KASAN: use-after-free Read in task_is_descendant

[Oleg Nesterov]

On 10/25, Kees Cook wrote:
>
> On Thu, Oct 25, 2018 at 12:13 PM, Oleg Nesterov <oleg at redhat.com> wrote:
> > So again, suppose that "child" is already dead. Its task_struct can't be freed,
> > but child->real_parent can point to the already freed memory.
>
> I can't find a path for "child" to be released. I see task_lock()
> always called on it before it ends up in Yama.

Are you saying that yama_ptrace_access_check() is always called under
task_lock(child) ? Yes, it seems so

So what? Say, ptrace_attach() can hit a dead task. It should notice this
and fail later, but security_ptrace_access_check() is called before that.


> > This means that the 1st walker = rcu_dereference(walker->real_parent) is fine,
> > this simply reads the child->real_parent pointer, but on the second iteration
> >
> >         walker = rcu_dereference(walker->real_parent);
> >
> > reads the alredy freed memory.
>
> What does rcu_read_lock() protect actually protect here? I thought
> none of the task structs would be freed until after all
> rcu_read_unlock() finished.

See another email I sent you a minute ago.

Oleg.