Re: [PATCH RESEND2] ptrace: make ptrace() fail if the tracee changed its pid unexpectedly

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Jan Kratochvil <jan.kratochvil@redhat.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Pedro Alves <palves@redhat.com>,
	Simon Marchi <simon.marchi@efficios.com>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RESEND2] ptrace: make ptrace() fail if the tracee changed its pid unexpectedly
Date: Wed, 12 May 2021 09:56:49 -0400 (EDT)	[thread overview]
Message-ID: <1879292286.40455.1620827809948.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20210512133615.GA19594@redhat.com>

----- On May 12, 2021, at 9:36 AM, Oleg Nesterov oleg@redhat.com wrote:

> On 05/11, Oleg Nesterov wrote:
>>
>> On 05/11, Oleg Nesterov wrote:
>> >
>> > On 05/11, Linus Torvalds wrote:
>> >
>> > > That said, why this:
>> > >
>> > > > +       rcu_read_lock();
>> > > > +       pid = task_pid_nr_ns(task, task_active_pid_ns(task->parent));
>> > > > +       rcu_read_unlock();
>> > >
>> > > I don't see why the RCU read lock would be needed? task_pid_nr_ns()
>> > > does any required locking itself, afaik.
>> > >
>> > > And even if it wasn't, this all happens with siglock held, can
>> > > anything actually change.
>> >
>> > ... and with tasklist_lock held.
>> >
>> > Hmm. Linus, I am shy to admit I can't answer immediately, I'll recheck
>> > tomorrow after sleep. But it seems you are right.
>>
>> most probably to protect task->parent, not sure, this was 6 month ago...
>> but in this case we can use "current". I'll recheck.
> 
> Of course you are right, rcu_read_lock() is not needed. Plus we can use
> task_pid_vnr() rather than task_pid_nr_ns(). I've sent v2.

Out of curiosity: what makes it OK to use either the current task or its
parent's pid namespace in this specific case ? What happens if they are
in different pid namespaces ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com