In the Linux kernel, the following vulnerability has been resolved:
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().