Re: [PATCH] ptrace: being capable wrt a process requires mapped uids/gids

[ebiederm@xmission.com (Eric W. Biederman)]

Josh Boyer <jwboyer@fedoraproject.org> writes:

> On Sat, Dec 26, 2015 at 9:03 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Sat, Dec 26, 2015 at 1:51 PM, Serge E. Hallyn
>> <serge.hallyn@ubuntu.com> wrote:
>>> On Sat, Dec 26, 2015 at 03:52:31AM +0100, Jann Horn wrote:
>>>> ptrace_has_cap() checks whether the current process should be
>>>> treated as having a certain capability for ptrace checks
>>>> against another process. Until now, this was equivalent to
>>>> has_ns_capability(current, target_ns, CAP_SYS_PTRACE).
>>>>
>>>> However, if a root-owned process wants to enter a user
>>>> namespace for some reason without knowing who owns it and
>>>> therefore can't change to the namespace owner's uid and gid
>>>> before entering, as soon as it has entered the namespace,
>>>> the namespace owner can attach to it via ptrace and thereby
>>>> gain access to its uid and gid.
>>>>
>>>> While it is possible for the entering process to switch to
>>>> the uid of a claimed namespace owner before entering,
>>>> causing the attempt to enter to fail if the claimed uid is
>>>> wrong, this doesn't solve the problem of determining an
>>>> appropriate gid.
>>>>
>>>> With this change, the entering process can first enter the
>>>> namespace and then safely inspect the namespace's
>>>> properties, e.g. through /proc/self/{uid_map,gid_map},
>>>> assuming that the namespace owner doesn't have access to
>>>> uid 0.
>>>>
>>>> Changed in v2: The caller needs to be capable in the
>>>> namespace into which tcred's uids/gids can be mapped.
>>>>
>>>> Signed-off-by: Jann Horn <jann@thejh.net>
>>>
>>> Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>
>>
>> Acked-by: Andy Lutomirski <luto@kernel.org>
>>
>> Who's going to apply this?  Linus?  Eric?
>
> An Ack from Oleg would be nice too.  I'm guessing this got lost in the
> holidays but it has an assigned CVE now.  Would be good to get it in
> 4.4 final.

If people are going to go around and refuse to understand the problem
and assign CVEs to the kernel when they can't understand what is
necessary to safely write code I am inclined to nack the entire mess.

Whatever (if anything) that is calling setns in this problematic way is
the problem today.

This thread is about a feature request to make it easier to write secure
code not about a vulnerability in user namespaces.

Eric