Re: [PATCH v2 14/25] evm: add support for fscaps security hooks

Seth Forshee (DigitalOcean)
2024-03-01T14:39:49Z

On Fri, Mar 01, 2024 at 10:19:13AM +0100, Roberto Sassu wrote:
> On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > Support the new fscaps security hooks by converting the vfs_caps to raw
> > xattr data and then handling them the same as other xattrs.
>
> Hi Seth
>
> I started looking at this patch set.
>
> The first question I have is if you are also going to update libcap
> (and also tar, I guess), since both deal with the raw xattr.

There are no changes needed for userspace; it will still deal with raw
xattrs. As I mentioned in the cover letter, capabilities tests from
libcap2, libcap-ng, ltp, and xfstests all pass against this sereies.
That's with no modifications to userspace.

> From IMA/EVM perspective (Mimi will add on that), I guess it is
> important that files with a signature/HMAC continue to be accessible
> after applying this patch set.
>
> Looking at the code, it seems the case (if I understood correctly,
> vfs_getxattr_alloc() is still allowed).

So this is something that would change based on Christian's request to
stop using the xattr handlers entirely for fscaps as was done for acls.
I see how this would impact EVM, but we should be able to deal with it.

I am a little curious now about this code in evm_calc_hmac_or_hash():

size = vfs_getxattr_alloc(&nop_mnt_idmap, dentry, xattr->name,
&xattr_value, xattr_size, GFP_NOFS);
if (size == -ENOMEM) {
error = -ENOMEM;
goto out;
}
if (size < 0)
continue;

user_space_size = vfs_getxattr(&nop_mnt_idmap, dentry,
xattr->name, NULL, 0);
if (user_space_size != size)
pr_debug("file %s: xattr %s size mismatch (kernel: %d, user: %d)\n",
dentry->d_name.name, xattr->name, size,
user_space_size);

Because with the current fscaps code you actually could end up getting
different sizes from these two interfaces, as vfs_getxattr_alloc() reads
the xattr directly from disk but vfs_getxattr() goes through
cap_inode_getsecurity(), which may do conversion between v2 and v3
formats which are different sizes.

Thanks,
Seth