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

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 10 Dec 2009, Oleg Nesterov wrote:
> 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.

I guess we have whole bunch of auditing to do all over the place. And
we definitely need some debugging aid (preferrably a lockdep
extension) which allows us to detect such nasty details.

Thanks,

	tglx