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

[Serge E. Hallyn]

Quoting Andrew Morton (akpm at linux-foundation.org):
> On Thu, 17 Feb 2011 15:03:33 +0000
> "Serge E. Hallyn" <serge at hallyn.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.

Thanks for the review, Andrew.  Unfortunately this one is hard to turn
into a function beecause it uses security_real_capable(), which is
sometimes defined in security/security.c as a real function, and
other times as a static inline in include/linux/security.h.  So
I'd have to #include security.h in capability.h, but security.h
already #includes capability.h.

All the other comments affect same_or_ancestor_user_ns(), which
following Eric's feedback is going away.

> >  /**
> >   * 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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