[PATCH 4.4 059/312] mlxsw: pci: Correctly determine if descriptor queue is full

[Greg Kroah-Hartman]

From: Ido Schimmel <idosch@xxxxxxxxxxxx>

commit 5091730d7795ccb21eb880699b5194730641c70b upstream.

The descriptor queues for sending (SDQs) and receiving (RDQs) packets
are managed by two counters - producer and consumer - which are both
16-bit in size. A queue is considered full when the difference between
the two equals the queue's maximum number of descriptors.

However, if the producer counter overflows, then it's possible for the
full queue check to fail, as it doesn't take the overflow into account.
In such a case, descriptors already passed to the device - but for which
a completion has yet to be posted - will be overwritten, thereby causing
undefined behavior. The above can be achieved under heavy load (~30
netperf instances).

Fix that by casting the subtraction result to u16, preventing it from
being treated as a signed integer.

Fixes: eda6500a987a ("mlxsw: Add PCI bus implementation")
Signed-off-by: Ido Schimmel <idosch@xxxxxxxxxxxx>
Signed-off-by: Jiri Pirko <jiri@xxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/net/ethernet/mellanox/mlxsw/pci.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/net/ethernet/mellanox/mlxsw/pci.c
+++ b/drivers/net/ethernet/mellanox/mlxsw/pci.c
@@ -215,7 +215,7 @@ mlxsw_pci_queue_elem_info_producer_get(s
 {
 	int index = q->producer_counter & (q->count - 1);
 
-	if ((q->producer_counter - q->consumer_counter) == q->count)
+	if ((u16) (q->producer_counter - q->consumer_counter) == q->count)
 		return NULL;
 	return mlxsw_pci_queue_elem_info_get(q, index);
 }