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

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Date:   Thu, 06 Apr 2023 18:09:41 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Stefan Berger <stefanb@...ux.ibm.com>,
        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 Thu, 2023-04-06 at 17:58 -0400, Stefan Berger wrote:
> 
> 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.
> 

getattr is fairly cheap on a local filesystem. It's more expensive with
something networked, but that's the price of correctness.

> > 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?

They absolutely do. Their change attributes are mediated by the server,
so they can't use the kernel's mechanism for IS_I_VERSION inodes. They
can report that field in their ->getattr routines however.

-- 
Jeff Layton <jlayton@...nel.org>