net: gro: Fix nesting of TCP GSO SKBs in skb_gro_receive_list()

Fraglist GRO and hardware GRO can create an fraglist of
HW-GRO packets. This cannot be segmented back into
the original form on TCP tethering scenario.

Avoid constructing such a GSO packet, by flushing an already
built fraglist GRO packet if a hardware GRO packet arrives.

Scenario (Tethering/Forwarding):
1.Driver submits a single TCP packet, P1. P1 is kept in the
gro_list as the first packet.

2. The driver submits a TCP GSO skb, P2. P2 has already aggregated
multiple TCP packets by HW_GRO, and its non-linear data is stored in
frags[].

3. P1 and P2 match the GRO rules, and since there is no local socket,
they are aggregated by skb_gro_receive_list(). The resulting skb,
P3, has a frag_list entry that still contains frags[]:
P3: [ Linear Data ] -> frag_list -> [ Linear Data ]
                                    [ frag[1] ]
                                    [ frag[2] ]
                                    ...
4. Later, tcp4_gso_segment() or tcp6_gso_segment() calls
skb_segment_list() to segment P3. However, skb_segment_list() only
segments the entries in frag_list. It does not segment the frags[]
inside P2, so P3 is not restored to the original packets, which leads
to IP fragmentation or packet drop in the following path.

Check skb_is_gso(skb) and current GRO method, make sure fraglist GRO
applies to consecutive non-GSO skb, others adopt regular GRO path.

Fixes: 8d95dc474f85 ("net: add code for TCP fraglist GRO")
Signed-off-by: Zhaoping Shu <zhaoping.shu@mediatek.com>
Signed-off-by: HW He <hw.he@mediatek.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260901082312.14596-1-zhaoping.shu@mediatek.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

2 files changed, 29 insertions, 8 deletions

diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
index 3b1fdcd3cb29..e74d99ca9fac 100644
--- a/net/ipv4/tcp_offload.c
+++ b/net/ipv4/tcp_offload.c
@@ -332,6 +332,7 @@ struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb,
 		flush |= skb->ip_summed != p->ip_summed;
 		flush |= skb->csum_level != p->csum_level;
 		flush |= NAPI_GRO_CB(p)->count >= 64;
+		flush |= NAPI_GRO_CB(p)->is_flist != NAPI_GRO_CB(skb)->is_flist;
 		skb_set_network_header(skb, skb_gro_receive_network_offset(skb));
 
 		if (flush || skb_gro_receive_list(p, skb))
@@ -395,12 +396,20 @@ static void tcp4_check_fraglist_gro(struct list_head *head, struct sk_buff *skb,
 	struct net *net;
 	int iif, sdif;
 
-	if (likely(!(skb->dev->features & NETIF_F_GRO_FRAGLIST)))
-		return;
-
 	p = tcp_gro_lookup(head, th);
 	if (p) {
-		NAPI_GRO_CB(skb)->is_flist = NAPI_GRO_CB(p)->is_flist;
+		/* flist GRO applies to consecutive non-GSO skbs */
+		if (!skb_is_gso(skb) || !NAPI_GRO_CB(p)->is_flist) {
+			NAPI_GRO_CB(skb)->is_flist = NAPI_GRO_CB(p)->is_flist;
+			return;
+		}
+
+		/* Fall back to the regular GRO path */
+		if (NAPI_GRO_CB(p)->count == 1)
+			NAPI_GRO_CB(p)->is_flist = 0;
+
+		NAPI_GRO_CB(skb)->is_flist = 0;
+
 		return;
 	}
 
@@ -410,7 +419,7 @@ static void tcp4_check_fraglist_gro(struct list_head *head, struct sk_buff *skb,
 	sk = __inet_lookup_established(net, iph->saddr, th->source,
 				       iph->daddr, ntohs(th->dest),
 				       iif, sdif);
-	NAPI_GRO_CB(skb)->is_flist = !sk;
+	NAPI_GRO_CB(skb)->is_flist = !sk && !skb_is_gso(skb);
 	if (sk)
 		sock_gen_put(sk);
 }
@@ -430,7 +439,8 @@ struct sk_buff *tcp4_gro_receive(struct list_head *head, struct sk_buff *skb)
 	if (!th)
 		goto flush;
 
-	tcp4_check_fraglist_gro(head, skb, th);
+	if (unlikely(skb->dev->features & NETIF_F_GRO_FRAGLIST))
+		tcp4_check_fraglist_gro(head, skb, th);
 
 	return tcp_gro_receive(head, skb, th);
 
diff --git a/net/ipv6/tcpv6_offload.c b/net/ipv6/tcpv6_offload.c
index f2a659cd6183..eec3778855eb 100644
--- a/net/ipv6/tcpv6_offload.c
+++ b/net/ipv6/tcpv6_offload.c
@@ -26,7 +26,18 @@ static void tcp6_check_fraglist_gro(struct list_head *head, struct sk_buff *skb,
 
 	p = tcp_gro_lookup(head, th);
 	if (p) {
-		NAPI_GRO_CB(skb)->is_flist = NAPI_GRO_CB(p)->is_flist;
+		/* flist GRO applies to consecutive non-GSO skbs */
+		if (!skb_is_gso(skb) || !NAPI_GRO_CB(p)->is_flist) {
+			NAPI_GRO_CB(skb)->is_flist = NAPI_GRO_CB(p)->is_flist;
+			return;
+		}
+
+		/* Fall back to the regular GRO path */
+		if (NAPI_GRO_CB(p)->count == 1)
+			NAPI_GRO_CB(p)->is_flist = 0;
+
+		NAPI_GRO_CB(skb)->is_flist = 0;
+
 		return;
 	}
 
@@ -36,7 +47,7 @@ static void tcp6_check_fraglist_gro(struct list_head *head, struct sk_buff *skb,
 	sk = __inet6_lookup_established(net, &hdr->saddr, th->source,
 					&hdr->daddr, ntohs(th->dest),
 					iif, sdif);
-	NAPI_GRO_CB(skb)->is_flist = !sk;
+	NAPI_GRO_CB(skb)->is_flist = !sk && !skb_is_gso(skb);
 	if (sk)
 		sock_gen_put(sk);
 #endif /* IS_ENABLED(CONFIG_IPV6) */