Re: KASAN: use-after-free Read in task_is_descendant
From: Kees Cook
Date: Thu Oct 25 2018 - 07:36:18 EST
On Thu, Oct 25, 2018 at 12:13 PM, Oleg Nesterov <oleg@xxxxxxxxxx> 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.
(Well, I can't find the lock for switch_mm(), but I assume that's safe
because it's switching to the task.)
> 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
> OK. Lets ignore ptracer_exception_found() for the moment. Why do you think the
> patch below can't help?
> --- x/security/yama/yama_lsm.c
> +++ x/security/yama/yama_lsm.c
> @@ -368,7 +368,8 @@ static int yama_ptrace_access_check(stru
> case YAMA_SCOPE_RELATIONAL:
> - if (!task_is_descendant(current, child) &&
> + if (!pid_alive(child) ||
> + !task_is_descendant(current, child) &&
> !ptracer_exception_found(current, child) &&
> !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
> rc = -EPERM;
Hm, documentation there says:
* If pid_alive fails, then pointers within the task structure
* can be stale and must not be dereferenced.
What is the safe pattern for checking vs rcu?