linux-kernel - Re: [PATCH 5/9] Allow ptrace from non-init user namespaces

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Date:	Fri, 18 Feb 2011 15:59:25 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	LSM <linux-security-module@...r.kernel.org>,
	James Morris <jmorris@...ei.org>,
	Kees Cook <kees.cook@...onical.com>,
	containers@...ts.linux-foundation.org,
	kernel list <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Michael Kerrisk <mtk.manpages@...il.com>, xemul@...allels.com,
	dhowells@...hat.com
Subject: Re: [PATCH 5/9] Allow ptrace from non-init user namespaces

On Thu, 17 Feb 2011 15:03:33 +0000
"Serge E. Hallyn" <serge@...lyn.com> wrote:

> ptrace is allowed to tasks in the same user namespace according to
> the usual rules (i.e. the same rules as for two tasks in the init
> user namespace).  ptrace is also allowed to a user namespace to
> which the current task the has CAP_SYS_PTRACE capability.
> 
>
> ...
>
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -546,6 +546,8 @@ extern const kernel_cap_t __cap_init_eff_set;
>   */
>  #define has_capability(t, cap) (security_real_capable((t), &init_user_ns, (cap)) == 0)
>  
> +#define has_ns_capability(t, ns, cap) (security_real_capable((t), (ns), (cap)) == 0)

macroitis.

>  /**
>   * has_capability_noaudit - Determine if a task has a superior capability available (unaudited)
>   * @t: The task in question
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index faf4679..862fc59 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -39,6 +39,9 @@ static inline void put_user_ns(struct user_namespace *ns)
>  uid_t user_ns_map_uid(struct user_namespace *to, const struct cred *cred, uid_t uid);
>  gid_t user_ns_map_gid(struct user_namespace *to, const struct cred *cred, gid_t gid);
>  
> +int same_or_ancestor_user_ns(struct task_struct *task,
> +	struct task_struct *victim);

bool.

>  #else
>  
>  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
>
> ...
>
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -129,6 +129,22 @@ gid_t user_ns_map_gid(struct user_namespace *to, const struct cred *cred, gid_t
>  	return overflowgid;
>  }
>  
> +int same_or_ancestor_user_ns(struct task_struct *task,
> +	struct task_struct *victim)
> +{
> +	struct user_namespace *u1 = task_cred_xxx(task, user)->user_ns;
> +	struct user_namespace *u2 = task_cred_xxx(victim, user)->user_ns;
> +	for (;;) {
> +		if (u1 == u2)
> +			return 1;
> +		if (u1 == &init_user_ns)
> +			return 0;
> +		u1 = u1->creator->user_ns;
> +	}
> +	/* We never get here */
> +	return 0;

Remove?

> +}
> +
>  static __init int user_namespaces_init(void)
>  {
>  	user_ns_cachep = KMEM_CACHE(user_namespace, SLAB_PANIC);
>
> ...
>
>  int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
>  {
>  	int ret = 0;
> +	const struct cred *cred, *tcred;
>  
>  	rcu_read_lock();
> -	if (!cap_issubset(__task_cred(child)->cap_permitted,
> -			  current_cred()->cap_permitted) &&
> -	    !capable(CAP_SYS_PTRACE))
> -		ret = -EPERM;
> +	cred = current_cred();
> +	tcred = __task_cred(child);
> +	/*
> +	 * The ancestor user_ns check may be gratuitous, as I think
> +	 * we've already guaranteed that in kernel/ptrace.c.
> +	 */

?

> +	if (same_or_ancestor_user_ns(current, child) &&
> +	    cap_issubset(tcred->cap_permitted, cred->cap_permitted))
> +		goto out;
> +	if (ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE))
> +		goto out;
> +	ret = -EPERM;
> +out:
>  	rcu_read_unlock();
>  	return ret;
>  }
>
> ...
>

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