Re: [PATCH 1/1 V2] mqueue: Implment generic xattr support

[Paul Moore]

On Mon, Dec 5, 2016 at 11:15 AM, David Graziano
<david.graziano@xxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Dec 5, 2016 at 9:37 AM, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>> On Mon, Nov 28, 2016 at 3:04 PM, David Graziano
>> <david.graziano@xxxxxxxxxxxxxxxxxxx> wrote:
>>> On Wed, Nov 9, 2016 at 4:25 PM, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>>>> On Wed, Nov 9, 2016 at 11:25 AM, David Graziano
>>>> <david.graziano@xxxxxxxxxxxxxxxxxxx> wrote:
>>>>> On Mon, Nov 7, 2016 at 4:23 PM, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>>>>>> On Mon, Nov 7, 2016 at 3:46 PM, David Graziano
>>>>>> <david.graziano@xxxxxxxxxxxxxxxxxxx> wrote:
>>>>>>> This patch adds support for generic extended attributes within the
>>>>>>> POSIX message queues filesystem and setting them by consulting the LSM.
>>>>>>> This is needed so that the security.selinux extended attribute can be
>>>>>>> set via a SELinux named type transition on file inodes created within
>>>>>>> the filesystem. The implementation and LSM call back function are based
>>>>>>> off tmpfs/shmem.
>>>>>>>
>>>>>>> Signed-off-by: David Graziano <david.graziano@xxxxxxxxxxxxxxxxxxx>
>>>>>>> ---
>>>>>>>  ipc/mqueue.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>>  1 file changed, 46 insertions(+)
>>>>>>
>>>>>> Hi David,
>>>>>>
>>>>>> At first glance this looks reasonable to me, I just have a two
>>>>>> questions/comments:
>>>>>>
>>>>>> * Can you explain your current need for this functionality?  For
>>>>>> example, what are you trying to do that is made easier by allowing
>>>>>> greater message queue labeling flexibility?  This helps put things in
>>>>>> context and helps people review and comment on your patch.
>>>>>>
>>>>>> * How have you tested this?  While this patch is not SELinux specific,
>>>>>> I think adding a test to the selinux-testsuite[1] would be worthwhile.
>>>>>> The other LSM maintainers may suggest something similar if they have
>>>>>> an established public testsuite.
>>>>>>
>>>>>> [1] https://github.com/SELinuxProject/selinux-testsuite
>>>>>
>>>>> Hi Paul,
>>>>>
>>>>> I needed to write a selinux policy for a set of custom applications that use
>>>>> POSIX message queues for their IPC. The queues are created by one
>>>>> application and we needed a way for selinux to enforce which of the other
>>>>> apps are able to read/write to each individual queue. Uniquely labeling them
>>>>> based on the app that created them and the file name seemed to be our best
>>>>> solution since itâs an embedded system and we donât have restorecond to
>>>>> handle any relabeling.
>>>>
>>>> In the future putting things like the above in the patch description
>>>> can be helpful.  In other words, instead of simply saying this allows
>>>> you to better control the labels assigned to message queues, you could
>>>> expand upon it by saying that this patch allows you to better control
>>>> which applications have access to a given queue.  Yes, I realize that
>>>> is implied by better control over the labels, but being explicit is
>>>> rarely a bad thing when it comes to patch descriptions.
>>>>
>>>> I've never rejected a patch for a description that was too lengthy,
>>>> but I have rejected patches that need better descriptions ;)
>>>>
>>>>> To test this patch I used both a selinux enabled, buildroot based qemu target
>>>>> with a customized selinux policy and test C app to create the mqueues. I also
>>>>> tested with our real apps and selinux policy on our target hardware. I can
>>>>> certainly look at adding a test to the selinux-testsuite if that would
>>>>> be helpful.
>>>>
>>>> Please do.  I've been requiring tests for all new SELinux
>>>> functionality lately; this isn't strictly a SELinux patch but I think
>>>> it is a good practice regardless.
>>>
>>> Sorry for the delay. I have created a pull request within the
>>> selinux-testsuite github
>>> project with a set of mqueue tests.
>>
>> For anyone who is curious:
>>
>> * https://github.com/SELinuxProject/selinux-testsuite/pull/10
>>
>> Aside from a naming nit, the tests look good to me and I have no
>> problem with the kernel patch; it doesn't appear any of the other LSM
>> maintainers do either.  I'm happy to pull this into the SELinux tree
>> (for v4.11, it's a little late for v4.10 I think), but I think
>> Christoph made a good point about consolidation, have you had a chance
>> to look at that?
>
> I've made the update for the naming nit in the pull request.

I saw that, thanks.

> I agree with Christoph's point but doing so is a bit outside my expertise at
> this point. I would be open to suggestions as to where the function should be
> consolidated and work on a second patchset with the update. Maybe in
> fs/xattr.c as a simple_xattr_initxattrs function?

That seems to make the most sense, doesn't it?  Looking at
{shmem,mqueue}_initxattrs() the only fs specific bit is the
{shmem,mqueue}_inode_info struct pointer; considering that the fs_info
parameter is currently unused in this case, you could pass a reference
to the simple_xattr struct via the fs_info parameter.

I'd CC Andreas Gruenbacher <agruenba@xxxxxxxxxx> on the patch(set),
he's recently done a bunch of work around xattrs and the LSM, he may
have some additional thoughts.

-- 
paul moore
www.paul-moore.com