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

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Date:   Fri, 07 Apr 2023 09:29:29 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Christian Brauner <brauner@...nel.org>,
        Amir Goldstein <amir73il@...il.com>
Cc:     Stefan Berger <stefanb@...ux.ibm.com>,
        Paul Moore <paul@...l-moore.com>, 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
Subject: Re: [PATCH] overlayfs: Trigger file re-evaluation by IMA / EVM
 after writes

> > > > 
> > > > 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
> 
> We should cool it with the quick hacks to fix things. :)
> 

Yeah. It might fix this specific testcase, but I think the way it uses
the i_version is "gameable" in other situations. Then again, I don't
know a lot about IMA in this regard.

When is it expected to remeasure? If it's only expected to remeasure on
a close(), then that's one thing. That would be a weird design though.

> > > > 
> > > > 
> > > 
> > > 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:
> > 
> > -----------------------8<---------------------------
> > 
> > [PATCH] IMA: use vfs_getattr_nosec to get the i_version
> > 
> > IMA currently accesses the i_version out of the inode directly when it
> > does a measurement. This is fine for most simple filesystems, but can be
> > problematic with more complex setups (e.g. overlayfs).
> > 
> > Make IMA instead call vfs_getattr_nosec to get this info. This allows
> > the filesystem to determine whether and how to report the i_version, and
> > should allow IMA to work properly with a broader class of filesystems in
> > the future.
> > 
> > Reported-by: Stefan Berger <stefanb@...ux.ibm.com>
> > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > ---
> 
> So, I think we want both; we want the ovl_copyattr() and the
> vfs_getattr_nosec() change:
> 
> (1) overlayfs should copy up the inode version in ovl_copyattr(). That
>     is in line what we do with all other inode attributes. IOW, the
>     overlayfs inode's i_version counter should aim to mirror the
>     relevant layer's i_version counter. I wouldn't know why that
>     shouldn't be the case. Asking the other way around there doesn't
>     seem to be any use for overlayfs inodes to have an i_version that
>     isn't just mirroring the relevant layer's i_version.

It's less than ideal to do this IMO, particularly with an IS_I_VERSION
inode.

You can't just copy up the value from the upper. You'll need to call
inode_query_iversion(upper_inode), which will flag the upper inode for a
logged i_version update on the next write. IOW, this could create some
(probably minor) metadata write amplification in the upper layer inode
with IS_I_VERSION inodes.


> (2) Jeff's changes for ima to make it rely on vfs_getattr_nosec().
>     Currently, ima assumes that it will get the correct i_version from
>     an inode but that just doesn't hold for stacking filesystem.
> 
> While (1) would likely just fix the immediate bug (2) is correct and
> _robust_. If we change how attributes are handled vfs_*() helpers will
> get updated and ima with it. Poking at raw inodes without using
> appropriate helpers is much more likely to get ima into trouble.

This will fix it the right way, I think (assuming it actually works),
and should open the door for IMA to work properly with networked
filesystems that support i_version as well.

Note that there Stephen is correct that calling getattr is probably
going to be less efficient here since we're going to end up calling
generic_fillattr unnecessarily, but I still think it's the right thing
to do.

If it turns out to cause measurable performance regressions though,
maybe we can look at adding a something that still calls ->getattr if it
exists but only returns the change_cookie value.
-- 
Jeff Layton <jlayton@...nel.org>