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

[Roberto Sassu]

On Mon, 2023-12-11 at 09:36 -0600, Seth Forshee wrote:
> On Mon, Dec 11, 2023 at 03:56:06PM +0100, Roberto Sassu wrote:
> > Ok, I will try.
> > 
> > I explain first how EVM works in general, and then why EVM does not
> > work with overlayfs.
> > 
> > EVM gets called before there is a set/removexattr operation, and after,
> > if that operation is successful. Before the set/removexattr operation
> > EVM calculates the HMAC on current inode metadata (i_ino, i_generation,
> > i_uid, i_gid, i_mode, POSIX ACLs, protected xattrs). Finally, it
> > compares the calculated HMAC with the one in security.evm.
> > 
> > If the verification and the set/removexattr operation are successful,
> > EVM calculates again the HMAC (in the post hooks) based on the updated
> > inode metadata, and sets security.evm with the new HMAC.
> > 
> > The problem is the combination of: overlayfs inodes have different
> > metadata than the lower/upper inodes; overlayfs calls the VFS to
> > set/remove xattrs.
> 
> I don't know all of the inner workings of overlayfs in detail, but is it
> not true that whatever metadata an overlayfs mount presents for a given
> inode is stored in the lower and/or upper filesystem inodes? If the
> metadata for those inodes is verified with EVM, why is it also necessary
> to verify the metadata at the overlayfs level? If some overlayfs
> metadata is currently omitted from the checks on the lower/upper inodes,
> is there any reason EVM couldn't start including that its checksums?

Currently, the metadata where there is a misalignment are:
i_generation, s_uuid, (i_ino?). Maybe there is more?

If metadata are aligned, there is no need to store two separate HMACs.

Thanks

Roberto

> Granted that there could be some backwards compatibility issues, but
> maybe inclusion of the overlayfs metadata could be opt-in.
> 
> Thanks,
> Seth