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

On Wed, Oct 27, 2021 at 3:23 PM Jason Xing [off-list ref] wrote:

On Tue, Oct 26, 2021 at 9:19 PM [off-list ref] wrote:

quoted

From: Jason Xing <redacted>

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 <redacted>
---
 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 is really odd.

Thanks,
Jason

quoted

        __skb_header_release(skb);
        lp = p;
--
1.8.3.1