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Date:   Wed, 6 May 2020 11:46:40 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jay Vosburgh <jay.vosburgh@...onical.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        syzbot <syzbot+e73ceacfd8560cc8a3ca@...kaller.appspotmail.com>,
        syzbot+c2fb6f9ddcea95ba49b5@...kaller.appspotmail.com,
        Jarod Wilson <jarod@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [Patch net] net: fix a potential recursive NETDEV_FEAT_CHANGE

On Tue, May 5, 2020 at 3:42 PM Jay Vosburgh <jay.vosburgh@...onical.com> wrote:
>
> Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> >syzbot managed to trigger a recursive NETDEV_FEAT_CHANGE event
> >between bonding master and slave. I managed to find a reproducer
> >for this:
> >
> >  ip li set bond0 up
> >  ifenslave bond0 eth0
> >  brctl addbr br0
> >  ethtool -K eth0 lro off
> >  brctl addif br0 bond0
> >  ip li set br0 up
>
>         Presumably this is tied to the LRO feature being special in
> netdev_sync_lower_features (via NETIF_F_UPPER_DISABLES), but why doesn't
> LRO become disabled and stop the recursion once the test
>
>                 if (!(features & feature) && (lower->features & feature)) {
>
>         no longer evalutes to true (in theory)?

Good point!

Actually the LRO feature fails to disable:

[   62.559537] netdevice: bond0: failed to disable 0x0000000000008000 on eth0!
...
[   78.312003] netdevice: eth0: failed to disable LRO!

It seems we should only skip netdev_update_features() for such case,
like below. Note __netdev_update_features() intentionally returns -1
for this failure, so I am afraid we just have to live with it.

diff --git a/net/core/dev.c b/net/core/dev.c
index 522288177bbd..8040b07214fa 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8907,11 +8907,13 @@ static void netdev_sync_lower_features(struct
net_device *upper,
                        netdev_dbg(upper, "Disabling feature %pNF on
lower dev %s.\n",
                                   &feature, lower->name);
                        lower->wanted_features &= ~feature;
-                       netdev_update_features(lower);
+                       __netdev_update_features(lower);

                        if (unlikely(lower->features & feature))
                                netdev_WARN(upper, "failed to disable
%pNF on %s!\n",
                                            &feature, lower->name);
+                       else
+                               netdev_update_features(lower);
                }
        }
 }

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