Re: [PATCH v3 0/6] Composefs: an opportunistically sharing verified image filesystem

[Amir Goldstein]

On Mon, Feb 6, 2023 at 9:32 PM Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>
> On Mon, 6 Feb 2023 at 18:16, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
> > it's not overlay{erofs+erofs}
> > it's overlay{erofs+ext4} (or another fs-verity [1] supporting fs)
> > the lower layer is a mutable fs with /objects/ dir containing
> > the blobs.
> >
> > The way to ensure the integrity of erofs is to setup dm-verity at
> > erofs mount time.
> >
> > The way to ensure the integrity of the blobs is to store an fs-verity
> > signature of each blob file in trusted.overlay.verify xattr on the
> > metacopy and for overlayfs to enable fsverity on the blob file before
> > allowing access to the lowerdata.
> >
> > At least this is my understanding of the security model.
>
> So this should work out of the box, right?
>

Mostly. IIUC, overlayfs just needs to verify the signature on
open to fulfill the chain of trust, see cfs_open_file():
https://lore.kernel.org/linux-fsdevel/9b799ec7e403ba814e7bc097b1e8bd5f7662d596.1674227308.git.alexl@xxxxxxxxxx/

Thanks,
Amir.