netdev - Re: [PATCH] netlink: enable skb header refcounting before sending first broadcast

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Date:	Fri, 10 Jul 2015 15:49:47 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH] netlink: enable skb header refcounting before sending
 first broadcast

On Fri, 2015-07-10 at 14:51 +0300, Konstantin Khlebnikov wrote:
> This fixes race between non-atomic updates of adjacent bit-fields:
> skb->cloned could be lost because netlink broadcast clones skb after
> sending it to the first listener who sets skb->peeked at the same skb.
> As a result atomic refcounting of skb header stays disabled and
> skb_release_data() frees it twice. Race leads to double-free in kmalloc-xxx.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> Fixes: b19372273164 ("net: reorganize sk_buff for faster __copy_skb_header()")
> ---
>  net/netlink/af_netlink.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index dea925388a5b..921e0d8dfe3a 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -2028,6 +2028,12 @@ int netlink_broadcast_filtered(struct sock *ssk, struct sk_buff *skb, u32 portid
>  	info.tx_filter = filter;
>  	info.tx_data = filter_data;
>  
> +	/* Enable atomic refcounting in skb_release_data() before first send:
> +	 * non-atomic set of that bit-field in __skb_clone() could race with
> +	 * __skb_recv_datagram() which touches the same set of bit-fields.
> +	 */
> +	skb->cloned = 1;
> +
>  	/* While we sleep in clone, do not allow to change socket list */
>  
>  	netlink_lock_table();

Wow, this is tricky.

I wonder how you found this bug ????

Acked-by: Eric Dumazet <edumazet@...gle.com>



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