Re: [PATCH v3 1/3] kernel/notifier.c: avoid duplicate registration

[Vasily Averin]

On 7/10/19 6:09 AM, Xiaoming Ni wrote:
> Registering the same notifier to a hook repeatedly can cause the hook
> list to form a ring or lose other members of the list.
> 
> case1: An infinite loop in notifier_chain_register() can cause soft lockup
>         atomic_notifier_chain_register(&test_notifier_list, &test1);
>         atomic_notifier_chain_register(&test_notifier_list, &test1);
>         atomic_notifier_chain_register(&test_notifier_list, &test2);
> 
> case2: An infinite loop in notifier_chain_register() can cause soft lockup
>         atomic_notifier_chain_register(&test_notifier_list, &test1);
>         atomic_notifier_chain_register(&test_notifier_list, &test1);
>         atomic_notifier_call_chain(&test_notifier_list, 0, NULL);
> 
> case3: lose other hook test2
>         atomic_notifier_chain_register(&test_notifier_list, &test1);
>         atomic_notifier_chain_register(&test_notifier_list, &test2);
>         atomic_notifier_chain_register(&test_notifier_list, &test1);
> 
> case4: Unregister returns 0, but the hook is still in the linked list,
>         and it is not really registered. If you call notifier_call_chain
>         after ko is unloaded, it will trigger oops.
> 
> If the system is configured with softlockup_panic and the same
> hook is repeatedly registered on the panic_notifier_list, it
> will cause a loop panic.
> 
> Add a check in notifier_chain_register() to avoid duplicate registration
> 
> Signed-off-by: Xiaoming Ni <nixiaoming@xxxxxxxxxx>
Reviewed-by: Vasily Averin <vvs@xxxxxxxxxxxxx>
> ---
>  kernel/notifier.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/notifier.c b/kernel/notifier.c
> index d9f5081..30bedb8 100644
> --- a/kernel/notifier.c
> +++ b/kernel/notifier.c
> @@ -23,7 +23,10 @@ static int notifier_chain_register(struct notifier_block **nl,
>  		struct notifier_block *n)
>  {
>  	while ((*nl) != NULL) {
> -		WARN_ONCE(((*nl) == n), "double register detected");
> +		if (unlikely((*nl) == n)) {
> +			WARN(1, "double register detected");
> +			return 0;
> +		}
>  		if (n->priority > (*nl)->priority)
>  			break;
>  		nl = &((*nl)->next);
>