net: prevent skb corruption on frag list segmentation
CVE: CVE-2023-54094
Component: kernel
Severity: Moderate
CVSS v3: 6.4/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Release date: 2023-01-01T00:00:00+00:00

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

net: prevent skb corruption on frag list segmentation

Ian reported several skb corruptions triggered by rx-gro-list,
collecting different oops alike:

[   62.624003] BUG: kernel NULL pointer dereference, address: 00000000000000c0
[   62.631083] #PF: supervisor read access in kernel mode
[   62.636312] #PF: error_code(0x0000) - not-present page
[   62.641541] PGD 0 P4D 0
[   62.644174] Oops: 0000 [#1] PREEMPT SMP NOPTI
[   62.648629] CPU: 1 PID: 913 Comm: napi/eno2-79 Not tainted 6.4.0 #364
[   62.655162] Hardware name: Supermicro Super Server/A2SDi-12C-HLN4F, BIOS 1.7a 10/13/2022
[   62.663344] RIP: 0010:__udp_gso_segment (./include/linux/skbuff.h:2858
./include/linux/udp.h:23 net/ipv4/udp_offload.c:228 net/ipv4/udp_offload.c:261
net/ipv4/udp_offload.c:277)
[   62.687193] RSP: 0018:ffffbd3a83b4f868 EFLAGS: 00010246
[   62.692515] RAX: 00000000000000ce RBX: 0000000000000000 RCX: 0000000000000000
[   62.699743] RDX: ffffa124def8a000 RSI: 0000000000000079 RDI: ffffa125952a14d4
[   62.706970] RBP: ffffa124def8a000 R08: 0000000000000022 R09: 00002000001558c9
[   62.714199] R10: 0000000000000000 R11: 00000000be554639 R12: 00000000000000e2
[   62.721426] R13: ffffa125952a1400 R14: ffffa125952a1400 R15: 00002000001558c9
[   62.728654] FS:  0000000000000000(0000) GS:ffffa127efa40000(0000)
knlGS:0000000000000000
[   62.736852] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   62.742702] CR2: 00000000000000c0 CR3: 00000001034b0000 CR4: 00000000003526e0
[   62.749948] Call Trace:
[   62.752498]  <TASK>
[   62.779267] inet_gso_segment (net/ipv4/af_inet.c:1398)
[   62.787605] skb_mac_gso_segment (net/core/gro.c:141)
[   62.791906] __skb_gso_segment (net/core/dev.c:3403 (discriminator 2))
[   62.800492] validate_xmit_skb (./include/linux/netdevice.h:4862
net/core/dev.c:3659)
[   62.804695] validate_xmit_skb_list (net/core/dev.c:3710)
[   62.809158] sch_direct_xmit (net/sched/sch_generic.c:330)
[   62.813198] __dev_queue_xmit (net/core/dev.c:3805 net/core/dev.c:4210)
net/netfilter/core.c:626)
[   62.821093] br_dev_queue_push_xmit (net/bridge/br_forward.c:55)
[   62.825652] maybe_deliver (net/bridge/br_forward.c:193)
[   62.829420] br_flood (net/bridge/br_forward.c:233)
[   62.832758] br_handle_frame_finish (net/bridge/br_input.c:215)
[   62.837403] br_handle_frame (net/bridge/br_input.c:298
net/bridge/br_input.c:416)
[   62.851417] __netif_receive_skb_core.constprop.0 (net/core/dev.c:5387)
[   62.866114] __netif_receive_skb_list_core (net/core/dev.c:5570)
[   62.871367] netif_receive_skb_list_internal (net/core/dev.c:5638
net/core/dev.c:5727)
[   62.876795] napi_complete_done (./include/linux/list.h:37
./include/net/gro.h:434 ./include/net/gro.h:429 net/core/dev.c:6067)
[   62.881004] ixgbe_poll (drivers/net/ethernet/intel/ixgbe/ixgbe_main.c:3191)
[   62.893534] __napi_poll (net/core/dev.c:6498)
[   62.897133] napi_threaded_poll (./include/linux/netpoll.h:89
net/core/dev.c:6640)
[   62.905276] kthread (kernel/kthread.c:379)
[   62.913435] ret_from_fork (arch/x86/entry/entry_64.S:314)
[   62.917119]  </TASK>

In the critical scenario, rx-gro-list GRO-ed packets are fed, via a
bridge, both to the local input path and to an egress device (tun).

The segmentation of such packets unsafely writes to the cloned skbs
with shared heads.

This change addresses the issue by uncloning as needed the
to-be-segmented skbs.

Statement: A vulnerability in the Linux networking stack can lead to skb memory corruption when segmenting GRO frag list packets that share a cloned skb head. The issue occurs in skb_segment_list, which adjusts headers and then updates skb_shared_info fields such as frag_list. In a critical scenario, the rx-gro-list GRO aggregated packets are received and then processed by a bridge in a way that delivers the same underlying skb data both to the local input path and to an egress device that needs segmentation. When GSO segmentation is attempted on the forwarded path, the code may operate on a cloned skb with a shared head. The segmentation logic then writes to the shared skb head metadata. This corruption has been observed to trigger kernel NULL pointer dereferences and other crashes in protocol offload functions such as __udp_gso_segment.

Remediation: