Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

[Marian Marinov <mm@1h.com>]

On 04/29/2014 09:52 PM, Serge Hallyn wrote:
> Quoting Theodore Ts'o (tytso@mit.edu):
>> 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.

Marian

>> No?  Otherwise, pretty much every single use of capable() would be
>> broken, not just this once instances in ext4/ioctl.c.
>>
>> 					- Ted
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>


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Marian Marinov
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