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

[Amir Goldstein]

On Mon, Dec 11, 2023 at 7:15 PM Seth Forshee <sforshee at kernel.org> wrote:
>
> On Mon, Dec 11, 2023 at 04:41:46PM +0100, Roberto Sassu wrote:
> > 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.
>
> I can only think of three possible sources for the metadata overlayfs
> presents:
>
>  1. It comes directly from the underlying filesystems
>  2. overlayfs synthesizes if from the underlying filesystem data
>  3. It's purely generated at runtime
>
> Are there others?

3.b. purely generated and persisted in overlay private xattr

but IIRC only s_uuid fits in that category

>
> 1 and 2 should be covered by EVM on the underlying filesystems. If 3 is
> happening then it seems like hashing that data is just confirming that
> overlayfs consistently generates the same values for that data, and
> verifying code behavior doesn't seem in-scope for EVM.

I agree.
I don't think that IMA/EVM has a reason to assets overlayfs specific
metadata.

Thanks,
Amir.