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 10:32:39 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Jann Horn <jann@...jh.net>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Michal Hocko <mhocko@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-mm@...ck.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.

On Wed, Oct 19, 2016 at 10:29 AM, Jann Horn <jann@...jh.net> wrote:
> On Wed, Oct 19, 2016 at 11:52:50AM -0500, Eric W. Biederman wrote:
>> Andy Lutomirski <luto@...capital.net> writes:
>> > 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.
>
> Nope. check_unsafe_exec() sets LSM_UNSAFE_* flags in bprm->unsafe, and then
> the flags are checked by the LSMs and cap_bprm_set_creds() in commoncap.c.
> cap_bprm_set_creds() just degrades the execution to a non-setuid-ish one,
> and e.g. ptracers stay attached.

I think you're right.  I ought to be completely sure because I rewrote
that code back in 2005 or so back when I thought kernel programming
was only for the cool kids.  It was probably my first kernel patch
ever and it closed an awkward-to-exploit root hole.  But it's been a
while.  (Too bad my second (IIRC) kernel patch was more mundane and
fixed the mute button on "new" Lenovo X60-era laptops and spend
several years in limbo...)

--Andy