Re: [PATCH net] net: gro: set the last skb->next to NULL when it get merged

[Yunsheng Lin]

On 2021/10/27 16:56, Jason Xing wrote:
> On Wed, Oct 27, 2021 at 4:07 PM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
>>
>> On Wed, Oct 27, 2021 at 3:23 PM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
>>>
>>> On Tue, Oct 26, 2021 at 9:19 PM <kerneljasonxing@xxxxxxxxx> wrote:
>>>>
>>>> From: Jason Xing <xingwanli@xxxxxxxxxxxx>
>>>>
>>>> Setting the @next of the last skb to NULL to prevent the panic in future
>>>> when someone does something to the last of the gro list but its @next is
>>>> invalid.
>>>>
>>>> For example, without the fix (commit: ece23711dd95), a panic could happen
>>>> with the clsact loaded when skb is redirected and then validated in
>>>> validate_xmit_skb_list() which could access the error addr of the @next
>>>> of the last skb. Thus, "general protection fault" would appear after that.
>>>>
>>>> Signed-off-by: Jason Xing <xingwanli@xxxxxxxxxxxx>
>>>> ---
>>>>  net/core/skbuff.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>>> index 2170bea..7b248f1 100644
>>>> --- a/net/core/skbuff.c
>>>> +++ b/net/core/skbuff.c
>>>> @@ -4396,6 +4396,7 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
>>>>                 skb_shinfo(p)->frag_list = skb;
>>>>         else
>>>>                 NAPI_GRO_CB(p)->last->next = skb;
>>>> +       skb->next = NULL;
>>>>         NAPI_GRO_CB(p)->last = skb;
>>>
>>> Besides, I'm a little bit confused that this operation inserts the
>>> newest skb into the tail of the flow, so the tail of flow is the
>>> newest, head oldest. The patch (commit: 600adc18) introduces the flush
>>> of the oldest when the flow is full to lower the latency, but actually
>>> it fetches the tail of the flow. Do I get something wrong here? I feel
>>
>> I have to update this part. The commit 600adc18 evicts and flushes the
>> oldest flow. But for the current kernel, when
>> "napi->gro_hash[hash].count >= MAX_GRO_SKBS" happens, the
>> gro_flush_oldest() flushes the oldest skb of one certain flow,
>> actually it is the newest skb because it is at the end of the list.

it seems the below is more matched with the gro_flush_oldest() instead
of the above code block:
https://elixir.bootlin.com/linux/v5.15-rc3/source/net/core/dev.c#L6118

> 
> I just submitted another patch to explain how it happens, please help
> me review both patches.
> 
> Link: https://lore.kernel.org/lkml/20211027084944.4508-1-kerneljasonxing@xxxxxxxxx/
> 
> Thanks again,
> Jason
> 
>>
>>> it is really odd.
>>>
>>> Thanks,
>>> Jason
>>>
>>>>         __skb_header_release(skb);
>>>>         lp = p;
>>>> --
>>>> 1.8.3.1
>>>>
> .
>