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

[Andy Lutomirski]

On Tue, Jan 5, 2016 at 5:17 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> Josh Boyer <jwboyer@xxxxxxxxxxxxxxxxx> writes:
>
>> On Sat, Dec 26, 2015 at 9:03 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Sat, Dec 26, 2015 at 1:51 PM, Serge E. Hallyn
>>> <serge.hallyn@xxxxxxxxxx> 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@xxxxxxxxx>
>>>>
>>>> Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxx>
>>>
>>> Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
>>>
>>> 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.
>

Even if we were to grant that the setns caller is buggy, this patch
seems like a good hardening measure.  Is there any case where ptrace
access *should* be granted but would not be granted with this patch
applied?

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

So what?

--Andy
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