ip6_tunnel: use skb_vlan_inet_prepare() in __ip6_tnl_rcv()
CVE: CVE-2026-23003
Component: kernel
CWE: CWE-457
Severity: Moderate
CVSS v3: 6.2/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H
Release date: 2026-01-25T00:00:00+00:00

In the Linux kernel, the following vulnerability has been resolved:

ip6_tunnel: use skb_vlan_inet_prepare() in __ip6_tnl_rcv()

Blamed commit did not take care of VLAN encapsulations
as spotted by syzbot [1].

Use skb_vlan_inet_prepare() instead of pskb_inet_may_pull().

[1]
 BUG: KMSAN: uninit-value in __INET_ECN_decapsulate include/net/inet_ecn.h:253 [inline]
 BUG: KMSAN: uninit-value in INET_ECN_decapsulate include/net/inet_ecn.h:275 [inline]
 BUG: KMSAN: uninit-value in IP6_ECN_decapsulate+0x7a8/0x1fa0 include/net/inet_ecn.h:321
  __INET_ECN_decapsulate include/net/inet_ecn.h:253 [inline]
  INET_ECN_decapsulate include/net/inet_ecn.h:275 [inline]
  IP6_ECN_decapsulate+0x7a8/0x1fa0 include/net/inet_ecn.h:321
  ip6ip6_dscp_ecn_decapsulate+0x16f/0x1b0 net/ipv6/ip6_tunnel.c:729
  __ip6_tnl_rcv+0xed9/0x1b50 net/ipv6/ip6_tunnel.c:860
  ip6_tnl_rcv+0xc3/0x100 net/ipv6/ip6_tunnel.c:903
 gre_rcv+0x1529/0x1b90 net/ipv6/ip6_gre.c:-1
  ip6_protocol_deliver_rcu+0x1c89/0x2c60 net/ipv6/ip6_input.c:438
  ip6_input_finish+0x1f4/0x4a0 net/ipv6/ip6_input.c:489
  NF_HOOK include/linux/netfilter.h:318 [inline]
  ip6_input+0x9c/0x330 net/ipv6/ip6_input.c:500
  ip6_mc_input+0x7ca/0xc10 net/ipv6/ip6_input.c:590
  dst_input include/net/dst.h:474 [inline]
  ip6_rcv_finish+0x958/0x990 net/ipv6/ip6_input.c:79
  NF_HOOK include/linux/netfilter.h:318 [inline]
  ipv6_rcv+0xf1/0x3c0 net/ipv6/ip6_input.c:311
  __netif_receive_skb_one_core net/core/dev.c:6139 [inline]
  __netif_receive_skb+0x1df/0xac0 net/core/dev.c:6252
  netif_receive_skb_internal net/core/dev.c:6338 [inline]
  netif_receive_skb+0x57/0x630 net/core/dev.c:6397
  tun_rx_batched+0x1df/0x980 drivers/net/tun.c:1485
  tun_get_user+0x5c0e/0x6c60 drivers/net/tun.c:1953
  tun_chr_write_iter+0x3e9/0x5c0 drivers/net/tun.c:1999
  new_sync_write fs/read_write.c:593 [inline]
  vfs_write+0xbe2/0x15d0 fs/read_write.c:686
  ksys_write fs/read_write.c:738 [inline]
  __do_sys_write fs/read_write.c:749 [inline]
  __se_sys_write fs/read_write.c:746 [inline]
  __x64_sys_write+0x1fb/0x4d0 fs/read_write.c:746
  x64_sys_call+0x30ab/0x3e70 arch/x86/include/generated/asm/syscalls_64.h:2
  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
  do_syscall_64+0xd3/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was created at:
  slab_post_alloc_hook mm/slub.c:4960 [inline]
  slab_alloc_node mm/slub.c:5263 [inline]
  kmem_cache_alloc_node_noprof+0x9e7/0x17a0 mm/slub.c:5315
  kmalloc_reserve+0x13c/0x4b0 net/core/skbuff.c:586
  __alloc_skb+0x805/0x1040 net/core/skbuff.c:690
  alloc_skb include/linux/skbuff.h:1383 [inline]
  alloc_skb_with_frags+0xc5/0xa60 net/core/skbuff.c:6712
  sock_alloc_send_pskb+0xacc/0xc60 net/core/sock.c:2995
  tun_alloc_skb drivers/net/tun.c:1461 [inline]
  tun_get_user+0x1142/0x6c60 drivers/net/tun.c:1794
  tun_chr_write_iter+0x3e9/0x5c0 drivers/net/tun.c:1999
  new_sync_write fs/read_write.c:593 [inline]
  vfs_write+0xbe2/0x15d0 fs/read_write.c:686
  ksys_write fs/read_write.c:738 [inline]
  __do_sys_write fs/read_write.c:749 [inline]
  __se_sys_write fs/read_write.c:746 [inline]
  __x64_sys_write+0x1fb/0x4d0 fs/read_write.c:746
  x64_sys_call+0x30ab/0x3e70 arch/x86/include/generated/asm/syscalls_64.h:2
  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
  do_syscall_64+0xd3/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 0 UID: 0 PID: 6465 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(none)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025

Statement: The ip6 tunnel receive path could read uninitialized skb data when VLAN encapsulation was present, leading to use of uninitialized values during ECN/DSCP decapsulation. This was detected by KMSAN as an uninitialized memory read in __INET_ECN_decapsulate().

Remediation: