linux-kernel - Re: [lxc-devel] Kernel bug? Setuid apps and user namespaces

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Date:	Sat, 12 Apr 2014 00:46:29 +0200
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Sean Pajot <sean.pajot@...culink.com>,
	lxc-devel@...ts.linuxcontainers.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [lxc-devel] Kernel bug? Setuid apps and user namespaces

Quoting Andy Lutomirski (luto@...capital.net):
> On Fri, Apr 11, 2014 at 3:29 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> > Quoting Andy Lutomirski (luto@...capital.net):
> >> On Fri, Apr 11, 2014 at 2:52 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> >> > Quoting Andy Lutomirski (luto@...capital.net):
> >> >> On Mon, Apr 7, 2014 at 11:13 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
> >> >> > Quoting Andy Lutomirski (luto@...capital.net):
> >> >> >> I'm starting to think that we need to extend dumpable to something
> >> >> >> much more general like a list of struct creds that someone needs to be
> >> >> >> able to ptrace, *in addition to current creds* in order to access
> >> >> >> sensitive /proc files, coredumps, etc.  If you get started as setuid,
> >> >> >
> >> >> > Hm, yeah, this sort of makes sense.
> >> >> >
> >> >> >> then you start with two struct creds in the list (or maybe just your
> >> >> >> euid and uid).  If you get started !setuid, then your initial creds
> >> >> >> are in the list.  It's possible that few or no things will need to
> >> >> >> change that list after execve.
> >> >> >>
> >> >> >> If all of the entries and current->cred are in the same user_ns, then
> >> >> >> we can dump as userns root.  If they're in different usernses, then we
> >> >> >> dump as global root or maybe the common ancestor root.
> >> >> >> setuid(getuid()) and other such nastiness may have to empty the list,
> >> >> >> or maybe we can just use a prctl for that.
> >> >> >
> >> >> > A few questions,
> >> >> >
> >> >> > 1. is there any other action which would trigger adding a new cred to
> >> >> > the ist?
> >> >>
> >> >> I don't think so.  Anyone who can ptrace you from the start can
> >> >> corrupt you such that you leak rights even if some future action
> >> >> prevents new ptracers from attaching.
> >> >>
> >> >> OTOH, it might be nice for something like an HTTPS server to be able
> >> >> to fork and shove its private key into the child, while preventing
> >> >> anyone from ptracing the child.  But doing this securely without help
> >> >> from someone with a different uid might be impossible anyway.
> >> >>
> >> >> >
> >> >> > 2. would execve clear (and re-init) the list of creds?
> >> >>
> >> >> Probably.  Thoughts?
> >> >
> >> > Yeah it seems to me it should be re-initialized, with a cred added
> >> > to the list for every open fd.
> >>
> >> What do you mean "every fd"?
> >>
> >> It seems odd to me that execve of anything that isn't setuid would add
> >> anything to the list -- attackers can always ptrace before the execve
> >> happens.
> >
> > Maybe you're right.  Maybe I shouldn't reason about this on a friday
> > afternoon.
> >
> > My *thought* was setuid-root program opens /etc/shadow, then execs a
> > regular program keeping that open.  Attaching to that fails now though,
> > presumably due to dumpable.
> >
> 
> Why would it fail?

I had expected it to succeed when I tried it, but it did in fact fail.

>   Isn't dumpable cleared on execve of a non-setuid
> program?  Maybe I need to look this stuff up again.

I guess this particular case was handled by setup_new_exec:

        if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), current_gid()))
                set_dumpable(current->mm, SUID_DUMP_USER);
        else
                set_dumpable(current->mm, suid_dumpable);

since my euid was 0 and uid 1000, when I did the exec.

-serge
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