Re: [RFC PATCH] mm, oom: fix use-after-free in oom_kill_process

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2019/01/19 9:50, Shakeel Butt wrote:
> On looking further it seems like the process selected to be oom-killed
> has exited even before reaching read_lock(&tasklist_lock) in
> oom_kill_process(). More specifically the tsk->usage is 1 which is due
> to get_task_struct() in oom_evaluate_task() and the put_task_struct
> within for_each_thread() frees the tsk and for_each_thread() tries to
> access the tsk. The easiest fix is to do get/put across the
> for_each_thread() on the selected task.

Good catch. p->usage can become 1 while printk()ing a lot at dump_header().

> @@ -981,6 +981,13 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
>  	 * still freeing memory.
>  	 */
>  	read_lock(&tasklist_lock);
> +
> +	/*
> +	 * The task 'p' might have already exited before reaching here. The
> +	 * put_task_struct() will free task_struct 'p' while the loop still try
> +	 * to access the field of 'p', so, get an extra reference.
> +	 */
> +	get_task_struct(p);
>  	for_each_thread(p, t) {
>  		list_for_each_entry(child, &t->children, sibling) {
>  			unsigned int child_points;
> @@ -1000,6 +1007,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
>  			}
>  		}
>  	}
> +	put_task_struct(p);

Moving put_task_struct(p) to after read_unlock(&tasklist_lock) will reduce
latency of a write_lock(&tasklist_lock) waiter.

>  	read_unlock(&tasklist_lock);
>  
>  	/*
> 

By the way, p->usage is already 1 implies that p->mm == NULL due to already
completed exit_mm(p). Then, process_shares_mm(child, p->mm) might fail to
return true for some of children. Not critical but might lead to unnecessary
oom_badness() calls for child selection. Maybe we want to use same logic
__oom_kill_process() uses (i.e. bail out if find_task_lock_mm(p) failed)?