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

[Jason Xing]

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.

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
> > >