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Date:   Thu, 07 May 2020 18:19:30 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     xiyou.wangcong@...il.com
Cc:     netdev@...r.kernel.org,
        syzbot+e73ceacfd8560cc8a3ca@...kaller.appspotmail.com,
        syzbot+c2fb6f9ddcea95ba49b5@...kaller.appspotmail.com,
        jarod@...hat.com, nikolay@...ulusnetworks.com, jpoimboe@...hat.com,
        jannh@...gle.com, jay.vosburgh@...onical.com
Subject: Re: [Patch net v3] net: fix a potential recursive
 NETDEV_FEAT_CHANGE

From: Cong Wang <xiyou.wangcong@...il.com>
Date: Thu,  7 May 2020 12:19:03 -0700

> 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
> exhausted.
> 
> 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: fd867d51f889 ("net/core: generic support for disabling netdev features down stack")
> Reported-by: syzbot+e73ceacfd8560cc8a3ca@...kaller.appspotmail.com
> Reported-by: syzbot+c2fb6f9ddcea95ba49b5@...kaller.appspotmail.com
> Cc: Jarod Wilson <jarod@...hat.com>
> Cc: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: Jann Horn <jannh@...gle.com>
> Reviewed-by: Jay Vosburgh <jay.vosburgh@...onical.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>

Applied and queued up for -stable, thanks Cong.

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