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SubjectRe: [RFC 5/5] user namespaces: Allow ptrace from non-init user namespaces
Thanks for reviewing, Eric.

Quoting Eric W. Biederman (ebiederm@xmission.com):
> > +static inline int may_ptrace_ns(struct task_struct *t)
>
> Can we name this ptrace_capable? Since you are only
> wrapping the capability check? With a name like may_ptrace_ns
> I imagine very different semantics.

Hm, the whole structure here could probably stand to be improved
anyway. I just can't quite think how. I'll rename it as you
suggest for starters, just not sure if it'll continue to exist.

>
> > +{
> > + struct user_namespace *ns;
> > + int ret;
> > +
> > + rcu_read_lock();
> > + ns = task_cred_xxx(t, user)->user_ns;
> > + ret = ns_capable(ns, CAP_SYS_PTRACE);
> > + rcu_read_unlock();
> > +
> > + return ret;
> > +}
> > +
> > int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> > {
> > const struct cred *cred = current_cred(), *tcred;
> > @@ -134,21 +147,24 @@ 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;
>
> This needs to be:
> > + 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;

Hm, I started to explain why it doesn't, but you're right.
If any of the uids are different, then you must have
CAP_SYS_PTRACE or be denied.

> > --- a/security/commoncap.c
> > +++ b/security/commoncap.c
> > @@ -136,12 +136,20 @@ int cap_settime(struct timespec *ts, struct timezone *tz)
> > int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
> > {
> > int ret = 0;
> > + struct cred *cred, *tcred;
> >
> > rcu_read_lock();
> > - if (!cap_issubset(__task_cred(child)->cap_permitted,
> > - current_cred()->cap_permitted) &&
> > + cred = current_cred();
> > + tcred = __task_cred(child);
> > + if (cred->user->user_ns != tcred->user->user_ns) {
>
> This probably deserves a comment about why cap_issubset isn't
> needed here. Aka we implicitly have all caps in child user namespaces
> so if we have CAP_SYS_PTRACE we know we have them all.

(going strictly by the rules which fall out from the original intent
of ns_capable) :

There is a case where that isn't true - if I'm user B in userns 3, and
user A in userns 3 created the userns 4 in which this target task, owned
by user C, sits. Then user B does not have all capabilities to userns 4,
but any calculated capabilities which B has, are also valid in userns 4.

I'd still claim that capabilities aren't really comparable (because
they are targeted at different user namespaces), and therefore the
CAP_SYS_PTRACE should be sufficient for this case. But maybe that's
not as practical. Maybe the cap_issubset check should be there after
all.

> > + if (!ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE))
> > + ret = -EPERM;
> > + goto out;
> > + }
> > + if (!cap_issubset(tcred->cap_permitted, cred->cap_permitted) &&
> > !capable(CAP_SYS_PTRACE))
> > ret = -EPERM;
> > +out:
> > rcu_read_unlock();
> > return ret;
> > }
> > @@ -156,12 +164,20 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
> > int cap_ptrace_traceme(struct task_struct *parent)
> > {
> > int ret = 0;
> > + struct cred *cred, *tcred;
> >
> > rcu_read_lock();
> > - if (!cap_issubset(current_cred()->cap_permitted,
> > - __task_cred(parent)->cap_permitted) &&
> > - !has_capability(parent, CAP_SYS_PTRACE))
> > + cred = __task_cred(parent);
> > + tcred = current_cred();
> > + if (cred->user->user_ns != tcred->user->user_ns) {
> > + if (!has_ns_capability(parent, tcred->user->user_ns, CAP_SYS_PTRACE))
> > + ret = -EPERM;
> > + goto out;
> > + }
> > + if (!cap_issubset(tcred->cap_permitted, cred->cap_permitted) &&
> > + !has_ns_capability(parent, tcred->user->user_ns, CAP_SYS_PTRACE))
> > ret = -EPERM;
> > +out:
> > rcu_read_unlock();
> > return ret;
> > }
> --
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