Re: [patch 1/9] sys: Fix missing rcu protection for __task_cred() access

Oleg Nesterov
2009-12-10T14:26:54Z

On 12/10, Thomas Gleixner wrote:
>
> commit c69e8d9 (CRED: Use RCU to access another task's creds and to
> release a task's own creds) added non rcu_read_lock() protected access
> to task creds of the target task in set_prio_one().
>
> The comment above the function says:
> * - the caller must hold the RCU read lock
>
> The calling code in sys_setpriority does read_lock(&tasklist_lock) but
> not rcu_read_lock(). This works only when CONFIG_TREE_PREEMPT_RCU=n.
> With CONFIG_TREE_PREEMPT_RCU=y the rcu_callbacks can run in the tick
> interrupt when they see no read side critical section.
> ...
> --- linux-2.6-tip.orig/kernel/sys.c
> +++ linux-2.6-tip/kernel/sys.c
> @@ -163,6 +163,7 @@ SYSCALL_DEFINE3(setpriority, int, which,
> if (niceval > 19)
> niceval = 19;
>
> + rcu_read_lock();
> read_lock(&tasklist_lock);

Off-topic, but can't resist...

This also fixes another bug here. find_task_by_vpid() is not safe
without rcu_read_lock(). I do not mean it is not safe to use the
result, just find_pid_ns() by itself is not safe.

Usually tasklist gives enough protection, but if copy_process() fails
it calls free_pid() lockless and does call_rcu(delayed_put_pid().
This means, without rcu lock find_pid_ns() can't scan the hash table
safely.

Oleg.