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From:   Jann Horn <jannh@google.com>
Date:   Wed, 29 May 2019 23:02:37 +0200
Message-ID: <CAG48ez3PCoZTqKeUJ-jLu8HtiXRf_mOcnC6AtgTc0QM0Q6VrSA@mail.gmail.com>
Subject: Re: [PATCH] ptrace: restore smp_rmb() in __ptrace_may_access()
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     "Eric W . Biederman" <ebiederm@xmission.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        David Howells <dhowells@redhat.com>,
        kernel list <linux-kernel@vger.kernel.org>
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On Wed, May 29, 2019 at 6:21 PM Oleg Nesterov <oleg@redhat.com> wrote:
> On 05/29, Jann Horn wrote:
> > --- a/kernel/ptrace.c
> > +++ b/kernel/ptrace.c
> > @@ -324,6 +324,16 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
[...]
> >       mm = task->mm;
>
> while at it, could you also change this into mm = READ_ONCE(task->mm) ?

Actually, that shouldn't be necessary. The caller of
__ptrace_may_access() holds the task_lock() on the task, and that
should prevent concurrent updates of ->mm. If concurrent updates of
->mm *were* possible, we'd probably be in deep trouble here (and by
that I mean use-after-free).