Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

Marian Marinov
2014-04-30T00:11:06Z

On 04/30/2014 03:01 AM, Stéphane Graber wrote:
> On Tue, Apr 29, 2014 at 04:51:54PM -0700, Andy Lutomirski wrote:
>> On Tue, Apr 29, 2014 at 4:47 PM, Stéphane Graber <stgraber@ubuntu.com> wrote:
>>> On Tue, Apr 29, 2014 at 04:22:55PM -0700, Andy Lutomirski wrote:
>>>> On Tue, Apr 29, 2014 at 4:20 PM, Marian Marinov <mm@1h.com> wrote:
>>>>> On 04/30/2014 01:45 AM, Andy Lutomirski wrote:
>>>>>>
>>>>>> On 04/29/2014 03:29 PM, Serge Hallyn wrote:
>>>>>>>
>>>>>>> Quoting Marian Marinov (mm-108MBtLGafw@public.gmane.org):
>>>>>>>>
>>>>>>>> On 04/30/2014 01:02 AM, Serge Hallyn wrote:
>>>>>>>>>
>>>>>>>>> Quoting Marian Marinov (mm-108MBtLGafw@public.gmane.org):
>>>>>>>>>>
>>>>>>>>>> On 04/29/2014 09:52 PM, Serge Hallyn wrote:
>>>>>>>>>>>
>>>>>>>>>>> Quoting Theodore Ts'o (tytso-3s7WtUTddSA@public.gmane.org):
>>>>>>>>>>>>
>>>>>>>>>>>> On Tue, Apr 29, 2014 at 04:49:14PM +0300, Marian Marinov wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> I'm proposing a fix to this, by replacing the
>>>>>>>>>>>>> capable(CAP_LINUX_IMMUTABLE)
>>>>>>>>>>>>> check with ns_capable(current_cred()->user_ns,
>>>>>>>>>>>>> CAP_LINUX_IMMUTABLE).
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Um, wouldn't it be better to simply fix the capable() function?
>>>>>>>>>>>>
>>>>>>>>>>>> /**
>>>>>>>>>>>> * capable - Determine if the current task has a superior
>>>>>>>>>>>> capability in effect
>>>>>>>>>>>> * @cap: The capability to be tested for
>>>>>>>>>>>> *
>>>>>>>>>>>> * Return true if the current task has the given superior
>>>>>>>>>>>> capability currently
>>>>>>>>>>>> * available for use, false if not.
>>>>>>>>>>>> *
>>>>>>>>>>>> * This sets PF_SUPERPRIV on the task if the capability is
>>>>>>>>>>>> available on the
>>>>>>>>>>>> * assumption that it's about to be used.
>>>>>>>>>>>> */
>>>>>>>>>>>> bool capable(int cap)
>>>>>>>>>>>> {
>>>>>>>>>>>> return ns_capable(&init_user_ns, cap);
>>>>>>>>>>>> }
>>>>>>>>>>>> EXPORT_SYMBOL(capable);
>>>>>>>>>>>>
>>>>>>>>>>>> The documentation states that it is for "the current task", and I
>>>>>>>>>>>> can't imagine any use case, where user namespaces are in effect,
>>>>>>>>>>>> where
>>>>>>>>>>>> using init_user_ns would ever make sense.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> the init_user_ns represents the user_ns owning the object, not the
>>>>>>>>>>> subject.
>>>>>>>>>>>
>>>>>>>>>>> The patch by Marian is wrong. Anyone can do 'clone(CLONE_NEWUSER)',
>>>>>>>>>>> setuid(0), execve, and end up satisfying
>>>>>>>>>>> 'ns_capable(current_cred()->userns,
>>>>>>>>>>> CAP_SYS_IMMUTABLE)' by definition.
>>>>>>>>>>>
>>>>>>>>>>> So NACK to that particular patch. I'm not sure, but IIUC it should
>>>>>>>>>>> be
>>>>>>>>>>> safe to check against the userns owning the inode?
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> So what you are proposing is to replace
>>>>>>>>>> 'ns_capable(current_cred()->userns, CAP_SYS_IMMUTABLE)' with
>>>>>>>>>> 'inode_capable(inode, CAP_SYS_IMMUTABLE)' ?
>>>>>>>>>>
>>>>>>>>>> I agree that this is more sane.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Right, and I think the two operations you're looking at seem sane
>>>>>>>>> to allow.
>>>>>>>>
>>>>>>>>
>>>>>>>> If you are ok with this patch, I will fix all file systems and send
>>>>>>>> patches.
>>>>>>>
>>>>>>>
>>>>>>> Sounds good, thanks.
>>>>>>>
>>>>>>>> Signed-off-by: Marian Marinov <mm-NV7Lj0SOnH0@public.gmane.org>
>>>>>>>
>>>>>>>
>>>>>>> Acked-by: Serge E. Hallyn
>>>>>>> <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>
>>>>>>
>>>>>>
>>>>>> Wait, what?
>>>>>>
>>>>>> Inodes aren't owned by user namespaces; they're owned by users. And any
>>>>>> user can arrange to have a user namespace in which they pass an
>>>>>> inode_capable check on any inode that they own.
>>>>>>
>>>>>> Presumably there's a reason that CAP_SYS_IMMUTABLE is needed. If this
>>>>>> gets merged, then it would be better to just drop CAP_SYS_IMMUTABLE
>>>>>> entirely.
>>>>>
>>>>>
>>>>> The problem I'm trying to solve is this:
>>>>>
>>>>> container with its own user namespace and CAP_SYS_IMMUTABLE should be able
>>>>> to use chattr on all files witch this container has access to.
>>>>>
>>>>> Unfortunately with the capable(CAP_SYS_IMMUTABLE) check this is not working.
>>>>>
>>>>> With the proposed two fixes CAP_SYS_IMMUTABLE started working in the
>>>>> container.
>>>>>
>>>>> The first solution got its user namespace from the currently running process
>>>>> and the second gets its user namespace from the currently opened inode.
>>>>>
>>>>> So what would be the best solution in this case?
>>>>
>>>> I'd suggest adding a mount option like fs_owner_uid that names a uid
>>>> that owns, in the sense of having unlimited access to, a filesystem.
>>>> Then anyone with caps on a namespace owned by that uid could do
>>>> whatever.
>>>>
>>>> Eric?
>>>>
>>>> --Andy
>>>
>>> The most obvious problem I can think of with "do whatever" is that this
>>> will likely include mknod of char and block devices which you can then
>>> chown/chmod as you wish and use to access any devices on the system from
>>> an unprivileged container.
>>> This can however be mitigated by using the devices cgroup controller.
>>
>> Or 'nodev'. setuid/setgid may have the same problem, too.
>>
>> Implementing something like this would also make CAP_DAC_READ_SEARCH
>> and CAP_DAC_OVERRIDE work.
>>
>> Arguably it should be impossible to mount such a thing in the first
>> place without global privilege.
>>
>>>
>>> You also probably wouldn't want any unprivileged user from the host to
>>> find a way to access that mounted filesytem but so long as you do the
>>> mount in a separate mountns and don't share uids between the host and
>>> the container, that should be fine too.
>>
>> This part should be a nonissue -- an unprivileged user who has the
>> right uid owns the namespace anyway, so this is the least of your
>> worries.
>>
>> --Andy
>
> It should be a nonissue so long as we make sure that a file owned by a
> uid outside the scope of the container may not be changed even though
> fs_owner_uid is set. Otherwise, it's just a matter of chmod +S on say
> a shell and anyone who can see the fs from the host will be getting a
> root shell (assuming said file is owned by the host's uid 0).
>
> So that's restricting slightly what "do whatever" would do in this case.
>

In my case I give an LVM volume to each container and limit the container to only this block device using the devices
cgroup.
So the inode_capable() fix worked like a charm for me.
The container can not see any filesystem other then its own.
And I have another patch for my kernel that prohibits setns from cgroup other then / which prevents programs from the
container from getting out. clone() can be used to create new namespaces but can not be used to attach to already
created namespaces.

Marian

--
Marian Marinov
Founder & CEO of 1H Ltd.
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