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

JIRA: https://issues.redhat.com/browse/RHEL-115630
Upstream Status: commit 18cdb3d

commit 18cdb3d
Author: Eric Dumazet <edumazet@google.com>
Date:   Mon Jul 7 12:45:17 2025 +0000

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

    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>

Signed-off-by: Florian Westphal <fwestpha@redhat.com>

Author: fw-strlen

include/net/netfilter/nf_flow_table.h

@@ -353,7 +353,7 @@ 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);