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

[Paolo Abeni]

On 9/25/24 20:33, Simon Horman wrote:
> 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 <dhowells at redhat.com>
>> Reviewed-by: Eric Dumazet <edumazet at google.com>
> 
> ...: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.

sk->sk_user_data is cleared just before io_thread by rxrpc_io_thread(), 
I think accessing a NULL 'local' here should be possible.

@David, could you please respin addressing the above?

Thanks!

Paolo