hw/net/smc91c111: Sanitize packet length on tx

hw/net/smc91c111: Sanitize packet length on tx

When the smc91c111 transmits a packet, it must read a control byte
which is at the end of the data area and CRC.  However, we don't
sanitize the length field in the packet buffer, so if the guest sets
the length field to something large we will try to read past the end
of the packet data buffer when we access the control byte.

As usual, the datasheet says nothing about the behaviour of the
hardware if the guest misprograms it in this way.  It says only that
the maximum valid length is 2048 bytes.  We choose to log the guest
error and silently drop the packet.

This requires us to factor out the "mark the tx packet as complete"
logic, so we can call it for this "drop packet" case as well as at
the end of the loop when we send a valid packet.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2742
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250228174802.1945417-3-peter.maydell@linaro.org>
[PMD: Update smc91c111_do_tx() as len > MAX_PACKET_SIZE]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>