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Date:   Wed, 6 May 2020 23:31:16 +0300
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     syzbot+e73ceacfd8560cc8a3ca@...kaller.appspotmail.com,
        syzbot+c2fb6f9ddcea95ba49b5@...kaller.appspotmail.com,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jay Vosburgh <j.vosburgh@...il.com>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [Patch net v2] net: fix a potential recursive NETDEV_FEAT_CHANGE

On 06/05/2020 22:46, 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
> 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: 17b85d29e82c ("net/core: revert "net: fix __netdev_update_features return.." and add comment")
> Reported-by: syzbot+e73ceacfd8560cc8a3ca@...kaller.appspotmail.com
> Reported-by: syzbot+c2fb6f9ddcea95ba49b5@...kaller.appspotmail.com
> Cc: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: Jay Vosburgh <j.vosburgh@...il.com>
> Cc: Jann Horn <jannh@...gle.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---

The patch looks good, but note that __netdev_update_features() used to return -1
before the commit in the Fixes tag above (between 6cb6a27c45ce and 00ee59271777).
It only restored that behaviour.

>  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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