Re: [PATCH] vfs: Fix possible NULL pointer dereference in inode_permission()

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 9 Jan 2014 15:45:37 -0800
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> >  static void inode_free_security(struct inode *inode)
> >  {
> >  	struct inode_security_struct *isec = inode->i_security;
> > @@ -244,8 +252,7 @@ static void inode_free_security(struct i
> >  		list_del_init(&isec->list);
> >  	spin_unlock(&sbsec->isec_lock);
> > 
> > -	inode->i_security = NULL;
> > -	kmem_cache_free(sel_inode_cache, isec);
> > +	call_rcu(&isec->rcu, inode_free_rcu);
> 
> Does not clearing ->i_security mean that RCU readers can traverse
> this pointer after the invocation of call_rcu()?  If so, this is
> problematic.  (If something else already prevents readers from getting
> here, no problem.)

This is called when we are about to free the inode. Look at
destroy_inode(). Basically, this is the same as doing:

	call_rcu(&isec->rcu, inode_free_rcu);
	call_rcu(&inode->i_rcu, i_callback);

Where i_callback() does the free of the inode.

If you can access inode->i_security, after a call_rcu, then you can
also access the inode itself that has just been freed.

Yes, technically, having two separate call_rcu(), the first grace
period can end before the second, but everything to remove the inode
from sight has already been set up before that first call_rcu() is
made. That means when the first call_rcu() is executed, the inode
should already be invisible to the readers.


- Steve

> 
> 							Thanx, Paul
> 
> >  }
> >