Fwd: Since 6.1: flow_dissector.c __skb_flow_dissect+0xa91/0x1cd0 raises WARNING in specific circumstances

Fwd: Since 6.1: flow_dissector.c __skb_flow_dissect+0xa91/0x1cd0 raises WARNING in specific circumstances

[Bagas Sanjaya]

Hi,

I notice a regression report on Bugzilla [1]. Quoting from it:

> ## Overall description
> 
> After migration from kernel 5.10 to 6.1, i started to encounter the following warning although they do not seem to have any significant functional impact.
> Following further investigation, it appears that the warning appears when both of the following conditions are:
>  - At least one policy based routing rules with L4 selectors (among others, L3 matchers and fwmark DO NOT trigger the issue)
>  - A nftable or iptable filtering policy with at least one action rejecting traffic with TCP reset (drop action, or reject with ICMP unreachable DO NOT trigger)
> 
> The first time a packet matches the reject policy, and that a RST segment is forged and sent, a kernel WARNING as follows will show up in the dmesg:
> ```
> WARNING: CPU: 0 PID: 0 at net/core/flow_dissector.c:1016 __skb_flow_dissect+0xa91/0x1cd0
> ```
> 
> Traffic does not appear to be affected, including the RST segment that is reliably delivered in spite of the WARNING.
> 
> 
> ## Expected behaviour
> 
> The warning should not appear, in this particular setup the namespace of the RST packet should be resolved to the default one.
> 
> 
> ## Reproducing steps
> 
> The following minimalistic configuration will reproduce the issue:
> 
> - Running last 6.1 "vanilla" kernel at the time of writing:
> ```
> root@v-debian12:~# uname -a
> Linux v-debian12 6.1.47 #1 SMP PREEMPT_DYNAMIC Fri Aug 25 13:23:01 CEST 2023 x86_64 GNU/Linux
> ```
> NB: Issue won't happen on different 5.10 kernels that i tried
> 
> - A routing policy with a rule that uses L4 selectors:
> ```
> root@v-debian12:~# cat /etc/iproute2/rt_tables.d/wanalt.conf
> 100 wanalt
> 
> root@v-debian12:~# ip rule ls
> 0:	from all lookup local
> 100:	from all ipproto udp dport 27000-27999 lookup wanalt
> 32766:	from all lookup main
> 32767:	from all lookup default
> 
> root@v-debian12:~# ip route ls
> default via 192.168.56.1 dev enp0s8
> 192.168.56.0/24 dev enp0s8 proto kernel scope link src 192.168.56.3
> 
> root@v-debian12:~# ip route ls table wanalt
> default via 192.168.56.2 dev enp0s8
> ```
> 
> - A nftables or iptables rules that rejects some traffic with tcp reset:
> ```
> root@v-debian12:~# nft list ruleset
> table inet filter {
> 	chain input {
> 		type filter hook input priority filter; policy accept;
> 		tcp dport 12345 log reject with tcp reset
> 	}
> 
> 	chain output {
> 		type filter hook output priority filter; policy accept;
> 		tcp sport 12345 tcp flags rst / fin,syn,rst log
> 	}
> }
> ```
> (NB: the rule in thhhe output chain is only for debugging purpose, it let us see what packet caused the warning in the dmesg)
> 
> - from another VM, we attempt to connect to port 12345 on the reproducing host:
> ```
> root@v-debian11:~# nc 192.168.56.3 12345
> ```
> 
> - the following appears in the dmesg of the reproducing host:
> ```
> # the packet that matched the reject rule
> 2023-08-26T14:04:35.619764+02:00 v-debian12 kernel: [ 3335.550250] IN=enp0s8 OUT= MAC=08:00:27:76:64:62:08:00:27:69:c9:e5:08:00 SRC=192.168.56.4 DST=192.168.56.3 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=30150 DF PROTO=TCP SPT=48576 DPT=12345 WINDOW=64240 RES=0x00 SYN URGP=0
> 
> # the warning showing up
> 2023-08-26T14:04:35.619791+02:00 v-debian12 kernel: [ 3335.550267] ------------[ cut here ]------------
> 2023-08-26T14:04:35.619793+02:00 v-debian12 kernel: [ 3335.550269] WARNING: CPU: 0 PID: 0 at net/core/flow_dissector.c:1016 __skb_flow_dissect+0xa91/0x1cd0
> 2023-08-26T14:04:35.619795+02:00 v-debian12 kernel: [ 3335.550277] Modules linked in: nf_log_syslog(E) nft_reject_inet(E) nf_reject_ipv4(E) nf_reject_ipv6(E) nft_reject(E) nft_log(E) nf_tables(E) libcrc32c(E) nfnetlink(E) intel_rapl_msr(E) intel_rapl_common(E) intel_pmc_core(E) intel_powerclamp(E) ghash_clmulni_intel(E) sha512_ssse3(E) sha512_generic(E) vmwgfx(E) aesni_intel(E) drm_ttm_helper(E) snd_pcm(E) crypto_simd(E) iTCO_wdt(E) cryptd(E) intel_pmc_bxt(E) ttm(E) iTCO_vendor_support(E) rapl(E) watchdog(E) snd_timer(E) drm_kms_helper(E) snd(E) joydev(E) soundcore(E) vboxguest(E) pcspkr(E) rng_core(E) ac(E) button(E) evdev(E) serio_raw(E) sg(E) binfmt_misc(E) drm(E) fuse(E) dm_mod(E) loop(E) efi_pstore(E) configfs(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc32c_generic(E) crc16(E) mbcache(E) jbd2(E) sd_mod(E) t10_pi(E) crc64_rocksoft(E) crc64(E) crc_t10dif(E) sr_mod(E) cdrom(E) crct10dif_generic(E) ata_generic(E) ahci(E) libahci(E) ata_piix(E) crct10dif_pclmul(E) crct10dif_common(E) crc32_pclmul(E) crc32c_intel(E) psmouse(E)
> 2023-08-26T14:04:35.619796+02:00 v-debian12 kernel: [ 3335.550336]  libata(E) scsi_mod(E) scsi_common(E) lpc_ich(E) i2c_piix4(E) e1000(E) battery(E) video(E) wmi(E)
> 2023-08-26T14:04:35.619797+02:00 v-debian12 kernel: [ 3335.550345] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G            E      6.1.47 #1
> 2023-08-26T14:04:35.619797+02:00 v-debian12 kernel: [ 3335.550348] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
> 2023-08-26T14:04:35.619798+02:00 v-debian12 kernel: [ 3335.550350] RIP: 0010:__skb_flow_dissect+0xa91/0x1cd0
> 2023-08-26T14:04:35.619799+02:00 v-debian12 kernel: [ 3335.550354] Code: 00 bf 02 00 00 00 49 89 0c 12 66 89 78 02 41 8b 55 00 e9 e7 f6 ff ff 49 8b 44 24 18 48 85 c0 74 09 4c 8b 78 30 e9 30 f7 ff ff <0f> 0b e9 f6 f7 ff ff f6 c2 20 0f 84 e4 f6 ff ff 41 0f b7 45 0e 4c
> 2023-08-26T14:04:35.619809+02:00 v-debian12 kernel: [ 3335.550356] RSP: 0018:ffffc130800036b0 EFLAGS: 00010246
> 2023-08-26T14:04:35.619810+02:00 v-debian12 kernel: [ 3335.550358] RAX: 0000000000000000 RBX: 0000000000000008 RCX: ffffc13080003810
> 2023-08-26T14:04:35.619811+02:00 v-debian12 kernel: [ 3335.550360] RDX: ffffffff94240e40 RSI: ffff9b53c770ee00 RDI: 0000000000000000
> 2023-08-26T14:04:35.619812+02:00 v-debian12 kernel: [ 3335.550361] RBP: ffffc13080003798 R08: 0000000000000000 R09: 0000000000000000
> 2023-08-26T14:04:35.619813+02:00 v-debian12 kernel: [ 3335.550363] R10: ffffc13080003810 R11: ffff9b53e812ba80 R12: ffff9b53c770ee00
> 2023-08-26T14:04:35.619814+02:00 v-debian12 kernel: [ 3335.550364] R13: ffffffff94240e40 R14: ffff9b53c6456080 R15: 0000000000000000
> 2023-08-26T14:04:35.619814+02:00 v-debian12 kernel: [ 3335.550366] FS:  0000000000000000(0000) GS:ffff9b53fdc00000(0000) knlGS:0000000000000000
> 2023-08-26T14:04:35.619815+02:00 v-debian12 kernel: [ 3335.550368] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> 2023-08-26T14:04:35.619816+02:00 v-debian12 kernel: [ 3335.550369] CR2: 00007ffdda939440 CR3: 000000004d326005 CR4: 00000000000706f0
> 2023-08-26T14:04:35.619817+02:00 v-debian12 kernel: [ 3335.550371] Call Trace:
> 2023-08-26T14:04:35.619817+02:00 v-debian12 kernel: [ 3335.550374]  <IRQ>
> 2023-08-26T14:04:35.619818+02:00 v-debian12 kernel: [ 3335.550378]  ? __warn+0x7d/0xc0
> 2023-08-26T14:04:35.619819+02:00 v-debian12 kernel: [ 3335.550382]  ? __skb_flow_dissect+0xa91/0x1cd0
> 2023-08-26T14:04:35.619820+02:00 v-debian12 kernel: [ 3335.550385]  ? report_bug+0xe6/0x170
> 2023-08-26T14:04:35.619821+02:00 v-debian12 kernel: [ 3335.550388]  ? handle_bug+0x41/0x70
> 2023-08-26T14:04:35.619821+02:00 v-debian12 kernel: [ 3335.550391]  ? exc_invalid_op+0x13/0x60
> 2023-08-26T14:04:35.619822+02:00 v-debian12 kernel: [ 3335.550393]  ? asm_exc_invalid_op+0x16/0x20
> 2023-08-26T14:04:35.619832+02:00 v-debian12 kernel: [ 3335.550397]  ? __skb_flow_dissect+0xa91/0x1cd0
> 2023-08-26T14:04:35.619833+02:00 v-debian12 kernel: [ 3335.550401]  ? __inet_dev_addr_type+0xe8/0x180
> 2023-08-26T14:04:35.619834+02:00 v-debian12 kernel: [ 3335.550405]  ip_route_me_harder+0x143/0x330
> 2023-08-26T14:04:35.619835+02:00 v-debian12 kernel: [ 3335.550411]  nf_send_reset+0x17c/0x2d0 [nf_reject_ipv4]
> 2023-08-26T14:04:35.619835+02:00 v-debian12 kernel: [ 3335.550417]  nft_reject_inet_eval+0xa9/0xf2 [nft_reject_inet]
> 2023-08-26T14:04:35.619836+02:00 v-debian12 kernel: [ 3335.550422]  nft_do_chain+0x198/0x5d0 [nf_tables]
> 2023-08-26T14:04:35.619837+02:00 v-debian12 kernel: [ 3335.550439]  nft_do_chain_inet+0xa4/0x110 [nf_tables]
> 2023-08-26T14:04:35.619838+02:00 v-debian12 kernel: [ 3335.550454]  nf_hook_slow+0x41/0xc0
> 2023-08-26T14:04:35.619838+02:00 v-debian12 kernel: [ 3335.550458]  ip_local_deliver+0xce/0x110
> 2023-08-26T14:04:35.619839+02:00 v-debian12 kernel: [ 3335.550461]  ? ip_protocol_deliver_rcu+0x230/0x230
> 2023-08-26T14:04:35.619840+02:00 v-debian12 kernel: [ 3335.550464]  ip_sublist_rcv_finish+0x7e/0x90
> 2023-08-26T14:04:35.619840+02:00 v-debian12 kernel: [ 3335.550466]  ip_sublist_rcv+0x183/0x230
> 2023-08-26T14:04:35.619841+02:00 v-debian12 kernel: [ 3335.550468]  ? __netif_receive_skb_core.constprop.0+0xaf/0xf00
> 2023-08-26T14:04:35.619842+02:00 v-debian12 kernel: [ 3335.550472]  ip_list_rcv+0x139/0x170
> 2023-08-26T14:04:35.619843+02:00 v-debian12 kernel: [ 3335.550475]  __netif_receive_skb_list_core+0x29e/0x2c0
> 2023-08-26T14:04:35.619843+02:00 v-debian12 kernel: [ 3335.550479]  netif_receive_skb_list_internal+0x1cd/0x300
> 2023-08-26T14:04:35.619844+02:00 v-debian12 kernel: [ 3335.550483]  ? e1000_clean_rx_irq+0x369/0x500 [e1000]
> 2023-08-26T14:04:35.619845+02:00 v-debian12 kernel: [ 3335.550492]  napi_complete_done+0x6d/0x1a0
> 2023-08-26T14:04:35.619845+02:00 v-debian12 kernel: [ 3335.550495]  e1000_clean+0x296/0x6a0 [e1000]
> 2023-08-26T14:04:35.619846+02:00 v-debian12 kernel: [ 3335.550505]  ? vbg_heartbeat_host_config+0x90/0x90 [vboxguest]
> 2023-08-26T14:04:35.619847+02:00 v-debian12 kernel: [ 3335.550512]  __napi_poll+0x2b/0x160
> 2023-08-26T14:04:35.619848+02:00 v-debian12 kernel: [ 3335.550515]  net_rx_action+0x2a5/0x360
> 2023-08-26T14:04:35.619848+02:00 v-debian12 kernel: [ 3335.550518]  ? __napi_schedule+0x8e/0xb0
> 2023-08-26T14:04:35.619849+02:00 v-debian12 kernel: [ 3335.550521]  __do_softirq+0xf0/0x2fe
> 2023-08-26T14:04:35.619850+02:00 v-debian12 kernel: [ 3335.550525]  __irq_exit_rcu+0xc7/0x130
> 2023-08-26T14:04:35.619850+02:00 v-debian12 kernel: [ 3335.550528]  common_interrupt+0xb9/0xd0
> 2023-08-26T14:04:35.619851+02:00 v-debian12 kernel: [ 3335.550531]  </IRQ>
> 2023-08-26T14:04:35.619852+02:00 v-debian12 kernel: [ 3335.550532]  <TASK>
> 2023-08-26T14:04:35.619852+02:00 v-debian12 kernel: [ 3335.550533]  asm_common_interrupt+0x22/0x40
> 2023-08-26T14:04:35.619853+02:00 v-debian12 kernel: [ 3335.550535] RIP: 0010:mwait_idle+0x54/0x80
> 2023-08-26T14:04:35.619854+02:00 v-debian12 kernel: [ 3335.550539] Code: 31 d2 48 89 d1 65 48 8b 04 25 c0 fb 01 00 0f 01 c8 48 8b 00 a8 08 75 14 66 90 0f 00 2d 05 f5 5c 00 31 c0 48 89 c1 fb 0f 01 c9 <eb> 06 fb 0f 1f 44 00 00 65 48 8b 04 25 c0 fb 01 00 3e 80 60 02 df
> 2023-08-26T14:04:35.619855+02:00 v-debian12 kernel: [ 3335.550540] RSP: 0018:ffffffff94003e98 EFLAGS: 00000246
> 2023-08-26T14:04:35.619855+02:00 v-debian12 kernel: [ 3335.550542] RAX: 0000000000000000 RBX: ffffffff9401aa40 RCX: 0000000000000000
> 2023-08-26T14:04:35.619856+02:00 v-debian12 kernel: [ 3335.550544] RDX: 0000000000000000 RSI: ffffffff9399cba6 RDI: ffffffff939761d1
> 2023-08-26T14:04:35.619857+02:00 v-debian12 kernel: [ 3335.550545] RBP: 0000000000000000 R08: 000003a6bb03287e R09: 0000000000000000
> 2023-08-26T14:04:35.619857+02:00 v-debian12 kernel: [ 3335.550546] R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000000
> 2023-08-26T14:04:35.619858+02:00 v-debian12 kernel: [ 3335.550547] R13: 0000000000000000 R14: ffffffff9401a118 R15: 0000000000000000
> 2023-08-26T14:04:35.619859+02:00 v-debian12 kernel: [ 3335.550550]  ? default_idle+0x10/0x10
> 2023-08-26T14:04:35.619859+02:00 v-debian12 kernel: [ 3335.550553]  default_idle_call+0x36/0xf0
> 2023-08-26T14:04:35.619860+02:00 v-debian12 kernel: [ 3335.550556]  do_idle+0x225/0x2b0
> 2023-08-26T14:04:35.619861+02:00 v-debian12 kernel: [ 3335.550560]  cpu_startup_entry+0x19/0x20
> 2023-08-26T14:04:35.619862+02:00 v-debian12 kernel: [ 3335.550562]  rest_init+0xcb/0xd0
> 2023-08-26T14:04:35.619862+02:00 v-debian12 kernel: [ 3335.550565]  arch_call_rest_init+0xa/0x14
> 2023-08-26T14:04:35.619863+02:00 v-debian12 kernel: [ 3335.550572]  start_kernel+0x6fe/0x727
> 2023-08-26T14:04:35.619873+02:00 v-debian12 kernel: [ 3335.550575]  secondary_startup_64_no_verify+0xe5/0xeb
> 2023-08-26T14:04:35.619875+02:00 v-debian12 kernel: [ 3335.550580]  </TASK>
> 2023-08-26T14:04:35.619875+02:00 v-debian12 kernel: [ 3335.550581] ---[ end trace 0000000000000000 ]---
> 
> # the RST packet that caused the warning
> 2023-08-26T14:04:35.619876+02:00 v-debian12 kernel: [ 3335.550593] IN= OUT=enp0s8 SRC=192.168.56.3 DST=192.168.56.4 LEN=40 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=TCP SPT=12345 DPT=48576 WINDOW=0 RES=0x00 ACK RST URGP=0
> ```
> 
> 
> ## Known workarounds
> 
> The issue is benign and does not prevent the traffic from being sent correctly so there is no strict necessity for a workaround.
> On my affected setup i could prevent the warning from showing up by marking traffic for the alternate uplink using nftables and rewriting the routing policy to only use L3 and fwmark selectors.

See Bugzilla for the full thread.

Anyway, I'm adding it to be tracked by regzbot:

#regzbot introduced: v5.10..v6.1 https://bugzilla.kernel.org/show_bug.cgi?id=217826
#regzbot title: flow_dissector warning but netfilter traffic unaffected

Thanks.

[1]: https://bugzilla.kernel.org/show_bug.cgi?id=217826

-- 
An old man doll... just what I always wanted! - Clara