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Date: Thu, 9 Jan 2014 16:44:11 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>, Dave Chinner <david@fromorbit.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
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        Paul Moore <paul@paul-moore.com>, LKML <linux-kernel@vger.kernel.org>,
        Matthew Wilcox <matthew@wil.cx>, Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH] vfs: Fix possible NULL pointer dereference in
 inode_permission()
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On Thu, Jan 09, 2014 at 06:59:07PM -0500, Steven Rostedt wrote:
> 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.

Got it, should be fine then, sorry for the noise.

							Thanx, Paul