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

From: Oleg Nesterov <oleg@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Jan Kratochvil <jan.kratochvil@redhat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Pedro Alves <palves@redhat.com>,
	Simon Marchi <simon.marchi@efficios.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RESEND2] ptrace: make ptrace() fail if the tracee changed its pid unexpectedly
Date: Tue, 11 May 2021 19:53:42 +0200	[thread overview]
Message-ID: <20210511175341.GA14488@redhat.com> (raw)
In-Reply-To: <CAHk-=whLqbTNc1T+rHCm-kxbVAuhK3hjo5fOgDVf5-z--x1mvQ@mail.gmail.com>

On 05/11, Linus Torvalds wrote:
>
> On Tue, May 11, 2021 at 9:56 AM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > This patch makes ptrace() fail in this case until debugger does wait()
> > and consumes PTHREAD_EVENT_EXEC which reports old_pid.
>
> I'm ok with the patch, just wondering which way it's supposed to come
> to me. Should I just apply it directly?

would be nice!

> 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.

Thanks!

Oleg.