Re: [PATCH v3] Bluetooth: call lock_sock() outside of spinlock section

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2021/07/22 13:47, LinMa wrote:
> Hi Tetsuo,
> 
> Just find out another interesting function: sock_owned_by_user(). (I am just a noob of kernel locks)
> 
> Hence I think the following patch has the same 'effect' as the old patch e305509e678b3 ("Bluetooth: use correct lock to prevent UAF of hdev object")
> 
> diff --git a/net/bluetooth/hci_sock.c b/net/bluetooth/hci_sock.c
> index b04a5a02ecf3..0cc4b88daa96 100644
> --- a/net/bluetooth/hci_sock.c
> +++ b/net/bluetooth/hci_sock.c
> @@ -762,7 +762,11 @@ void hci_sock_dev_event(struct hci_dev *hdev, int event)
>                 /* Detach sockets from device */
>                 read_lock(&hci_sk_list.lock);
>                 sk_for_each(sk, &hci_sk_list.head) {
> -                       lock_sock(sk);
> +                       bh_lock_sock_nested(sk);
> +busywait:
> +                       if (sock_owned_by_user(sk))
> +                               goto busywait;
> +
>                         if (hci_pi(sk)->hdev == hdev) {
>                                 hci_pi(sk)->hdev = NULL;
>                                 sk->sk_err = EPIPE;
> @@ -771,7 +775,7 @@ void hci_sock_dev_event(struct hci_dev *hdev, int event)
> 
>                                 hci_dev_put(hdev);
>                         }
> -                       release_sock(sk);
> +                       bh_unlock_sock(sk);
>                 }
>                 read_unlock(&hci_sk_list.lock);
>         }
> 
> The sad thing is that it seems will cost CPU resource to do meaningless wait...
> 
> What do you think? Can this sock_owned_by_user() function do any help?

I don't think it helps.

One of problems we are seeing (and my patch will fix) is a race window that
this sk_for_each() loop needs to wait using lock_sock() because
"hci_pi(sk)->hdev = hdev;" is called by hci_sock_bind() under lock_sock().
Doing hci_pi(sk)->hdev == hdev comparison without lock_sock() will lead to
failing to "/* Detach sockets from device */".