Re: [RFC net-next] net: guard drivers against shared skbs

[Eric Dumazet <eric.dumazet@gmail.com>]

On 11/15/21 9:35 AM, Jakub Kicinski wrote:
> On Mon, 15 Nov 2021 08:56:10 -0800 Eric Dumazet wrote:
>> On 11/15/21 8:32 AM, Jakub Kicinski wrote:
>>> Commit d8873315065f ("net: add IFF_SKB_TX_SHARED flag to priv_flags")
>>> introduced IFF_SKB_TX_SHARED to protect drivers which are not ready
>>> for getting shared skbs from pktgen sending such frames.
>>>
>>> Some drivers dutifully clear the flag but most don't, even though
>>> they modify the skb or call skb helpers which expect private skbs.
>>>
>>> syzbot has also discovered more sources of shared skbs than just
>>> pktgen (e.g. llc).
>>>
>>> I think defaulting to opt-in is doing more harm than good, those
>>> who care about fast pktgen should inspect their drivers and opt-in.
>>> It's far too risky to enable this flag in ether_setup().
> 
>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>> index 15ac064b5562..476a826bb4f0 100644
>>> --- a/net/core/dev.c
>>> +++ b/net/core/dev.c
>>> @@ -3661,6 +3661,10 @@ static struct sk_buff *validate_xmit_skb(struct sk_buff *skb, struct net_device
>>>  	if (unlikely(!skb))
>>>  		goto out_null;
>>>  
>>> +	if (unlikely(skb_shared(skb)) &&
>>> +	    !(dev->priv_flags & IFF_TX_SKB_SHARING))
>>> +		goto out_kfree_skb;  
>>
>> So this will break llc, right ?
> 
> Likely. I haven't checked why LLC thinks it's a good idea to send
> shared skbs, probably convenience.
> 
>> I am sad we are adding so much tests in fast path.
> 
> What's our general stance on shared skbs in the Tx path? If we think
> that it's okay maybe it's time to turn the BUG_ON(shared_skb)s in pskb
> functions into return -EINVALs?

Yes, I think that a WARN_ON_ONCE() should be enough to keep syzbot reports
from alerting us, while not crashing regular hosts.

> 
> The IFF_TX_SKB_SHARING flag is pretty toothless as it stands.
> 

skb_padto() needs to be replaced by something better.
so that skb can be cloned if needed.


static inline int skb_padto(struct sk_buff *skb, unsigned int len)

->

static inline struct sk_buff *skb_padto(struct sk_buff *skb, unsigned int len)
{
}