linux-kernel - Re: [PATCH 4/5] evm: Use the real inode's metadata to calculate metadata hash

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Date: Fri, 2 Feb 2024 17:51:40 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Stefan Berger <stefanb@...ux.ibm.com>
Cc: linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org, 
	linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com, zohar@...ux.ibm.com, 
	roberto.sassu@...wei.com, miklos@...redi.hu
Subject: Re: [PATCH 4/5] evm: Use the real inode's metadata to calculate
 metadata hash

On Fri, Feb 2, 2024 at 4:59 PM Stefan Berger <stefanb@...ux.ibm.com> wrote:
>
>
>
> On 2/2/24 04:24, Amir Goldstein wrote:
> > On Thu, Feb 1, 2024 at 10:35 PM Stefan Berger <stefanb@...ux.ibm.com> wrote:
>
> >
> >>
> >> and your suggested change to this patch :
> >>
> >> -       struct inode *inode = d_real_inode(dentry);
> >> +       struct inode *inode = d_inode(d_real(dentry, false));;
> >>
> >
> > In the new version I change the API to use an enum instead of bool, e.g:
> >
> >         struct inode *inode = d_inode(d_real(dentry, D_REAL_METADATA));
>
> Thanks. I will use it.
>
> >
> > This catches in build time and in run time, callers that were not converted
> > to the new API.
> >
> >> The test cases are now passing with and without metacopy enabled. Yay!
> >
> > Too soon to be happy.
> > I guess you are missing a test for the following case:
> > 1. file was meta copied up (change is detected)
> > 2. the lower file that contains the data is being changed (change is
> > not detected)
>
> Right. Though it seems there's something wrong with overlayfs as well
> after appending a byte to the file on the lower.
>
> -rwxr-xr-x    1 0        0               25 Feb  2 14:55
> /ext4.mount/lower/test_rsa_portable2
> -rwxr-xr-x    1 0        0               24 Feb  2 14:55
> /ext4.mount/overlay/test_rsa_portable2
> bb16aa5350bcc8863da1a873c846fec9281842d9
> /ext4.mount/lower/test_rsa_portable2
> bb16aa5350bcc8863da1a873c846fec9281842d9
> /ext4.mount/overlay/test_rsa_portable2
>
> We have a hash collision on a file with 24 bytes and the underlying one
> with 25 byte. (-;  :-)

https://docs.kernel.org/filesystems/overlayfs.html#changes-to-underlying-filesystems

If you modify the lower file underneath overlayfs, you get no
guarantee from overlayfs about expected results.

This makes your work more challenging.

Thanks,
Amir.