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

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Message-ID: <20140109185907.20cddb57@gandalf.local.home>
Date:	Thu, 9 Jan 2014 18:59:07 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
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, 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.


- Steve

> 
> 							Thanx, Paul
> 
> >  }
> > 
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