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Date:   Wed, 06 May 2020 13:49:41 -0700
From:   Jay Vosburgh <>
To:     Cong Wang <>
        Nikolay Aleksandrov <>,
        Josh Poimboeuf <>,
        Jann Horn <>
Subject: Re: [Patch net v2] net: fix a potential recursive NETDEV_FEAT_CHANGE

Cong Wang <> 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
>When a NETDEV_FEAT_CHANGE event is triggered on a bonding slave,
>it captures this and calls bond_compute_features() to fixup its
>master's and other slaves' features. However, when syncing with
>its lower devices by netdev_sync_lower_features() this event is
>triggered again on slaves when the LRO feature fails to change,
>so it goes back and forth recursively until the kernel stack is
>Commit 17b85d29e82c intentionally lets __netdev_update_features()
>return -1 for such a failure case, so we have to just rely on
>the existing check inside netdev_sync_lower_features() and skip
>NETDEV_FEAT_CHANGE event only for this specific failure case.
>Fixes: 17b85d29e82c ("net/core: revert "net: fix __netdev_update_features return.." and add comment")
>Cc: Nikolay Aleksandrov <>
>Cc: Josh Poimboeuf <>
>Cc: Jay Vosburgh <>
>Cc: Jann Horn <>
>Signed-off-by: Cong Wang <>

Reviewed-by: Jay Vosburgh <>

> net/core/dev.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>diff --git a/net/core/dev.c b/net/core/dev.c
>index 522288177bbd..6d327b7aa813 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_features_change(lower);
> 		}
> 	}
> }

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