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

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Date:   Thu, 6 Apr 2023 17:58:59 -0400
From:   Stefan Berger <stefanb@...ux.ibm.com>
To:     Jeff Layton <jlayton@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Paul Moore <paul@...l-moore.com>
Cc:     zohar@...ux.ibm.com, linux-integrity@...r.kernel.org,
        miklos@...redi.hu, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-unionfs@...r.kernel.org,
        amir73il@...il.com
Subject: Re: [PATCH] overlayfs: Trigger file re-evaluation by IMA / EVM after
 writes



On 4/6/23 17:24, 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.

You mean compare known stat against current ? It seems more expensive to stat the file
rather than using the simple i_version-has-changed indicator.

> 
> That sort of design may also eventually help IMA work properly with more
> exotic filesystems, like NFS or Ceph.

And these don't support i_version at all?

    Stefan