linux-kernel - Re: [PATCH] vfs: Fix possible NULL pointer dereference in inode

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Message-ID: <20140110004411.GS10038@linux.vnet.ibm.com>
Date:	Thu, 9 Jan 2014 16:44:11 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel@...r.kernel.org,
	James Morris <james.l.morris@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	"Theodore Ts'o" <tytso@....edu>,
	Eric Paris <eparis@...isplace.org>,
	stable <stable@...r.kernel.org>,
	Paul Moore <paul@...l-moore.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Matthew Wilcox <matthew@....cx>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] vfs: Fix possible NULL pointer dereference in
 inode_permission()

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@...ux.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

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