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

[Amir Goldstein]

On Fri, Dec 8, 2023 at 7:25 PM Roberto Sassu
<roberto.sassu at huaweicloud.com> wrote:
>
> From: Roberto Sassu <roberto.sassu at huawei.com>
>
> EVM updates the HMAC in security.evm whenever there is a setxattr or
> removexattr operation on one of its protected xattrs (e.g. security.ima).
>
> Unfortunately, since overlayfs redirects those xattrs operations on the
> lower filesystem, the EVM HMAC cannot be calculated reliably, since lower
> inode attributes on which the HMAC is calculated are different from upper
> inode attributes (for example i_generation and s_uuid).
>
> Although maybe it is possible to align such attributes between the lower
> and the upper inode, another idea is to map security.evm to another name
> (security.evm_overlayfs)

If we were to accept this solution, this will need to be trusted.overlay.evm
to properly support private overlay xattr escaping.

> during an xattr operation, so that it does not
> collide with security.evm set by the lower filesystem.

You are using wrong terminology and it is very confusing to me.
see the overlay mount command has lowerdir= and upperdir=.
Seems that you are using lower filesystem to refer to the upper fs
and upper filesystem to refer to overlayfs.

>
> Whenever overlayfs wants to set security.evm, it is actually setting
> security.evm_overlayfs calculated with the upper inode attributes. The
> lower filesystem continues to update security.evm.
>

I understand why that works, but I am having a hard time swallowing
the solution, mainly because I feel that there are other issues on the
intersection of overlayfs and IMA and I don't feel confident that this
addresses them all.

If you want to try to convince me, please try to write a complete
model of how IMA/EVM works with overlayfs, using the section
"Permission model" in Documentation/filesystems/overlayfs.rst
as a reference.

Thanks,
Amir.