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

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Date:	Thu, 9 Jan 2014 18:27:56 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	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 18:25:23 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 10 Jan 2014 06:41:03 +0800
> Linus Torvalds <torvalds@...ux-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@...dmis.org>

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 */
--
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