Re: [RFC 20/20] ima: Setup securityfs_ns for IMA namespace

Stefan Berger
2021-12-01T22:09:18Z

On 12/1/21 17:01, James Bottomley wrote:
> On Wed, 2021-12-01 at 16:34 -0500, Stefan Berger wrote:
>> On 12/1/21 16:11, James Bottomley wrote:
>>> On Wed, 2021-12-01 at 15:25 -0500, Stefan Berger wrote:
>>>> On 12/1/21 14:21, James Bottomley wrote:
>>>>> On Wed, 2021-12-01 at 13:11 -0500, Stefan Berger wrote:
>>>>>> On 12/1/21 12:56, James Bottomley wrote:
>>>>> [...]
>>>>>> I tried this with runc and a user namespace active mapping
>>>>>> uid
>>>>>> 1000 on the host to uid 0 in the container. There I run into
>>>>>> the
>>>>>> problem that all of the files and directories without the
>>>>>> above
>>>>>> work-around are mapped to 'nobody', just like all the files
>>>>>> in
>>>>>> sysfs in this case are also mapped to nobody. This code
>>>>>> resolved
>>>>>> the issue.
>>>>> So I applied your patches with the permission shift commented
>>>>> out
>>>>> and instrumented inode_alloc() to see where it might be failing
>>>>> and
>>>>> I actually find it all works as expected for me:
>>>>>
>>>>> ejb@testdeb:~> unshare -r --user --mount --ima
>>>>> root@testdeb:~# mount -t securityfs_ns none
>>>>> /sys/kernel/security
>>>>> root@testdeb:~# ls -l /sys/kernel/security/ima/
>>>>> total 0
>>>>> -r--r----- 1 root root 0 Dec 1 19:11
>>>>> ascii_runtime_measurements
>>>>> -r--r----- 1 root root 0 Dec 1 19:11
>>>>> binary_runtime_measurements
>>>>> -rw------- 1 root root 0 Dec 1 19:11 policy
>>>>> -r--r----- 1 root root 0 Dec 1 19:11
>>>>> runtime_measurements_count
>>>>> -r--r----- 1 root root 0 Dec 1 19:11 violations
>>>>>
>>>>> I think your problem is something to do with how runc is
>>>>> installing
>>>>> the uid/gid mappings. If it's installing them after the
>>>>> security_ns inodes are created then they get the -1 value
>>>>> (because
>>>>> no mappings exist in s_user_ns). I can even demonstrate this
>>>>> by
>>>>> forcing unshare to enter the IMA namespace before writing the
>>>>> mapping values and I'll see "nobody nogroup" above like you do.
>>>> I am surprised you get this mapping even after commenting the
>>>> permission adjustments... it doesn't work for me when I comment
>>>> them
>>>> out:
>>>>
>>>> [stefanb@ima-ns-dev rootfs]$ unshare -r --user --mount
>>>> [root@ima-ns-dev rootfs]# mount -t securityfs_ns none
>>>> /sys/kernel/security/
>>>> [root@ima-ns-dev rootfs]# cd /sys/kernel/security/ima/
>>>> [root@ima-ns-dev ima]# ls -l
>>>> total 0
>>>> -r--r-----. 1 nobody nobody 0 Dec 1 15:20
>>>> ascii_runtime_measurements
>>>> -r--r-----. 1 nobody nobody 0 Dec 1 15:20
>>>> binary_runtime_measurements
>>>> -rw-------. 1 nobody nobody 0 Dec 1 15:20 policy
>>>> -r--r-----. 1 nobody nobody 0 Dec 1 15:20
>>>> runtime_measurements_count
>>>> -r--r-----. 1 nobody nobody 0 Dec 1 15:20 violations
>>>> [root@ima-ns-dev ima]# cat /proc/self/uid_map
>>>> 0 1000 1
>>>> [root@ima-ns-dev ima]# cat /proc/self/gid_map
>>>> 0 1000 1
>>>>
>>>> The initialization of securityfs and setup of files and
>>>> directories
>>>> happens at the same time as the IMA namespace is created. At this
>>>> time there are no user mappings available, so that's why I need
>>>> to
>>>> make the adjustments 'late'.
>>> There is one other possible difference: To get the correct
>>> s_user_ns
>> I am currently wondering why I cannot re-create your setup while
>> disabling the remapping...
> OK, I think I figured it out. When I applied your patches, it was on
> top of my existing ones, so I had to massage them a bit.
>
> Your problem is the securityfs inode creation is triggered inside
> create_user_ns, which means it happens *before* ushare writes to the
> proc/self/uid_map file, so the securityfs_inodes are always created on
> an empty mapping and i_write_uid always sets the inode uid to -1.

Right, the initialization of the filesystem is quite early.

>
> I don't see this because my setup for everything is triggered off the
> first use of the IMA namespace. You'd need to have some type of lazy
> setup of the inodes as well to give unshare time to install the uid/gid
> mappings.

What could trigger that? A callback while mounting - but I am not sure
where to hook into then. What is your mechanisms to trigger as the
'first use of the IMA namespace'? What is 'use' here?

Stefan

>
> James
>
>