FAILED: patch "[PATCH] net: gro: fix double aggregation of flush-marked skbs" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x e751256486d0ded20f5a9f9863467f1dce65142f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072947-straw-entity-e1ff@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e751256486d0ded20f5a9f9863467f1dce65142f Mon Sep 17 00:00:00 2001
From: Shiming Cheng <[email protected]>
Date: Thu, 9 Jul 2026 09:46:39 +0800
Subject: [PATCH] net: gro: fix double aggregation of flush-marked skbs

Commit 0ab03f353d36 ("net-gro: Fix GRO flush when receiving a GSO packet.") added a flush check to skb_gro_receive(), but skb_gro_receive_list() lacks the same validation.

As a result, packets marked with NAPI_GRO_CB(skb)->flush may still be re-aggregated.

This allows already-GRO'd packets with existing frag_list to be re-aggregated into a new GRO session, corrupting the frag_list chain structure. When skb_segment() attempts to unpack these malformed packets, it encounters invalid state and triggers a kernel panic.

Scenario (Tethering/Device forwarding):
1. Driver: Generated aggregated packet P1 via LRO with frag_list
2. Dev A: Receives aggregated fraglist packet and flush flag set
3. Dev A: Re-enters GRO, skb_gro_receive_list() is called
4. Missing flush check allows re-aggregation despite flush flag
5. Frag_list chain becomes corrupted (loops or dangling refs)
6. Dev B: TX path calls skb_segment(), crashes on corrupted frag_list

Root cause in skb_segment():
The check at line ~4891:

    if (hsize <= 0 && i >= nfrags && skb_headlen(list_skb) &&
        (skb_headlen(list_skb) == len || sg)) {

When frag_list is corrupted by double aggregation, when list_skb is a NULL pointer from skb->next, skb_headlen(list_skb) dereference NULL/corrupted pointers occurs.

Call Trace:
 skb_headlen(NULL skb)
 skb_segment
 tcp_gso_segment
 tcp4_gso_segment
 inet_gso_segment
 skb_mac_gso_segment
 __skb_gso_segment
 skb_gso_segment
 validate_xmit_skb
 validate_xmit_skb_list
 sch_direct_xmit
 qdisc_restart
 __qdisc_run
 qdisc_run
 net_tx_action

Fix: Add NAPI_GRO_CB(skb)->flush validation to the early-return check in skb_gro_receive_list(), matching the defensive programming pattern of skb_gro_receive().

index 35f2f708f010..29b4d02bf519 100644
@@ -229,7 +229,9 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
 
 int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb)
 {
-	if (unlikely(p->len + skb->len >= 65536))
+	/* make sure to check flush flag and to not merge */
+	if (unlikely(p->len + skb->len >= 65536 ||
+		     NAPI_GRO_CB(skb)->flush))
 		return -E2BIG;
 
 	if (!pskb_may_pull(skb, skb_gro_offset(skb))) {