Subject: Re: [PATCH net] rxrpc: Fix a race between socket set up and I/O thread creation

From: Simon Horman <[email-masked]>
Date: Wed, 25 Sep 2024 19:33:27 +0100

On Tue, Sep 24, 2024 at 11:08:01PM +0100, David Howells wrote:
> In rxrpc_open_socket(), it sets up the socket and then sets up the I/O
> thread that will handle it.  This is a problem, however, as there's a gap
> between the two phases in which a packet may come into rxrpc_encap_rcv()
> from the UDP packet but we oops when trying to wake the not-yet created I/O
> thread.
> 
> As a quick fix, just make rxrpc_encap_rcv() discard the packet if there's
> no I/O thread yet.
> 
> A better, but more intrusive fix would perhaps be to rearrange things such
> that the socket creation is done by the I/O thread.
> 
> Fixes: a275da62e8c1 ("rxrpc: Create a per-local endpoint receive queue and I/O thread")
> Signed-off-by: David Howells <[email-masked]>
> Reviewed-by: Eric Dumazet <[email-masked]>

...:wq

> diff --git a/net/rxrpc/io_thread.c b/net/rxrpc/io_thread.c
> index 0300baa9afcd..5c0a5374d51a 100644
> --- a/net/rxrpc/io_thread.c
> +++ b/net/rxrpc/io_thread.c
> @@ -27,8 +27,9 @@ int rxrpc_encap_rcv(struct sock *udp_sk, struct sk_buff *skb)
>  {
>  	struct sk_buff_head *rx_queue;
>  	struct rxrpc_local *local = rcu_dereference_sk_user_data(udp_sk);
> +	struct task_struct *io_thread = READ_ONCE(local->io_thread);

Hi David,

The line above dereferences local.
But the line below assumes that it may be NULL.
This seems inconsistent.

Flagged by Smatch.

>  
> -	if (unlikely(!local)) {
> +	if (unlikely(!local || !io_thread)) {
>  		kfree_skb(skb);
>  		return 0;
>  	}