Subject: CVE-2024-36933: nsh: Restore skb->{protocol,data,mac_header} for outer header in nsh_gso_segment().

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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

nsh: Restore skb->{protocol,data,mac_header} for outer header in nsh_gso_segment().

syzbot triggered various splats (see [0] and links) by a crafted GSO
packet of VIRTIO_NET_HDR_GSO_UDP layering the following protocols:

  ETH_P_8021AD + ETH_P_NSH + ETH_P_IPV6 + IPPROTO_UDP

NSH can encapsulate IPv4, IPv6, Ethernet, NSH, and MPLS.  As the inner
protocol can be Ethernet, NSH GSO handler, nsh_gso_segment(), calls
skb_mac_gso_segment() to invoke inner protocol GSO handlers.

nsh_gso_segment() does the following for the original skb before
calling skb_mac_gso_segment()

  1. reset skb->network_header
  2. save the original skb->{mac_heaeder,mac_len} in a local variable
  3. pull the NSH header
  4. resets skb->mac_header
  5. set up skb->mac_len and skb->protocol for the inner protocol.

and does the following for the segmented skb

  6. set ntohs(ETH_P_NSH) to skb->protocol
  7. push the NSH header
  8. restore skb->mac_header
  9. set skb->mac_header + mac_len to skb->network_header
 10. restore skb->mac_len

There are two problems in 6-7 and 8-9.

  (a)
  After 6 & 7, skb->data points to the NSH header, so the outer header
  (ETH_P_8021AD in this case) is stripped when skb is sent out of netdev.

  Also, if NSH is encapsulated by NSH + Ethernet (so NSH-Ethernet-NSH),
  skb_pull() in the first nsh_gso_segment() will make skb->data point
  to the middle of the outer NSH or Ethernet header because the Ethernet
  header is not pulled by the second nsh_gso_segment().

  (b)
  While restoring skb->{mac_header,network_header} in 8 & 9,
  nsh_gso_segment() does not assume that the data in the linear
  buffer is shifted.

  However, udp6_ufo_fragment() could shift the data and change
  skb->mac_header accordingly as demonstrated by syzbot.

  If this happens, even the restored skb->mac_header points to
  the middle of the outer header.

It seems nsh_gso_segment() has never worked with outer headers so far.

At the end of nsh_gso_segment(), the outer header must be restored for
the segmented skb, instead of the NSH header.

To do that, let's calculate the outer header position relatively from
the inner header and set skb->{data,mac_header,protocol} properly.

[0]:
BUG: KMSAN: uninit-value in ipvlan_process_outbound drivers/net/ipvlan/ipvlan_core.c:524 [inline]
BUG: KMSAN: uninit-value in ipvlan_xmit_mode_l3 drivers/net/ipvlan/ipvlan_core.c:602 [inline]
BUG: KMSAN: uninit-value in ipvlan_queue_xmit+0xf44/0x16b0 drivers/net/ipvlan/ipvlan_core.c:668
 ipvlan_process_outbound drivers/net/ipvlan/ipvlan_core.c:524 [inline]
 ipvlan_xmit_mode_l3 drivers/net/ipvlan/ipvlan_core.c:602 [inline]
 ipvlan_queue_xmit+0xf44/0x16b0 drivers/net/ipvlan/ipvlan_core.c:668
 ipvlan_start_xmit+0x5c/0x1a0 drivers/net/ipvlan/ipvlan_main.c:222
 __netdev_start_xmit include/linux/netdevice.h:4989 [inline]
 netdev_start_xmit include/linux/netdevice.h:5003 [inline]
 xmit_one net/core/dev.c:3547 [inline]
 dev_hard_start_xmit+0x244/0xa10 net/core/dev.c:3563
 __dev_queue_xmit+0x33ed/0x51c0 net/core/dev.c:4351
 dev_queue_xmit include/linux/netdevice.h:3171 [inline]
 packet_xmit+0x9c/0x6b0 net/packet/af_packet.c:276
 packet_snd net/packet/af_packet.c:3081 [inline]
 packet_sendmsg+0x8aef/0x9f10 net/packet/af_packet.c:3113
 sock_sendmsg_nosec net/socket.c:730 [inline]
 __sock_sendmsg net/socket.c:745 [inline]
 __sys_sendto+0x735/0xa10 net/socket.c:2191
 __do_sys_sendto net/socket.c:2203 [inline]
 __se_sys_sendto net/socket.c:2199 [inline]
 __x64_sys_sendto+0x125/0x1c0 net/socket.c:2199
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Uninit was created at:
 slab_post_alloc_hook mm/slub.c:3819 [inline]
 slab_alloc_node mm/slub.c:3860 [inline]
 __do_kmalloc_node mm/slub.c:3980 [inline]
 __kmalloc_node_track_caller+0x705/0x1000 mm/slub.c:4001
 kmalloc_reserve+0x249/0x4a0 net/core/skbuff.c:582
 __alloc_skb+0x352/0x790 net/core/skbuff.c:651
 skb_segment+0x20aa/0x7080 net/core/skbuff.c:4647
 udp6_ufo_fragment+0xcab/0x1150 net/ipv6/udp_offload.c:109
 ipv6_gso_segment+0x14be/0x2ca0 net/ipv6/ip6_offload.c:152
 skb_mac_gso_segment+0x3e8/0x760 net/core/gso.c:53
 nsh_gso_segment+0x6f4/0xf70 net/nsh/nsh.c:108
 skb_mac_gso_segment+0x3e8/0x760 net/core/gso.c:53
 __skb_gso_segment+0x4b0/0x730 net/core/gso.c:124
 skb_gso_segment include/net/gso.h:83 [inline]
 validate_xmit_skb+0x107f/0x1930 net/core/dev.c:3628
 __dev_queue_xmit+0x1f28/0x51c0 net/core/dev.c:4343
 dev_queue_xmit include/linux/netdevice.h:3171 [inline]
 packet_xmit+0x9c/0x6b0 net/packet/af_packet.c:276
 packet_snd net/packet/af_packet.c:3081 [inline]
 packet_sendmsg+0x8aef/0x9f10 net/packet/af_packet.c:3113
 sock_sendmsg_nosec net/socket.c:730 [inline]
 __sock_sendmsg net/socket.c:745 [inline]
 __sys_sendto+0x735/0xa10 net/socket.c:2191
 __do_sys_sendto net/socket.c:2203 [inline]
 __se_sys_sendto net/socket.c:2199 [inline]
 __x64_sys_sendto+0x125/0x1c0 net/socket.c:2199
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

CPU: 1 PID: 5101 Comm: syz-executor421 Not tainted 6.8.0-rc5-syzkaller-00297-gf2e367d6ad3b #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024

The Linux kernel CVE team has assigned CVE-2024-36933 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 4.14 with commit c411ed854584 and fixed in 4.19.314 with commit a7c2c3c1caab
	Issue introduced in 4.14 with commit c411ed854584 and fixed in 5.4.276 with commit 46134031c20f
	Issue introduced in 4.14 with commit c411ed854584 and fixed in 5.10.217 with commit bbccf0caef2f
	Issue introduced in 4.14 with commit c411ed854584 and fixed in 5.15.159 with commit 5a4603fbc285
	Issue introduced in 4.14 with commit c411ed854584 and fixed in 6.1.91 with commit 37ed6f244ec5
	Issue introduced in 4.14 with commit c411ed854584 and fixed in 6.6.31 with commit 696d18bb5972
	Issue introduced in 4.14 with commit c411ed854584 and fixed in 6.8.10 with commit 29a07f2ee4d2
	Issue introduced in 4.14 with commit c411ed854584 and fixed in 6.9 with commit 4b911a9690d7

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-36933
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	net/nsh/nsh.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/a7c2c3c1caabcb4a3d6c47284c397507aaf54fe9
	https://git.kernel.org/stable/c/46134031c20fd313d03b90169d64b2e05ca6b65c
	https://git.kernel.org/stable/c/bbccf0caef2fa917d6d0692385a06ce3c262a216
	https://git.kernel.org/stable/c/5a4603fbc285752d19e4b415466db18ef3617e4a
	https://git.kernel.org/stable/c/37ed6f244ec5bda2e90b085084e322ea55d0aaa2
	https://git.kernel.org/stable/c/696d18bb59727a2e0526c0802a812620be1c9340
	https://git.kernel.org/stable/c/29a07f2ee4d273760c2acbfc756e29eccd82470a
	https://git.kernel.org/stable/c/4b911a9690d72641879ea6d13cce1de31d346d79