On 10/25, Kees Cook wrote:

On Thu, Oct 25, 2018 at 12:13 PM, Oleg Nesterov [off-list ref] wrote:

quoted

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.

quoted

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.