linux-kernel - Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

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Date:	Wed, 30 Apr 2014 02:20:09 +0300
From:	Marian Marinov <mm@...com>
To:	Andy Lutomirski <luto@...capital.net>,
	Serge Hallyn <serge.hallyn@...ntu.com>
CC:	containers@...ts.linux-foundation.org, Ted Ts'o <tytso@....edu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	lxc-devel <lxc-devel@...ts.sourceforge.net>
Subject: Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

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@...lic.gmane.org):
>>> On 04/30/2014 01:02 AM, Serge Hallyn wrote:
>>>> Quoting Marian Marinov (mm-108MBtLGafw@...lic.gmane.org):
>>>>> On 04/29/2014 09:52 PM, Serge Hallyn wrote:
>>>>>> Quoting Theodore Ts'o (tytso-3s7WtUTddSA@...lic.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@...lic.gmane.org>
>>
>> Acked-by: Serge E. Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@...lic.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?

Marian

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


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