linux-kernel - Re: [PATCH RFC] Introduce new security.nscapability xattr

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Date:	Wed, 20 Jan 2016 13:14:52 +0100
From:	Jann Horn <jann@...jh.net>
To:	"Serge E. Hallyn" <serge.hallyn@...ntu.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morgan <morgan@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	lxc-devel@...ts.linuxcontainers.org,
	Richard Weinberger <richard@....at>,
	LSM <linux-security-module@...r.kernel.org>,
	linux-api@...r.kernel.org, keescook@...omium.org
Subject: Re: [PATCH RFC] Introduce new security.nscapability xattr

On Mon, Nov 30, 2015 at 04:43:56PM -0600, Serge E. Hallyn wrote:
> +int get_vfs_ns_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps)
> +{
[...]
> +	/* find an applicable entry */
> +	/* a global entry (uid == -1) takes precedence */
> +	current_root = make_kuid(current_user_ns(), 0);
> +	if (!uid_valid(current_root)) {
> +		/* no root user in this namespace;  no capabilities */
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
> +	for (i = 0, cap = (void *) hdr + sizeof(*hdr); i < ncaps; cap += sizeof(*cap), i++) {
> +		uid_t uid = le32_to_cpu(cap->rootid);
> +		if (uid == -1) {
> +			nscap = cap;
> +			break;
> +		}
> +
> +		caprootuid = make_kuid(&init_user_ns, uid);
> +		if (uid_eq(caprootuid, current_root))
> +			nscap = cap;
> +	}

Wouldn't it be more consistent to check against the root uids of all parent
namespaces until one matches?

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