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 12/09, Paul E. McKenney wrote:
> >
> > On Thu, Dec 10, 2009 at 12:52:51AM -0000, 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.
> > >
> > > There is another instance of __task_cred() in sys_setpriority() itself
> > > which is equally unprotected.
> > >
> > > Wrap the whole code section into a rcu read side critical section to
> > > fix this quick and dirty.
> > >
> > > Will be revisited in course of the read_lock(&tasklist_lock) -> rcu
> > > crusade.
> >
> > OK, I will bite...  Don't the corresponding updates write-hold
> > tasklist_lock?  If so, then the fact that the code below is read-holding
> > tasklist_lock would prevent any of the data from changing, which would
> > remove the need to do the rcu_read_lock().
> >
> > Or are there updates that are carried out without write-holding
> > tasklist_lock that I am missing?
> 
> Yes, commit_creds() is called lockless.

Right, and that's what the problem is. commit_creds(), which rcu frees
the old creds, does not take tasklist lock write lock.

Thanks,

	tglx