Re: [PATCH] overlayfs: Trigger file re-evaluation by IMA / EVM after writes

Stefan Berger
2023-04-06T22:27:27Z

On 4/6/23 18:04, Jeff Layton wrote:
> On Thu, 2023-04-06 at 17:24 -0400, Jeff Layton wrote:
>> On Thu, 2023-04-06 at 16:22 -0400, Stefan Berger wrote:
>>>
>>> On 4/6/23 15:37, Jeff Layton wrote:
>>>> On Thu, 2023-04-06 at 15:11 -0400, Stefan Berger wrote:
>>>>>
>>>>> On 4/6/23 14:46, Jeff Layton wrote:
>>>>>> On Thu, 2023-04-06 at 17:01 +0200, Christian Brauner wrote:
>>>>>>> On Thu, Apr 06, 2023 at 10:36:41AM -0400, Paul Moore wrote:
>>>>>
>>>>>>
>>>>>> Correct. As long as IMA is also measuring the upper inode then it seems
>>>>>> like you shouldn't need to do anything special here.
>>>>>
>>>>> Unfortunately IMA does not notice the changes. With the patch provided in the other email IMA works as expected.
>>>>>
>>>>
>>>>
>>>> It looks like remeasurement is usually done in ima_check_last_writer.
>>>> That gets called from __fput which is called when we're releasing the
>>>> last reference to the struct file.
>>>>
>>>> You've hooked into the ->release op, which gets called whenever
>>>> filp_close is called, which happens when we're disassociating the file
>>>> from the file descriptor table.
>>>>
>>>> So...I don't get it. Is ima_file_free not getting called on your file
>>>> for some reason when you go to close it? It seems like that should be
>>>> handling this.
>>>
>>> I would ditch the original proposal in favor of this 2-line patch shown here:
>>>
>>> https://lore.kernel.org/linux-integrity/a95f62ed-8b8a-38e5-e468-ecbde3b221af@linux.ibm.com/T/#m3bd047c6e5c8200df1d273c0ad551c645dd43232
>>>
>>>
>>
>> Ok, I think I get it. IMA is trying to use the i_version from the
>> overlayfs inode.
>>
>> I suspect that the real problem here is that IMA is just doing a bare
>> inode_query_iversion. Really, we ought to make IMA call
>> vfs_getattr_nosec (or something like it) to query the getattr routine in
>> the upper layer. Then overlayfs could just propagate the results from
>> the upper layer in its response.
>>
>> That sort of design may also eventually help IMA work properly with more
>> exotic filesystems, like NFS or Ceph.
>>
>>
>>
>
> Maybe something like this? It builds for me but I haven't tested it. It
> looks like overlayfs already should report the upper layer's i_version
> in getattr, though I haven't tested that either:

Thank you! I will give it a try once I am back.

Stefan