Re: [RFC][PATCH] overlayfs: Redirect xattr ops on security.evm to security.evm_overlayfs

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On 12.12.23 11:44, Amir Goldstein wrote:
On Tue, Dec 12, 2023 at 12:25 PM Roberto Sassu
<roberto.sassu@xxxxxxxxxxxxxxx> wrote:

On 11.12.23 19:01, Christian Brauner wrote:
The second problem is that one security.evm is not enough. We need two,
to store the two different HMACs. And we need both at the same time,
since when overlayfs is mounted the lower/upper directories can be
still accessible.

"Changes to the underlying filesystems while part of a mounted overlay
filesystem are not allowed. If the underlying filesystem is changed, the
behavior of the overlay is undefined, though it will not result in a
crash or deadlock."

So I don't know why this would be a problem.

+ Eric Snowberg

Ok, that would reduce the surface of attack. However, when looking at:

       ovl: Always reevaluate the file signature for IMA

       Commit db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the
       partially closed an IMA integrity issue when directly modifying a file
       on the lower filesystem.  If the overlay file is first opened by a
       and later the lower backing file is modified by root, but the extended
       attribute is NOT updated, the signature validation succeeds with
the old
       original signature.

Ok, so if the behavior of overlayfs is undefined if the lower backing
file is modified by root, do we need to reevaluate? Or instead would be
better to forbid the write from IMA (legitimate, I think, since the
behavior is documented)? I just saw that we have d_real_inode(), we can
use it to determine if the write should be denied.

There may be several possible legitimate actions in this case, but the
overall concept IMO should be the same as I said about EVM -
overlayfs does not need an IMA signature of its own, because it
can use the IMA signature of the underlying file.

Whether overlayfs reads a file from lower fs or upper fs, it does not
matter, the only thing that matters is that the underlying file content
is attested when needed.

The only incident that requires special attention is copy-up.
This is what the security hooks security_inode_copy_up() and
security_inode_copy_up_xattr() are for.

When a file starts in state "lower" and has security.ima,evm xattrs
then before a user changes the file, it is copied up to upper fs
and suppose that security.ima,evm xattrs are copied as is?

When later the overlayfs file content is read from the upper copy
the security.ima signature should be enough to attest that file content
was not tampered with between going from "lower" to "upper".

security.evm may need to be fixed on copy up, but that should be
easy to do with the security_inode_copy_up_xattr() hook. No?

It is not yet clear to me. EVM will be seeing the creation of a new file, and for new files setting xattrs is already allowed.

Maybe the security_inode_copy_up*() would be useful for IMA/EVM to authorize writes by overlayfs, which would be otherwise denied to the others (according to my solution).

Still, would like to hear Mimi's opinion.



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