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

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Date: Fri, 1 Mar 2024 08:39:48 -0600
From: "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>
To: Roberto Sassu <roberto.sassu@...weicloud.com>
Cc: Christian Brauner <brauner@...nel.org>, Serge Hallyn <serge@...lyn.com>,
	Paul Moore <paul@...l-moore.com>, Eric Paris <eparis@...hat.com>,
	James Morris <jmorris@...ei.org>,
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	Stephen Smalley <stephen.smalley.work@...il.com>,
	Ondrej Mosnacek <omosnace@...hat.com>,
	Casey Schaufler <casey@...aufler-ca.com>,
	Mimi Zohar <zohar@...ux.ibm.com>,
	Roberto Sassu <roberto.sassu@...wei.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
	Eric Snowberg <eric.snowberg@...cle.com>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Miklos Szeredi <miklos@...redi.hu>,
	Amir Goldstein <amir73il@...il.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, audit@...r.kernel.org,
	selinux@...r.kernel.org, linux-integrity@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-unionfs@...r.kernel.org
Subject: Re: [PATCH v2 14/25] evm: add support for fscaps security hooks

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