linux-kernel - Re: [PATCH RFC] Allow introspection to already attached ptracer in __ptrace_may

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Date:	Tue, 6 Jan 2015 19:44:27 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Stijn Volckaert <Stijn.Volckaert@...s.ugent.be>,
	Roland McGrath <roland@...k.frob.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Casey Schaufler <casey@...aufler-ca.com>,
	John Johansen <john.johansen@...onical.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [PATCH RFC] Allow introspection to already attached ptracer in
	__ptrace_may_access

On 01/05, Kees Cook wrote:
>
> I'm nervous to add this (or Oleg's suggestion) generally to
> __ptrace_may_access, as it would mean other LSMs would stop seeing
> access checks even when attached. It does seem silly to deny ptrace
> checks when already attached, but it does change the behavior here.

Same here.

> If the other LSM folks don't see a problem here, then it should live
> in the general case. Otherwise, I'm happy to add this check only in
> Yama.

In this case this check should probably go into ptracer_exception_found().

Btw it looks buggy... RCU protects ptrace_relation object, but not
relation->tracer (the final __put_task_struct() calls yama_task_free()
and frees task_struct without rcu gp).

This means that task_is_descendant(parent, tracer) can dereference the
already freed/unmapped parent, no?

And the usage of ->group_leader looks strange. tracee->group_leader can
point to nowhere if the task is already dead. In this case
"relation->tracee == tracee" can be false-positive (the same task_struct
can be re-allocated), but probably this is not that bad exactly because
the task is dead anyway.

prctl(PR_SET_PTRACER) looks strange too wrt group_leader. What if the caller
execs later? The comment says "we want process-level granularity of control",
but this is only true if the main thread does exec.

And get/out_task_struct(myself) look unneeded.

And it seems that task_is_descendant() doesn't need ->group_leader at all,
it could simply do

	static int task_is_descendant(struct task_struct *parent,
				      struct task_struct *child)
	{
		int rc = 0;
		struct task_struct *walker = child;

		if (!parent || !child)
			return 0;

		rcu_read_lock();
		do {
			if (same_thread_group(walker, parent)) {
				rc = 1;
				break;
			}
			walker = rcu_dereference(walker->real_parent);
		} while (walker != &init_task);
		rcu_read_unlock();

		return rc;
	}

Oleg.

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