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

From: Konstantin Khlebnikov <[email-masked]>
Date: Fri, 10 Jul 2015 17:08:32 +0300

On 10.07.2015 16:49, Eric Dumazet wrote:
> 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 <[email-masked]>
>> 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 ????

In some setups race happens quite often: once or twice per hour.
I guess the main trigger was the openvswitch which generates a
lot of netlink traffic. Though debugging was a real pain.

>
> Acked-by: Eric Dumazet <[email-masked]>
>
>
>

-- 
Konstantin