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[0/3] rndis_host: handle bogus MAC addresses in ZTE RNDIS devices

Message ID 20220407001926.11252-1-lech.perczak@gmail.com
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Series rndis_host: handle bogus MAC addresses in ZTE RNDIS devices | expand

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Lech Perczak April 7, 2022, 12:19 a.m. UTC 
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When porting support of ZTE MF286R to OpenWrt [1], it was discovered,
that its built-in LTE modem fails to adjust its target MAC address,
when a random MAC address is assigned to the interface, due to detection of
"locally-administered address" bit. This leads to dropping of ingress
trafficat the host. The modem uses RNDIS as its primary interface,
with some variants exposing both of them simultaneously.

Then it was discovered, that cdc_ether driver contains a fixup for that
exact issue, also appearing on CDC ECM interfaces.
I discussed how to proceed with that with Bjørn Mork at OpenWrt forum [3],
with the first approach would be to trust the locally-administered MAC
again, and add a quirk for the problematic ZTE devices, as suggested by
Kristian Evensen. before [4], but reusing the fixup from cdc_ether looks
like a safer and more generic solution.

Finally, according to Bjørn's suggestion. limit the scope of bogus MAC
addressdetection to ZTE devices, the same way as it is done in cdc_ether,
as this trait wasn't really observed outside of ZTE devices.
Do that for both flavours of RNDIS devices, with interface classes
02/02/ff and e0/01/03, as both types are reported by different modems.

[1] https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=7ac8da00609f42b8aba74b7efc6b0d055b7cef3e
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bfe9b9d2df669a57a95d641ed46eb018e204c6ce
[3] https://forum.openwrt.org/t/problem-with-modem-in-zte-mf286r/120988
[4] https://lore.kernel.org/all/CAKfDRXhDp3heiD75Lat7cr1JmY-kaJ-MS0tt7QXX=s8RFjbpUQ@mail.gmail.com/T/

Cc: Kristian Evensen <kristian.evensen@gmail.com>
Cc: Bjørn Mork <bjorn@mork.no>
Cc: Oliver Neukum <oliver@neukum.org>

Lech Perczak (3):
  cdc_ether: export usbnet_cdc_zte_rx_fixup
  rndis_host: enable the bogus MAC fixup for ZTE devices from cdc_ether
  rndis_host: limit scope of bogus MAC address detection to ZTE devices

 drivers/net/usb/cdc_ether.c  |  3 ++-
 drivers/net/usb/rndis_host.c | 39 ++++++++++++++++++++++++++++++++----
 include/linux/usb/usbnet.h   |  1 +
 3 files changed, 38 insertions(+), 5 deletions(-)

Comments

Bjørn Mork April 7, 2022, 6:43 a.m. UTC | #1 
Lech Perczak <lech.perczak@gmail.com> writes:

> Reporting of bogus MAC addresses and ignoring configuration of new
> destination address wasn't observed outside of a range of ZTE devices,
> among which this seems to be the common bug. Align rndis_host driver
> with implementation found in cdc_ether, which also limits this workaround
> to ZTE devices.

Reviewed-by: Bjørn Mork <bjorn@mork.no>

Yes, this is a much better solution.

We have no business rejecting the address chosen by the device, even if
it is "locally administered".  The device has every right to use a local
address on a link with no other devices, which is the case for every
cellular modem for example.

And even if we believe the device is wrong there isn't much we can do
about that.

Rejecting the device address, with no way to inform the device about a
new address, implies that host and device disagrees about it.  This does
not fix anything.  It just makes the host to silentlig drop all packets,
leaving the user with a non-working device.

I take full responibility for coming up with the idea of over-
simplifying the original workaround proposed by Kristian. It wasn't very
well thought over.

Thanks to Lech for fixing this!



Bjørn