Re: [PATCH] ptrace: restore smp_rmb() in __ptrace_may_access()

[Jann Horn]

On Wed, May 29, 2019 at 6:21 PM Oleg Nesterov <oleg@xxxxxxxxxx> 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).