[lxc-devel] Kernel bug? Setuid apps and user namespaces

[Andy Lutomirski]

On Fri, Apr 11, 2014 at 3:46 PM, Serge E. Hallyn <serge at hallyn.com> wrote:
> Quoting Andy Lutomirski (luto at amacapital.net):
>> On Fri, Apr 11, 2014 at 3:29 PM, Serge E. Hallyn <serge at hallyn.com> wrote:
>> > Quoting Andy Lutomirski (luto at amacapital.net):
>> >> On Fri, Apr 11, 2014 at 2:52 PM, Serge E. Hallyn <serge at hallyn.com> wrote:
>> >> > Quoting Andy Lutomirski (luto at amacapital.net):
>> >> >> On Mon, Apr 7, 2014 at 11:13 AM, Serge E. Hallyn <serge at hallyn.com> wrote:
>> >> >> > Quoting Andy Lutomirski (luto at amacapital.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.
>

Then we need to keep this working.  I guess we can just keep the
dumpable bit around.

--Andy