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

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Date:   Wed, 27 Oct 2021 20:54:11 +0800
From:   Jason Xing <kerneljasonxing@...il.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     David Miller <davem@...emloft.net>, kuba@...nel.org,
        alobakin@...me, jonathan.lemon@...il.com,
        Willem de Bruijn <willemb@...gle.com>, pabeni@...hat.com,
        vvs@...tuozzo.com, cong.wang@...edance.com,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jason Xing <xingwanli@...ishou.com>
Subject: Re: [PATCH net] net: gro: set the last skb->next to NULL when it get merged

On Wed, Oct 27, 2021 at 8:40 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2021/10/27 16:56, Jason Xing wrote:
> > On Wed, Oct 27, 2021 at 4:07 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> >>
> >> On Wed, Oct 27, 2021 at 3:23 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> >>>
> >>> On Tue, Oct 26, 2021 at 9:19 PM <kerneljasonxing@...il.com> wrote:
> >>>>
> >>>> From: Jason Xing <xingwanli@...ishou.com>
> >>>>
> >>>> 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@...ishou.com>
> >>>> ---
> >>>>  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
>

What you said is the @skb->list but not the list between skbs which is
connected by skb->next when the new incoming skb needs to get merged.
The @skb->list->next/prev is not the same as @skb->next.

> >
> > 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@gmail.com/
> >
> > Thanks again,
> > Jason
> >
> >>
> >>> it is really odd.
> >>>
> >>> Thanks,
> >>> Jason
> >>>
> >>>>         __skb_header_release(skb);
> >>>>         lp = p;
> >>>> --
> >>>> 1.8.3.1
> >>>>
> > .
> >