linux-kernel - Re: [PATCH v2 00/25] fs: use type-safe uid representation for filesystem capabilities

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Date: Thu, 22 Feb 2024 16:27:50 +0100
From: Christian Brauner <brauner@...nel.org>
To: "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>
Cc: 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 00/25] fs: use type-safe uid representation for
 filesystem capabilities

On Wed, Feb 21, 2024 at 03:24:31PM -0600, Seth Forshee (DigitalOcean) wrote:
> This series converts filesystem capabilities from passing around raw
> xattr data to using a kernel-internal representation with type safe
> uids, similar to the conversion done previously for posix ACLs.
> Currently fscaps representations in the kernel have two different
> instances of unclear or confused types:
> 
> - fscaps are generally passed around in the raw xattr form, with the
>   rootid sometimes containing the user uid value and at other times
>   containing the filesystem value.
> - The existing kernel-internal representation of fscaps,
>   cpu_vfs_cap_data, uses the kuid_t type, but the value stored is
>   actually a vfsuid.
> 
> This series eliminates this confusion by converting the xattr data to
> the kernel representation near the userspace and filesystem boundaries,
> using the kernel representation within the vfs and commoncap code. The
> internal representation is renamed to vfs_caps to reflect this broader
> use, and the rootid is changed to a vfsuid_t to correctly identify the
> type of uid which it contains.
> 
> New vfs interfaces are added to allow for getting and setting fscaps
> using the kernel representation. This requires the addition of new inode
> operations to allow overlayfs to handle fscaps properly; all other
> filesystems fall back to a generic implementation. The top-level vfs
> xattr interfaces will now reject fscaps xattrs, though the lower-level
> interfaces continue to accept them for reading and writing the raw xattr
> data.
> 
> Based on previous feedback, new security hooks are added for fscaps
> operations. These are really only needed for EVM, and the selinux and
> smack implementations just peform the same operations that the
> equivalent xattr hooks would have done. Note too that this has not yet
> been updated based on the changes to make EVM into an LSM.
> 
> The remainder of the changes are preparatory work, addition of helpers
> for converting between the xattr and kernel fscaps representation, and
> various updates to use the kernel representation and new interfaces.

I still think that the generic_{get,set,remove}_fscaps() helpers falling
back to plain *vfs_*xattr() calls is a hackish. So ideally I'd like to
see this killed in a follow-up series and make all fses that support
them use the inode operation.

> 
> I have tested this code with xfstests, ltp, libcap2, and libcap-ng with
> no regressions found.

+1