
        Florian Weimer <fw@deneb.enyo.de>, Al Viro <viro@zeniv.linux.org.uk>,
        Serge Hallyn <serge.hallyn@ubuntu.com>, Jann Horn <jann@thejh.net>,
        "security\@kernel.org" <security@kernel.org>,
        "security\@ubuntu.com \>\> security" <security@ubuntu.com>,
        security@debian.org, Willy Tarreau <w@1wt.eu>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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	<CA+55aFwv7d8XfeN8KvYaVcSs7V9XoHtS-1cqtkG4gF4yJwB5nA@mail.gmail.com>
	<87twjcorwg.fsf@x220.int.ebiederm.org>
	<CA+55aFzCQ5MEs0C21SSobgpx-zGKxK67ur1Q45s+wn-nWZ56nw@mail.gmail.com>
Date: Fri, 08 Apr 2016 15:05:56 -0500
In-Reply-To: <CA+55aFzCQ5MEs0C21SSobgpx-zGKxK67ur1Q45s+wn-nWZ56nw@mail.gmail.com>
	(Linus Torvalds's message of "Fri, 8 Apr 2016 12:05:56 -0700")
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Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup
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Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Fri, Apr 8, 2016 at 11:51 AM, Eric W. Biederman
> <ebiederm@xmission.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