SMACK: Free the i_security blob in inode using RCU

There is race condition issue while freeing the i_security blob in SMACK
module. There is existing condition where i_security can be freed while
inode_permission is called from path lookup on second CPU. There has been
observed the page fault with such condition. VFS code and Selinux module
takes care of this condition by freeing the inode and i_security field
using RCU via call_rcu(). But in SMACK directly the i_secuirty blob is
being freed. Use call_rcu() to fix this race condition issue.

Signed-off-by: Himanshu Shukla <himanshu.sh@samsung.com>
Signed-off-by: Vishal Goel <vishal.goel@samsung.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack.h

@@ -114,6 +114,7 @@ struct inode_smack {
 	struct smack_known	*smk_mmap;	/* label of the mmap domain */
 	struct mutex		smk_lock;	/* initialization lock */
 	int			smk_flags;	/* smack inode flags */
+	struct rcu_head         smk_rcu;	/* for freeing inode_smack */
 };
 
 struct task_smack {

security/smack/smack_lsm.c

@@ -1006,15 +1006,39 @@ static int smack_inode_alloc_security(struct inode *inode)
 }
 
 /**
- * smack_inode_free_security - free an inode blob
+ * smack_inode_free_rcu - Free inode_smack blob from cache
+ * @head: the rcu_head for getting inode_smack pointer
+ *
+ *  Call back function called from call_rcu() to free
+ *  the i_security blob pointer in inode
+ */
+static void smack_inode_free_rcu(struct rcu_head *head)
+{
+	struct inode_smack *issp;
+
+	issp = container_of(head, struct inode_smack, smk_rcu);
+	kmem_cache_free(smack_inode_cache, issp);
+}
+
+/**
+ * smack_inode_free_security - free an inode blob using call_rcu()
  * @inode: the inode with a blob
  *
- * Clears the blob pointer in inode
+ * Clears the blob pointer in inode using RCU
  */
 static void smack_inode_free_security(struct inode *inode)
 {
-	kmem_cache_free(smack_inode_cache, inode->i_security);
-	inode->i_security = NULL;
+	struct inode_smack *issp = inode->i_security;
+
+	/*
+	 * The inode may still be referenced in a path walk and
+	 * a call to smack_inode_permission() can be made
+	 * after smack_inode_free_security() is called.
+	 * To avoid race condition free the i_security via RCU
+	 * and leave the current inode->i_security pointer intact.
+	 * The inode will be freed after the RCU grace period too.
+	 */
+	call_rcu(&issp->smk_rcu, smack_inode_free_rcu);
 }
 
 /**