linux-kernel - Re: [PATCH v2 06/25] capability: provide helpers for converting between xattrs and vfs

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Date: Mon, 4 Mar 2024 08:24:42 -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 06/25] capability: provide helpers for converting
 between xattrs and vfs_caps

On Mon, Mar 04, 2024 at 09:33:06AM +0100, Roberto Sassu wrote:
> On Fri, 2024-03-01 at 13:00 -0600, Seth Forshee (DigitalOcean) wrote:
> > On Fri, Mar 01, 2024 at 05:30:55PM +0100, Roberto Sassu wrote:
> > > > +/*
> > > > + * Inner implementation of vfs_caps_to_xattr() which does not return an
> > > > + * error if the rootid does not map into @dest_userns.
> > > > + */
> > > > +static ssize_t __vfs_caps_to_xattr(struct mnt_idmap *idmap,
> > > > +				   struct user_namespace *dest_userns,
> > > > +				   const struct vfs_caps *vfs_caps,
> > > > +				   void *data, size_t size)
> > > > +{
> > > > +	struct vfs_ns_cap_data *ns_caps = data;
> > > > +	struct vfs_cap_data *caps = (struct vfs_cap_data *)ns_caps;
> > > > +	kuid_t rootkuid;
> > > > +	uid_t rootid;
> > > > +
> > > > +	memset(ns_caps, 0, size);
> > > 
> > > size -> sizeof(*ns_caps) (or an equivalent change)
> > 
> > This is zeroing out the passed buffer, so it should use the size passed
> > for the buffer. sizeof(*ns_caps) could potentially be more than the size
> > of the buffer.
> 
> Uhm, then maybe the problem is that you are passing the wrong argument?
> 
> ssize_t
> do_getxattr(struct mnt_idmap *idmap, struct dentry *d,
> 	struct xattr_ctx *ctx)
> {
> 	ssize_t error;
> 	char *kname = ctx->kname->name;
> 
> 	if (is_fscaps_xattr(kname)) {
> 		struct vfs_caps caps;
> 		struct vfs_ns_cap_data data;
> 		int ret;
> 
> 		ret = vfs_get_fscaps(idmap, d, &caps);
> 		if (ret)
> 			return ret;
> 		/*
> 		 * rootid is already in the mount idmap, so pass nop_mnt_idmap
> 		 * so that it won't be mapped.
> 		 */
> 		ret = vfs_caps_to_user_xattr(&nop_mnt_idmap, current_user_ns(),
> 					     &caps, &data, ctx->size);
> 
> 
> ctx->size in my case is 1024 bytes.

Ah, yes that definitely isn't correct. I will fix it, thanks for finding
it.