[PATCH] Fix the kernel panic of audit_filter_task when key field is set

When calling audit_filter_task(), it calls audit_filter_rules() with audit_context is NULL.
If the key field is set, the result in audit_filter_rules() will be set to 1 and
ctx->filterkey will be set to key.
But the ctx is NULL in this condition, so kernel will panic.

Signed-off-by: Zhang Xiliang <zhangxiliang@cn.fujitsu.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/auditsc.c

@@ -610,7 +610,7 @@ static int audit_filter_rules(struct task_struct *tsk,
 		if (!result)
 			return 0;
 	}
-	if (rule->filterkey)
+	if (rule->filterkey && ctx)
 		ctx->filterkey = kstrdup(rule->filterkey, GFP_ATOMIC);
 	switch (rule->action) {
 	case AUDIT_NEVER:    *state = AUDIT_DISABLED;	    break;