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

[Andy Lutomirski]

On Fri, Apr 11, 2014 at 3:29 PM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
>> On Fri, Apr 11, 2014 at 2:52 PM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
>> > Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
>> >> On Mon, Apr 7, 2014 at 11:13 AM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
>> >> > Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
>> >> >> 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?  Isn't dumpable cleared on execve of a non-setuid
program?  Maybe I need to look this stuff up again.

--Andy
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