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

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Eric W. Biederman (ebiederm@xmission.com):
> "Serge E. Hallyn" <serge@hallyn.com> writes:
> 
> > 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.
> 
> 
> I don't see how it can go wrong at the moment but
> same_or_ancestore_user_ns is too permissive and potentially inefficient.
> Can you please replace it with a simple user namespace equality check.
> 
> Eric
> 
> 
> > Changelog:
> > 	Dec 31: Address feedback by Eric:
> > 		. Correct ptrace uid check
> > 		. Rename may_ptrace_ns to ptrace_capable
> > 		. Also fix the cap_ptrace checks.
> > 	Jan  1: Use const cred struct
> > 	Jan 11: use task_ns_capable() in place of ptrace_capable().
> >
> > Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
> > ---
> >  include/linux/capability.h     |    2 +
> >  include/linux/user_namespace.h |    9 +++++++
> >  kernel/ptrace.c                |   27 ++++++++++++----------
> >  kernel/user_namespace.c        |   16 +++++++++++++
> >  security/commoncap.c           |   48 +++++++++++++++++++++++++++++++++------
> >  5 files changed, 82 insertions(+), 20 deletions(-)
> >
> > diff --git a/include/linux/capability.h b/include/linux/capability.h
> > index cb3d2d9..bc0f262 100644
> > --- 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)
> > +
> >  /**
> >   * 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);
> > +
> >  #else
> >  
> >  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
> > @@ -66,6 +69,12 @@ static inline gid_t user_ns_map_gid(struct user_namespace *to,
> >  	return gid;
> >  }
> >  
> > +static inline int same_or_ancestor_user_ns(struct task_struct *task,
> > +	struct task_struct *victim)
> > +{
> > +	return 1;
> > +}
> > +
> >  #endif
> >  
> >  #endif /* _LINUX_USER_H */
> > diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> > index 1708b1e..cde4655 100644
> > --- a/kernel/ptrace.c
> > +++ b/kernel/ptrace.c
> > @@ -134,21 +134,24 @@ int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> >  		return 0;
> >  	rcu_read_lock();
> >  	tcred = __task_cred(task);
> > -	if ((cred->uid != tcred->euid ||
> > -	     cred->uid != tcred->suid ||
> > -	     cred->uid != tcred->uid  ||
> > -	     cred->gid != tcred->egid ||
> > -	     cred->gid != tcred->sgid ||
> > -	     cred->gid != tcred->gid) &&
> > -	    !capable(CAP_SYS_PTRACE)) {
> > -		rcu_read_unlock();
> > -		return -EPERM;
> > -	}
> > +	if (cred->user->user_ns == tcred->user->user_ns &&
> > +	    (cred->uid == tcred->euid &&
> > +	     cred->uid == tcred->suid &&
> > +	     cred->uid == tcred->uid  &&
> > +	     cred->gid == tcred->egid &&
> > +	     cred->gid == tcred->sgid &&
> > +	     cred->gid == tcred->gid))
> > +		goto ok;
> > +	if (ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE))
> > +		goto ok;
> > +	rcu_read_unlock();
> > +	return -EPERM;
> > +ok:
> >  	rcu_read_unlock();
> >  	smp_rmb();
> >  	if (task->mm)
> >  		dumpable = get_dumpable(task->mm);
> > -	if (!dumpable && !capable(CAP_SYS_PTRACE))
> > +	if (!dumpable && !task_ns_capable(task, CAP_SYS_PTRACE))
> >  		return -EPERM;
> >  
> >  	return security_ptrace_access_check(task, mode);
> > @@ -198,7 +201,7 @@ int ptrace_attach(struct task_struct *task)
> >  		goto unlock_tasklist;
> >  
> >  	task->ptrace = PT_PTRACED;
> > -	if (capable(CAP_SYS_PTRACE))
> > +	if (task_ns_capable(task, CAP_SYS_PTRACE))
> >  		task->ptrace |= PT_PTRACE_CAP;
> >  
> >  	__ptrace_link(task, current);
> > diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> > index 9da289c..0ef2258 100644
> > --- 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;
> > +}
> > +
> >  static __init int user_namespaces_init(void)
> >  {
> >  	user_ns_cachep = KMEM_CACHE(user_namespace, SLAB_PANIC);
> > diff --git a/security/commoncap.c b/security/commoncap.c
> > index 51fa9ec..12ff65c 100644
> > --- a/security/commoncap.c
> > +++ b/security/commoncap.c
> > @@ -130,18 +130,34 @@ int cap_settime(struct timespec *ts, struct timezone *tz)
> >   * @child: The process to be accessed
> >   * @mode: The mode of attachment.
> >   *
> > + * If we are in the same or an ancestor user_ns and have all the target
> > + * task's capabilities, then ptrace access is allowed.
> > + * If we have the ptrace capability to the target user_ns, then ptrace
> > + * access is allowed.
> > + * Else denied.
> > + *
> >   * Determine whether a process may access another, returning 0 if permission
> >   * granted, -ve if denied.
> >   */
> >  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;
> 
> I have commented on this before but I took a good hard look this time,
> and can comment more intelligently.

Thanks, Eric.

> The cap_issubset check is for the case where we don't use the
> CAP_SYS_PTRACE capability, as such is only valid in the same user
> namespace.  Furthermore capabilities really are not comparable between
> different user namespaces.  So can you please replace the
> same_or_ancestor_user_ns with a simple namespace equality check.
> Having the wrong logic in here will just be confusing in the future.

I see.  You're right, what's there is conceptually not quite right.
So I think we can just do:

	if (current_user_ns() == tcred->user->userns) &&
	    cap_issubset(child, current)
		goto out;
	if ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE)
		goto out;

This suffices since
	root in an ancestor ns will have CAP_SYS_PTRACE
	the user who created the user_ns will have CAP_SYS_PTRACE

Any user in an ancestor ns who does not have CAP_SYS_PTRACE will
be denied.

> Also could you name tcred child_cread I think that would be clearer in
> the test below.

Yes, will do, that'll be clearer.  Thanks.

> > +	if (ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE))
> > +		goto out;
> > +	ret = -EPERM;
> > +out:
> >  	rcu_read_unlock();
> >  	return ret;
> 
> I also find it strange that we allow CAP_SYS_PTRACE to allow us to
> ptrace processes with more capabilities than ourselves.  But that is an
> entirely different issue.

I concur.  I'm pretty sure it must stay, but it does feel icky.

I'll send out a new version

thanks
-serge