Re: [PATCH] cred - synchronize rcu before releasing cred

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Jul 27, 2010 at 10:56:20AM -0700, Linus Torvalds wrote:
> On Tue, Jul 27, 2010 at 9:46 AM, David Howells <dhowells@redhat.com> wrote:
> >
> > That's not the problem.
> >
> > The problem is that task_state() accesses the target task's credentials whilst
> > only holding the RCU read lock.  That means that the existence of the cred
> > struct so accessed can only be guaranteed up to the point that the RCU read
> > lock is released.
> 
> Umm. In that case, get_task_cred() is actively misleading.
> 
> What you are saying is that you cannot do
> 
>    rcu_read_lock()
>    __cred = (struct cred *) __task_cred((task));
>    get_cred(__cred);
>    rcu_read_unlock();
> 
> but that is _exactly_ what get_task_cred() does. And that
right, get_task_cred looks like source for similar bugs.. will check

> __task_cred() check checks that we have
> 
>    rcu_read_lock_held() || lockdep_tasklist_lock_is_held()
> 
> and what you are describing would require us to have a '&&' rather
> than a '||' in that test. Because it is _not_ sufficient to have just
> the rcu_read_lock held.
> 
> So it looks like the validation is simply wrong. The __task_cred()
> helper is buggy. It's used for two different cases, and they have
> totally different locking requirements.
> 
> Case #1:
>  - you can do __task_cred() with just read-lock held, but then you
> cannot add refs to it
> 
> Case #2:
>  - you can do __task_cred() with read-lock held _and_ guaranteeing
> that the task doesn't go away, and then you can hold a ref to it as
> long as you still guarantee the task is around.
> 
> And the comments are actively wrong. The comments talk about the "case
> #2" thing only. Ignoring case #1, except for the fact that the _check_
> allows case #1, so you never get a warning from the RCU "proving" code
> even for incorrect code.
> 
> So presumably Jiri's patch is correct, but the reason the bug happened
> in the first place is that all those accessor functions are totally
> confused about how they supposed to be used, with incorrect comments
> and incorrect access checks.
> 
> That should get fixed. Who knows how many other buggy users there are
> due to the confusion?
I'll see if I can find some other places

thanks,
jirka