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

[Roberto Sassu]

On Fri, 2024-03-01 at 08:39 -0600, Seth Forshee (DigitalOcean) wrote:
> 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.

Yes, figured it out after applying the patch set. Then yes, IMA/EVM
tests should work too.

> > 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.

Yes, that was another source of confusion. It happened that
security.selinux in the disk was without '\0', and the one from
vfs_getxattr() had it (of course the HMAC wouldn't match).

So, basically, you set something in user space and you get something
different.

Example:

# setfattr -n security.selinux -v "unconfined_u:object_r:admin_home_t:s0" test-file

SELinux active:
# getfattr -m - -d -e hex test-file
security.selinux=0x756e636f6e66696e65645f753a6f626a6563745f723a61646d696e5f686f6d655f743a733000

Smack active:
# getfattr -m - -d -e hex test-file
security.selinux=0x756e636f6e66696e65645f753a6f626a6563745f723a61646d696e5f686f6d655f743a7330


evmctl (will) allow to provide a hex xattr value for fscaps. That
should be the one to be used (and vfs_getxattr_alloc() does that).
However, I guess if the conversion happens, evmctl cannot correctly
verify anymore the file, unless the same string is specified for
verification (otherwise it reads the xattr through vfs_getxattr(),
which would be different).

Roberto