Re: [PATCH v2 16/24] locking/lockdep: Retain the class key and name while freeing a lock class

[Bart Van Assche <bvanassche@acm.org>]

On Tue, 2018-12-04 at 15:31 -0500, Waiman Long wrote:
> On 12/04/2018 02:08 PM, Bart Van Assche wrote:
> > On Tue, 2018-12-04 at 13:57 -0500, Waiman Long wrote:
> > > On 12/03/2018 07:28 PM, Bart Van Assche wrote:
> > > > The next patch in this series uses the class name in code that
> > > > detects lock class use-after-free. Hence retain the class name for
> > > > lock classes that are being freed.
> > > > 
> > > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > > Cc: Waiman Long <longman@redhat.com>
> > > > Cc: Johannes Berg <johannes@sipsolutions.net>
> > > > Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> > > > ---
> > > >  kernel/locking/lockdep.c | 4 +---
> > > >  1 file changed, 1 insertion(+), 3 deletions(-)
> > > > 
> > > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > > > index ecd92969674c..92bdb187987f 100644
> > > > --- a/kernel/locking/lockdep.c
> > > > +++ b/kernel/locking/lockdep.c
> > > > @@ -4147,10 +4147,8 @@ static void zap_class(struct lock_class *class)
> > > >  	 * Unhash the class and remove it from the all_lock_classes list:
> > > >  	 */
> > > >  	hlist_del_rcu(&class->hash_entry);
> > > > +	class->hash_entry.pprev = NULL;
> > > >  	list_del(&class->lock_entry);
> > > > -page 
> > > > -	RCU_INIT_POINTER(class->key, NULL);
> > > > -	RCU_INIT_POINTER(class->name, NULL);
> > > >  }
> > > >  
> > > >  static inline int within(const void *addr, void *start, unsigned long size)
> > > 
> > > Using the name after module unload can be problematic if it points to
> > > memory space occupied by the unloaded kernel module. The memory page may
> > > be invalid with the potential of crashing the kernel. You may have to
> > > duplicate the name if it is really from the module address space.
> > 
> > The class name shouldn't be used by any of the code I introduced in the
> > lockdep infrastructure after unloading a kernel module finished unless if
> > some of the kernel module code is still running after unloading the kernel
> > module finished. I think if that happens that the kernel module author is
> > to blame and not lockdep :-)
> > 
> > Bart.
> 
> You do use the name once in a WARN_ON check in check_prev_add(). You
> commit log did say the class name will be used.

Hi Waiman,

I will update the commit message and I will add a comment in check_prev_add()
that explains that the newly added warning may trigger a use-after-free.

Bart.