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

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Message-ID: <20210512133615.GA19594@redhat.com>
Date:   Wed, 12 May 2021 15:36:16 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Jan Kratochvil <jan.kratochvil@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Pedro Alves <palves@...hat.com>,
        Simon Marchi <simon.marchi@...icios.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND2] ptrace: make ptrace() fail if the tracee changed
 its pid unexpectedly

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.

Thanks again,

Oleg.