net: Disable NETIF_F_HW_TLS_TX when HW_CSUM is disabled

With NETIF_F_HW_TLS_TX packets are encrypted in HW. This cannot be
logically done when HW_CSUM offload is off.

Fixes: 2342a851 ("net: Add TLS TX offload features")
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Boris Pismenny <borisp@nvidia.com>
Link: https://lore.kernel.org/r/20201213143929.26253-1-tariqt@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/tls-offload.rst

@@ -524,7 +524,13 @@ on TCP retransmissions to handle corner cases is not acceptable.
 TLS device features
 -------------------
 
-Drivers should ignore the changes to TLS the device feature flags.
+Drivers should ignore the changes to the TLS device feature flags.
 These flags will be acted upon accordingly by the core ``ktls`` code.
 TLS device feature flags only control adding of new TLS connection
 offloads, old connections will remain active after flags are cleared.
+
+TLS encryption cannot be offloaded to devices without checksum calculation
+offload. Hence, TLS TX device feature flag requires NETIF_F_HW_CSUM being set.
+Disabling the latter implies clearing the former. Disabling TX checksum offload
+should not affect old connections, and drivers should make sure checksum
+calculation does not break for them.

net/core/dev.c

@@ -9643,6 +9643,11 @@ static netdev_features_t netdev_fix_features(struct net_device *dev,
 		}
 	}
 
+	if ((features & NETIF_F_HW_TLS_TX) && !(features & NETIF_F_HW_CSUM)) {
+		netdev_dbg(dev, "Dropping TLS TX HW offload feature since no CSUM feature.\n");
+		features &= ~NETIF_F_HW_TLS_TX;
+	}
+
 	return features;
 }