netdev - [PATCH net 0/5] net: xdp: execute xdp_do_flush() before napi_complete

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Message-Id: <20230117092533.5804-1-magnus.karlsson@gmail.com>
Date:   Tue, 17 Jan 2023 10:25:28 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     magnus.karlsson@...el.com, bjorn@...nel.org, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        jonathan.lemon@...il.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, toke@...hat.com, pabeni@...hat.com,
        davem@...emloft.net, aelior@...vell.com, manishc@...vell.com,
        horatiu.vultur@...rochip.com, UNGLinuxDriver@...rochip.com,
        mst@...hat.com, jasowang@...hat.com, ioana.ciornei@....com,
        madalin.bucur@....com
Cc:     Magnus Karlsson <magnus.karlsson@...il.com>, bpf@...r.kernel.org
Subject: [PATCH net 0/5] net: xdp: execute xdp_do_flush() before napi_complete_done()

Make sure that xdp_do_flush() is always executed before
napi_complete_done(). This is important for two reasons. First, a
redirect to an XSKMAP assumes that a call to xdp_do_redirect() from
napi context X on CPU Y will be follwed by a xdp_do_flush() from the
same napi context and CPU. This is not guaranteed if the
napi_complete_done() is executed before xdp_do_flush(), as it tells
the napi logic that it is fine to schedule napi context X on another
CPU. Details from a production system triggering this bug using the
veth driver can be found in [1].

The second reason is that the XDP_REDIRECT logic in itself relies on
being inside a single NAPI instance through to the xdp_do_flush() call
for RCU protection of all in-kernel data structures. Details can be
found in [2].

The drivers have only been compile-tested since I do not own any of
the HW below. So if you are a manintainer, please make sure I did not
mess something up. This is a lousy excuse for virtio-net though, but
it should be much simpler for the vitio-net maintainers to test this,
than me trying to find test cases, validation suites, instantiating a
good setup, etc. Michael and Jason can likely do this in minutes.

Note that these were the drivers I found that violated the ordering by
running a simple script and manually checking the ones that came up as
potential offenders. But the script was not perfect in any way. There
might still be offenders out there, since the script can generate
false negatives.

[1] https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
[2] https://lore.kernel.org/all/20210624160609.292325-1-toke@redhat.com/

Thanks: Magnus

Magnus Karlsson (5):
  qede: execute xdp_do_flush() before napi_complete_done()
  lan966x: execute xdp_do_flush() before napi_complete_done()
  virtio-net: execute xdp_do_flush() before napi_complete_done()
  dpaa_eth: execute xdp_do_flush() before napi_complete_done()
  dpaa2-eth: execute xdp_do_flush() before napi_complete_done()

 drivers/net/ethernet/freescale/dpaa/dpaa_eth.c        | 6 +++---
 drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c      | 9 ++++++---
 drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c | 6 +++---
 drivers/net/ethernet/qlogic/qede/qede_fp.c            | 7 ++++---
 drivers/net/virtio_net.c                              | 6 +++---
 5 files changed, 19 insertions(+), 15 deletions(-)

base-commit: 87b93b678e95c7d93fe6a55b0e0fbda26d8c7760
--
2.34.1