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

From: Eric Paris <eparis@redhat.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Dave Chinner <david@fromorbit.com>,
	linux-fsdevel@vger.kernel.org,
	James Morris <james.l.morris@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	"Theodore Ts'o" <tytso@mit.edu>, stable <stable@vger.kernel.org>,
	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()
Date: Thu, 09 Jan 2014 18:37:06 -0500	[thread overview]
Message-ID: <1389310626.15209.92.camel@localhost> (raw)
In-Reply-To: <20140109182756.17abaaa8@gandalf.local.home>

On Thu, 2014-01-09 at 18:27 -0500, Steven Rostedt wrote:
> On Thu, 9 Jan 2014 18:25:23 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > On Fri, 10 Jan 2014 06:41:03 +0800
> > Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > 
> > > I think the sane short term fix is to make the kfree() of the i_security
> > > member be a rcu free, and not clear the member.
> > 
> > You mean my first patch?
> > 
> > https://lkml.org/lkml/2014/1/9/349
> > 
> 
> Oh wait, you said not to clear the member. Thus, the patch would look
> like this:
> 
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

SMACK also needs this change somehow in smack_inode_free_security()

but at least from an SELinux PoV, I think it's quick and easy, but wrong
for maintainability...

> Index: linux-trace.git/security/selinux/hooks.c
> ===================================================================
> --- linux-trace.git.orig/security/selinux/hooks.c
> +++ linux-trace.git/security/selinux/hooks.c
> @@ -234,6 +234,14 @@ static int inode_alloc_security(struct i
>  	return 0;
>  }
>  
> +static void inode_free_rcu(struct rcu_head *head)
> +{
> +	struct inode_security_struct *isec;
> +
> +	isec = container_of(head, struct inode_security_struct, rcu);
> +	kmem_cache_free(sel_inode_cache, isec);
> +}
> +
>  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);
>  }
>  
>  static int file_alloc_security(struct file *file)
> Index: linux-trace.git/security/selinux/include/objsec.h
> ===================================================================
> --- linux-trace.git.orig/security/selinux/include/objsec.h
> +++ linux-trace.git/security/selinux/include/objsec.h
> @@ -38,7 +38,10 @@ struct task_security_struct {
>  
>  struct inode_security_struct {
>  	struct inode *inode;	/* back pointer to inode object */
> -	struct list_head list;	/* list of inode_security_struct */
> +	union {
> +		struct list_head list;	/* list of inode_security_struct */
> +		struct rcu_head rcu;	/* for freeing the inode_security_struct */
> +	};
>  	u32 task_sid;		/* SID of creating task */
>  	u32 sid;		/* SID of this object */
>  	u16 sclass;		/* security class of this object */