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 <redacted>
Suggested-by: Junvyyang, Tencent Zhuque Lab <redacted>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: LePremierHomme <redacted>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Willy Tarreau <w@1wt.eu>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
---
 net/rxrpc/call_accept.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/rxrpc/call_accept.c b/net/rxrpc/call_accept.c
index 7271977b1683..49fccee1a726 100644
--- a/net/rxrpc/call_accept.c
+++ b/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;