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From:   Josh Elsasser <jelsasser@appneta.com>
To:     davem@davemloft.net
Cc:     Josh Elsasser <jelsasser@appneta.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Eric Dumazet <edumazet@google.com>,
        Sasha Levin <alexander.levin@verizon.com>,
        Willem de Bruijn <willemb@google.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Myungho Jung <mhjungk@gmail.com>,
        =?UTF-8?q?Michal=20Kube=C4=8Dek?= <mkubecek@suse.cz>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 0/1] net: avoid a kernel panic during sk_busy_loop
Date:   Sun, 11 Mar 2018 12:22:54 -0700
Message-Id: <20180311192322.101598-1-jelsasser@appneta.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi Dave,

I stumbled across a reproducible kernel panic while playing around with
busy_poll on a Linux 4.9.86 kernel. There's an unfortunate interaction
between init_dummy_netdev, which doesn't bother to fill in netdev_ops, and
sk_busy_loop, which assumes netdev_ops is a valid pointer.

To reproduce on the device under test (DUT), I did:

  $ ip addr show dev wlan0
  8: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq [...]
      inet 172.16.122.6/23 brd 172.16.123.255 scope global wlan0
  $ sysctl -w net.core.busy_read=50
  $ nc -l 172.16.122.6 5001

Then transmitted some data to this socket from a second host:

  $ echo "foo" | nc 172.16.122.6 5001

The DUT immediately hits a kernel panic.

I've attached a patch that applies cleanly to the 4.9.87 stable release.
This fix isn't necessary for net/net-next (ndo_busy_poll was removed in
linux-4.11), but a further backport of this commit is likely required for
any stable releases older than linux-4.5.

I hope this is the right way to raise something like this. I couldn't find
a clear answer from the -stable and netdev howtos for bugs against features
that no longer exist in mainline.

Thanks,
Josh