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net: iptunnel: fix stale transport header during tunnel decapsulation

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Syzbot reported a crash in qdisc_pkt_len_segs_init() caused by a stale
transport_header offset after tunnel decapsulation.

BUG: unable to handle page fault for address: ffffed102091a42e
Oops: Oops: 0000 [#1] SMP KASAN NOPTI
CPU: 0 UID: 0 PID: 340 Comm: qdisc_uaf_repro Not tainted 7.2.0-rc4-00061-g248951ddc14d #256 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:__asan_load2
<IRQ>
qdisc_pkt_len_segs_init (net/core/dev.c:4145)
__dev_queue_xmit (net/core/dev.c:4787)
br_dev_queue_push_xmit (net/bridge/br_forward.c:53)
br_handle_frame_finish (net/bridge/br_input.c:229)
br_handle_frame (net/bridge/br_input.c:315)
__netif_receive_skb_core.constprop.0 (net/core/dev.c:6099)
__netif_receive_skb_list_core (net/core/dev.c:6287)
netif_receive_skb_list_internal (net/core/dev.c:6445)
napi_complete_done (net/core/dev.c:6813)
gro_cell_poll (net/core/gro_cells.c:74)
__napi_poll (net/core/dev.c:7735)
net_rx_action (net/core/dev.c:7798 net/core/dev.c:7955)
handle_softirqs (kernel/softirq.c:622)
do_softirq (kernel/softirq.c:523  kernel/softirq.c:510 )
__local_bh_enable_ip (kernel/softirq.c:450)
tun_get_user (drivers/net/tun.c:1986 (discriminator 1))
tun_chr_write_iter (drivers/net/tun.c:2032)

The issue is completely latent until qdisc read transport header in
commit 7fb4c19670 ("net: pull headers in qdisc_pkt_len_segs_init()").
The crash requires four conditions to line up:

1. The incoming packet is encapsulated and carries GSO metadata. The outer
   transport header offset is stored in skb->transport_header while the
   packet is still in the outer tunnel context.
2. The tunnel receiver strips the outer headers. skb->data is advanced to
   the inner frame, but skb->transport_header is left pointing to the
   now-removed outer L4 header, so it becomes a negative offset relative to
   the new data.
3. The inner frame is not delivered to the local IP stack. Instead, it
   is forwarded at L2 by a bridge or HSR, so ip_rcv_core() never runs and
   the transport header is not reset to the inner L4 offset.
4. The forwarding path calls __dev_queue_xmit(), which enters
   qdisc_pkt_len_segs_init(). That function computes the GSO header length
   from skb_transport_offset(skb). Because the offset is negative, the
   unsigned cast overflows and pskb_may_pull(skb, hdr_len +
   sizeof(struct tcphdr)) reads past the end of the skb, triggering a
   KASAN fault or page fault.

The issue specifically requires GSO packets (shinfo->gso_size != 0), which
are processed/aggregated through gro_cells. Fix this by clearing
transport_header to the ~0U sentinel in gro_cell for all tunnnel driver.
GTP does not support GRO/GSO, drop the evil GSO packets in GTP directly.

Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Reported-by: syzbot+83181a31faf9455499c5@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69de2bee.a00a0220.475f0.0041.GAE@google.com/T/
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Dong Chenchen <dongchenchen2@huawei.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260825123909.1463121-1-dongchenchen2@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Dong Chenchen 2026-08-25 20:39:09 +08:00 committed by Jakub Kicinski
parent b873bf8141
commit 28a57fb2c5
3 changed files with 12 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
drivers/net/gtp.c
View File

@ -318,6 +318,11 @@ static int gtp_inner_proto(struct sk_buff *skb, unsigned int hdrlen,
static int gtp_rx(struct pdp_ctx *pctx, struct sk_buff *skb,
unsigned int hdrlen, unsigned int role, __u16 inner_proto)
{
if (skb_is_gso(skb)) {
netdev_dbg(pctx->dev, "GSO is not supported in GTP\n");
goto err;
}
if (!gtp_check_ms(skb, pctx, hdrlen, role, inner_proto)) {
netdev_dbg(pctx->dev, "No PDP ctx for this MS\n");
return 1;

5
include/linux/skbuff.h
View File

@ -3082,6 +3082,11 @@ static inline bool skb_transport_header_was_set(const struct sk_buff *skb)
return skb->transport_header != (typeof(skb->transport_header))~0U;
}
static inline void skb_unset_transport_header(struct sk_buff *skb)
{
skb->transport_header = (typeof(skb->transport_header))~0U;
}
static inline unsigned char *skb_transport_header(const struct sk_buff *skb)
{
DEBUG_NET_WARN_ON_ONCE(!skb_transport_header_was_set(skb));

2
net/core/gro_cells.c
View File

@ -22,6 +22,8 @@ int gro_cells_receive(struct gro_cells *gcells, struct sk_buff *skb)
if (unlikely(!(dev->flags & IFF_UP)))
goto drop;
skb_unset_transport_header(skb);
if (!gcells->cells || skb_cloned(skb) || netif_elide_gro(dev)) {
res = netif_rx(skb);
goto unlock;