linux-kernel - Re: [REVIEW][PATCH] exec: Don't exec files the userns root can not read.

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Date:   Wed, 19 Oct 2016 11:52:50 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Jann Horn <jann@...jh.net>, Michal Hocko <mhocko@...nel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-mm\@kvack.org" <linux-mm@...ck.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [REVIEW][PATCH] exec: Don't exec files the userns root can not read.

Andy Lutomirski <luto@...capital.net> writes:

> On Tue, Oct 18, 2016 at 2:15 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>>
>> When the user namespace support was merged the need to prevent
>> ptracing an executable that is not readable was overlooked.
>
> Before getting too excited about this fix, isn't there a much bigger
> hole that's been there forever?

In this case it was a newish hole (2011) that the user namespace support
added that I am closing.  I am not super excited but I figure it is
useful to make the kernel semantics at least as secure as they were
before.

> Simply ptrace yourself, exec the
> program, and then dump the program out.  A program that really wants
> to be unreadable should have a stub: the stub is setuid and readable,
> but all the stub does is to exec the real program, and the real
> program should have mode 0500 or similar.
>
> ISTM the "right" check would be to enforce that the program's new
> creds can read the program, but that will break backwards
> compatibility.

Last I looked I had the impression that exec of a setuid program kills
the ptrace.

If we are talking about a exec of a simple unreadable executable (aka
something that sets undumpable but is not setuid or setgid).  Then I
agree it should break the ptrace as well and since those programs are as
rare as hens teeth I don't see any problem with changing the ptrace behavior
in that case.

Eric