share_pool: Check tsk->mm before use it

ascend inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4EUVI
CVE: NA

-------------------

The user could give a pid of daemon process when add task to group. That
daemon process has no mm_struct so we should check it before use.
Signed-off-by: NWang Wensheng <wangwensheng4@huawei.com>
Reviewed-by: NDing Tianhong <dingtianhong@huawei.com>
Reviewed-by: NTang Yizhou <tangyizhou@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NWeilong Chen <chenweilong@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

mm/share_pool.c

@@ -549,8 +549,6 @@ int sp_group_add_task(int pid, int spg_id)
 	tsk = find_task_by_vpid(pid);
 	if (!tsk || (tsk->flags & PF_EXITING))
 		ret = -ESRCH;
-	else if (tsk->mm->sp_group)	/* if it's already in a sp_group */
-		ret = -EEXIST;
 	else
 		get_task_struct(tsk);
 
@@ -560,6 +558,11 @@ int sp_group_add_task(int pid, int spg_id)
 		goto out_unlock;
 	}
 
+	if (!tsk->mm || tsk->mm->sp_group) {	/* if it's already in a sp_group */
+		ret = -EEXIST;
+		goto out_unlock;
+	}
+
 	spg = find_or_alloc_sp_group(spg_id);
 	if (IS_ERR(spg)) {
 		ret = PTR_ERR(spg);