rxrpc: Fix call timer start racing with call destruction

The rxrpc_call struct has a timer used to handle various timed events
relating to a call. This timer can get started from the packet input
routines that are run in softirq mode with just the RCU read lock held.
Unfortunately, because only the RCU read lock is held - and neither ref or
other lock is taken - the call can start getting destroyed at the same time
a packet comes in addressed to that call. This causes the timer - which
was already stopped - to get restarted. Later, the timer dispatch code may
then oops if the timer got deallocated first.

Fix this by trying to take a ref on the rxrpc_call struct and, if
successful, passing that ref along to the timer. If the timer was already
running, the ref is discarded.

The timer completion routine can then pass the ref along to the call's work
item when it queues it. If the timer or work item where already
queued/running, the extra ref is discarded.

Fixes: a158bdd3247b ("rxrpc: Fix call timeouts")
Reported-by: Marc Dionne <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Reviewed-by: Marc Dionne <[e-mail]>
Tested-by: Marc Dionne <[e-mail]>
cc: [e-mail]
Link: http://lists.infradead.org/pipermail/linux-afs/2022-March/005073.html
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

include/trace/events/rxrpc.h

···
 	rxrpc_call_error,
 	rxrpc_call_got,
 	rxrpc_call_got_kernel,
+	rxrpc_call_got_timer,
 	rxrpc_call_got_userid,
 	rxrpc_call_new_client,
 	rxrpc_call_new_service,
 	rxrpc_call_put,
 	rxrpc_call_put_kernel,
 	rxrpc_call_put_noqueue,
+	rxrpc_call_put_notimer,
+	rxrpc_call_put_timer,
 	rxrpc_call_put_userid,
 	rxrpc_call_queued,
 	rxrpc_call_queued_ref,
···
 	EM(rxrpc_call_error,			"*E*") \
 	EM(rxrpc_call_got,			"GOT") \
 	EM(rxrpc_call_got_kernel,		"Gke") \
+	EM(rxrpc_call_got_timer,		"GTM") \
 	EM(rxrpc_call_got_userid,		"Gus") \
 	EM(rxrpc_call_new_client,		"NWc") \
 	EM(rxrpc_call_new_service,		"NWs") \
 	EM(rxrpc_call_put,			"PUT") \
 	EM(rxrpc_call_put_kernel,		"Pke") \
-	EM(rxrpc_call_put_noqueue,		"PNQ") \
+	EM(rxrpc_call_put_noqueue,		"PnQ") \
+	EM(rxrpc_call_put_notimer,		"PnT") \
+	EM(rxrpc_call_put_timer,		"PTM") \
 	EM(rxrpc_call_put_userid,		"Pus") \
 	EM(rxrpc_call_queued,			"QUE") \
 	EM(rxrpc_call_queued_ref,		"QUR") \

net/rxrpc/ar-internal.h

···
 		       enum rxrpc_propose_ack_trace);
 void rxrpc_process_call(struct work_struct *);
 
-static inline void rxrpc_reduce_call_timer(struct rxrpc_call *call,
-					   unsigned long expire_at,
-					   unsigned long now,
-					   enum rxrpc_timer_trace why)
-{
-	trace_rxrpc_timer(call, why, now);
-	timer_reduce(&call->timer, expire_at);
-}
+void rxrpc_reduce_call_timer(struct rxrpc_call *call,
+			     unsigned long expire_at,
+			     unsigned long now,
+			     enum rxrpc_timer_trace why);
+
+void rxrpc_delete_call_timer(struct rxrpc_call *call);
 
 /*
  * call_object.c
···
 bool __rxrpc_queue_call(struct rxrpc_call *);
 bool rxrpc_queue_call(struct rxrpc_call *);
 void rxrpc_see_call(struct rxrpc_call *);
+bool rxrpc_try_get_call(struct rxrpc_call *call, enum rxrpc_call_trace op);
 void rxrpc_get_call(struct rxrpc_call *, enum rxrpc_call_trace);
 void rxrpc_put_call(struct rxrpc_call *, enum rxrpc_call_trace);
 void rxrpc_cleanup_call(struct rxrpc_call *);

net/rxrpc/call_event.c

···
 	}
 
 	if (call->state == RXRPC_CALL_COMPLETE) {
-		del_timer_sync(&call->timer);
+		rxrpc_delete_call_timer(call);
 		goto out_put;
 	}
 

net/rxrpc/call_object.c

···
 
 	if (call->state < RXRPC_CALL_COMPLETE) {
 		trace_rxrpc_timer(call, rxrpc_timer_expired, jiffies);
-		rxrpc_queue_call(call);
+		__rxrpc_queue_call(call);
+	} else {
+		rxrpc_put_call(call, rxrpc_call_put);
 	}
+}
+
+void rxrpc_reduce_call_timer(struct rxrpc_call *call,
+			     unsigned long expire_at,
+			     unsigned long now,
+			     enum rxrpc_timer_trace why)
+{
+	if (rxrpc_try_get_call(call, rxrpc_call_got_timer)) {
+		trace_rxrpc_timer(call, why, now);
+		if (timer_reduce(&call->timer, expire_at))
+			rxrpc_put_call(call, rxrpc_call_put_notimer);
+	}
+}
+
+void rxrpc_delete_call_timer(struct rxrpc_call *call)
+{
+	if (del_timer_sync(&call->timer))
+		rxrpc_put_call(call, rxrpc_call_put_timer);
 }
 
 static struct lock_class_key rxrpc_call_user_mutex_lock_class_key;
···
 	}
 }
 
+bool rxrpc_try_get_call(struct rxrpc_call *call, enum rxrpc_call_trace op)
+{
+	const void *here = __builtin_return_address(0);
+	int n = atomic_fetch_add_unless(&call->usage, 1, 0);
+
+	if (n == 0)
+		return false;
+	trace_rxrpc_call(call->debug_id, op, n, here, NULL);
+	return true;
+}
+
 /*
  * Note the addition of a ref on a call.
  */
···
 	spin_unlock_bh(&call->lock);
 
 	rxrpc_put_call_slot(call);
-
-	del_timer_sync(&call->timer);
+	rxrpc_delete_call_timer(call);
 
 	/* Make sure we don't get any more notifications */
 	write_lock_bh(&rx->recvmsg_lock);
···
 	struct rxrpc_call *call = container_of(work, struct rxrpc_call, processor);
 	struct rxrpc_net *rxnet = call->rxnet;
 
+	rxrpc_delete_call_timer(call);
+
 	rxrpc_put_connection(call->conn);
 	rxrpc_put_peer(call->peer);
 	kfree(call->rxtx_buffer);
···
 	_net("DESTROY CALL %d", call->debug_id);
 
 	memset(&call->sock_node, 0xcd, sizeof(call->sock_node));
-
-	del_timer_sync(&call->timer);
 
 	ASSERTCMP(call->state, ==, RXRPC_CALL_COMPLETE);
 	ASSERT(test_bit(RXRPC_CALL_RELEASED, &call->flags));