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

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Message-ID: <20161019172917.GE1210@laptop.thejh.net>
Date:   Wed, 19 Oct 2016 19:29:17 +0200
From:   Jann Horn <jann@...jh.net>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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 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.

Same thing happens if the fs struct is shared with another process or if
NO_NEW_PRIVS is active.

(Actually, it's still a bit like normal setuid execution: IIRC AT_SECURE
stays active, and the resulting process still won't be dumpable, so it's
not possible for a *new* ptracer to attach afterwards. But this is just
from memory, I'm not entirely sure.)