netdev - Re: [PATCH net] ipv4: fix suspicious RCU usage in fib_dump_info

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Date:   Wed, 26 Jun 2019 12:28:36 +0200
From:   Stefano Brivio <sbrivio@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        David Ahern <dsahern@...il.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ipv4: fix suspicious RCU usage in
 fib_dump_info_fnhe()

On Wed, 26 Jun 2019 03:04:50 -0700
Eric Dumazet <edumazet@...gle.com> wrote:

> sysbot reported that we lack appropriate rcu_read_lock()
> protection in fib_dump_info_fnhe()

Thanks for fixing this.

> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 6aee412a68bdd3c24a6a0eb9883e04b7a83998e0..59670fafcd2612b94c237cbe30109adb196cf3f0 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -2872,12 +2872,13 @@ int fib_dump_info_fnhe(struct sk_buff *skb, struct netlink_callback *cb,
>  		if (nhc->nhc_flags & RTNH_F_DEAD)
>  			continue;
>  
> +		rcu_read_lock();
>  		bucket = rcu_dereference(nhc->nhc_exceptions);
> -		if (!bucket)
> -			continue;
> -
> -		err = fnhe_dump_bucket(net, skb, cb, table_id, bucket, genid,
> -				       fa_index, fa_start);
> +		err = 0;

Could you perhaps move declaration and initialisation of 'err' outside
the block while at it? It looks a bit more readable at this point.

> +		if (bucket)
> +			err = fnhe_dump_bucket(net, skb, cb, table_id, bucket,
> +					       genid, fa_index, fa_start);
> +		rcu_read_unlock();
>  		if (err)
>  			return err;
>  	}

Either way,

Reviewed-by: Stefano Brivio <sbrivio@...hat.com>

-- 
Stefano