linux-kernel - Re: [PATCH 2/5] kernel: add a helper to get an owning user namespace for a namespace

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Date:	Sun, 24 Jul 2016 00:03:49 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrey Vagin <avagin@...nvz.org>
Cc:	linux-kernel@...r.kernel.org,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	Alexander Viro <viro@...iv.linux.org.uk>, criu@...nvz.org,
	linux-fsdevel@...r.kernel.org,
	"Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 2/5] kernel: add a helper to get an owning user namespace for a namespace

Andrey Vagin <avagin@...nvz.org> writes:

> Return -EPERM if an owning user namespace is outside of a process
> current user namespace.
>
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index a5bc78c..6382e5e 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -994,6 +994,30 @@ static int userns_install(struct nsproxy *nsproxy, struct ns_common *ns)
>  	return commit_creds(cred);
>  }
>  
> +struct ns_common *ns_get_owner(struct ns_common *ns)
> +{
> +	const struct cred *cred = current_cred();
> +	struct user_namespace *user_ns, *p;
> +
> +	user_ns = p = ns->user_ns;
> +	if (user_ns == NULL) { /* ns is init_user_ns */
> +		/* Unprivileged user should not know that it's init_user_ns. */
> +		if (capable(CAP_SYS_ADMIN))
> +			return ERR_PTR(-ENOENT);
> +		return ERR_PTR(-EPERM);
> +	}

This permission check is not what I meant to request.  This does not
handle nested user namespaces.

> +	for (;;) {
> +		if (p == cred->user_ns)
> +			break;
> +		if (p == &init_user_ns)
> +			return ERR_PTR(-EPERM);
> +		p = p->parent;
> +	}
> +

The permission check really needs to be down here. And be:

	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
        	return ERR_PTR(-EPERM).

That cleanly and easily handles more than a depth of a single user
namespace.

> +	return &get_user_ns(user_ns)->ns;
> +}
> +
>  const struct proc_ns_operations userns_operations = {
>  	.name		= "user",
>  	.type		= CLONE_NEWUSER,


Eric