bpf: Fix NULL dereference in bpf_task_storage

In bpf_pid_task_storage_update_elem(), it missed to
test the !task_storage_ptr(task) which then could trigger a NULL
pointer exception in bpf_local_storage_update().

Fixes: 4cf1bc1 ("bpf: Implement task local storage")
Signed-off-by: Martin KaFai Lau <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Tested-by: Roman Gushchin <[email protected]>
Acked-by: KP Singh <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: iamkafai

kernel/bpf/bpf_task_storage.c

@@ -150,7 +150,7 @@ static int bpf_pid_task_storage_update_elem(struct bpf_map *map, void *key,
 	 */
 	WARN_ON_ONCE(!rcu_read_lock_held());
 	task = pid_task(pid, PIDTYPE_PID);
-	if (!task) {
+	if (!task || !task_storage_ptr(task)) {
 		err = -ENOENT;
 		goto out;
 	}