r8169: Fix possible ring buffer corruption on fragmented Tx packets.

[ Upstream commit c71e3a5c ]

An issue was found on the RTL8125b when transmitting small fragmented
packets, whereby invalid entries were inserted into the transmit ring
buffer, subsequently leading to calls to dma_unmap_single() with a null
address.

This was caused by rtl8169_start_xmit() not noticing changes to nr_frags
which may occur when small packets are padded (to work around hardware
quirks) in rtl8169_tso_csum_v2().

To fix this, postpone inspecting nr_frags until after any padding has been
applied.

Fixes: 9020845f ("r8169: improve rtl8169_start_xmit")
Cc: stable@vger.kernel.org
Signed-off-by: Ken Milmore <ken.milmore@gmail.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/27ead18b-c23d-4f49-a020-1fc482c5ac95@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>