rxrpc: Fix oops due to non-existence of prealloc backlog struct

If an AF_RXRPC service socket is opened and bound, but calls are
preallocated, then rxrpc_alloc_incoming_call() will oops because the
rxrpc_backlog struct doesn't get allocated until the first preallocation is
made.

Fix this by returning NULL from rxrpc_alloc_incoming_call() if there is no
backlog struct.  This will cause the incoming call to be aborted.

Reported-by: Junvyyang, Tencent Zhuque Lab <[email protected]>
Suggested-by: Junvyyang, Tencent Zhuque Lab <[email protected]>
Signed-off-by: David Howells <[email protected]>
cc: LePremierHomme <[email protected]>
cc: Marc Dionne <[email protected]>
cc: Willy Tarreau <[email protected]>
cc: Simon Horman <[email protected]>
cc: [email protected]
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: dhowells

net/rxrpc/call_accept.c

@@ -255,6 +255,9 @@ static struct rxrpc_call *rxrpc_alloc_incoming_call(struct rxrpc_sock *rx,
 	unsigned short call_tail, conn_tail, peer_tail;
 	unsigned short call_count, conn_count;
 
+	if (!b)
+		return NULL;
+
 	/* #calls >= #conns >= #peers must hold true. */
 	call_head = smp_load_acquire(&b->call_backlog_head);
 	call_tail = b->call_backlog_tail;