Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

Andy Lutomirski
2014-04-29T22:45:28Z

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.

Nacked-by: Andy Lutomirski <luto@amacapital.net>