| | Date | Thu, 10 Dec 2009 15:20:36 +0100 | | From | Oleg Nesterov <> | | Subject | Re: [patch 1/9] sys: Fix missing rcu protection for __task_cred() access |
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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.
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