netfilter: flowtable: account for Ethernet header in nf_flow_pppoe_proto()

jira VULN-80080
cve CVE-2025-38441
commit-author Eric Dumazet <edumazet@google.com>
commit 18cdb3d

syzbot found a potential access to uninit-value in nf_flow_pppoe_proto()

Blamed commit forgot the Ethernet header.

BUG: KMSAN: uninit-value in nf_flow_offload_inet_hook+0x7e4/0x940 net/netfilter/nf_flow_table_inet.c:27
  nf_flow_offload_inet_hook+0x7e4/0x940 net/netfilter/nf_flow_table_inet.c:27
  nf_hook_entry_hookfn include/linux/netfilter.h:157 [inline]
  nf_hook_slow+0xe1/0x3d0 net/netfilter/core.c:623
  nf_hook_ingress include/linux/netfilter_netdev.h:34 [inline]
  nf_ingress net/core/dev.c:5742 [inline]
  __netif_receive_skb_core+0x4aff/0x70c0 net/core/dev.c:5837
  __netif_receive_skb_one_core net/core/dev.c:5975 [inline]
  __netif_receive_skb+0xcc/0xac0 net/core/dev.c:6090
  netif_receive_skb_internal net/core/dev.c:6176 [inline]
  netif_receive_skb+0x57/0x630 net/core/dev.c:6235
  tun_rx_batched+0x1df/0x980 drivers/net/tun.c:1485
  tun_get_user+0x4ee0/0x6b40 drivers/net/tun.c:1938
  tun_chr_write_iter+0x3e9/0x5c0 drivers/net/tun.c:1984
  new_sync_write fs/read_write.c:593 [inline]
  vfs_write+0xb4b/0x1580 fs/read_write.c:686
  ksys_write fs/read_write.c:738 [inline]
  __do_sys_write fs/read_write.c:749 [inline]

	Reported-by: syzbot+bf6ed459397e307c3ad2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/686bc073.a00a0220.c7b3.0086.GAE@google.com/T/#u
Fixes: 87b3593 ("netfilter: flowtable: validate pppoe header")
	Signed-off-by: Eric Dumazet <edumazet@google.com>
	Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>
Link: https://patch.msgid.link/20250707124517.614489-1-edumazet@google.com
	Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit 18cdb3d)
	Signed-off-by: CIQ Kernel Automation <ciq_kernel_automation@ciq.com>

Author: CIQ Kernel Automation

include/net/netfilter/nf_flow_table.h

@@ -340,7 +340,7 @@ static inline __be16 __nf_flow_pppoe_proto(const struct sk_buff *skb)
 
 static inline bool nf_flow_pppoe_proto(struct sk_buff *skb, __be16 *inner_proto)
 {
-	if (!pskb_may_pull(skb, PPPOE_SES_HLEN))
+	if (!pskb_may_pull(skb, ETH_HLEN + PPPOE_SES_HLEN))
 		return false;
 
 	*inner_proto = __nf_flow_pppoe_proto(skb);