[BUG] linux-azure kernel segfault in netfs module, netfs_rreq_unlock causes kernel panic on nodes

[kro-cat]

Describe the bug
This is a segmentation fault which exists in the netfs module of the linux-azure kernel (5.15.0-1075-azure). This was fixed in a later version, but not patched in the current AKS vm image. We've observed it on nodes with the cephfs module loaded.

To Reproduce
Steps to reproduce the behavior:

1. Load cephfs kernel module (may use rook-ceph provisioner).
2. Unknown system load or time characteristic. (may be correlated with high number of disk read operations but that's not confirmed)
3. Kernel panic shows in boot diagnostics for vmss instance, stateful workloads will experience ~5-10 minutes of downtime.

Expected behavior
Correct handling of XA_RETRY_ENTRY so that address 0000000000000402 is not dereferenced.
via https://github.com/torvalds/linux/blob/v5.15/fs/netfs/read_helper.c#L406 : On or after the first iteration of netfs_rreq_unlock, page can have the value XA_RETRY_ENTRY (returned by xas_find() in xas_for_each), which needs to be properly handled.

Screenshots

[87534.602454] BUG: kernel NULL pointer dereference, address: 0000000000000402
[87534.606859] #PF: supervisor read access in kernel mode
[87534.609959] #PF: error_code(0x0000) - not-present page
[87534.613243] PGD 0 P4D 0 
[87534.615278] Oops: 0000 [#1] SMP NOPTI
[87534.617686] CPU: 4 PID: 2688731 Comm: kworker/4:2 Not tainted 5.15.0-1075-azure #84-Ubuntu
[87534.622366] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.1 08/23/2024
[87534.628290] Workqueue: ceph-msgr ceph_con_workfn [libceph]
[87534.632247] RIP: 0010:netfs_rreq_unlock+0xf7/0x3c0 [netfs]
[87534.635624] Code: 7d a0 4c 89 fe 45 31 f6 e8 c6 7d c8 ed 4c 8b 45 90 48 85 c0 48 89 c7 0f 84 33 01 00 00 4d 8d 48 50 4c 89 fa 45 31 e4 4d 89 cf <48> 8b 0f 48 8b 47 20 48 2b 45 98 48 c1 e9 10 c1 e0 0c 83 e1 01 80
[87534.645989] RSP: 0018:ffffb1cc927bfac0 EFLAGS: 00010246
...

6.8.0-1018-azure/kernel/fs/netfs/netfs.ko (correct handling of signal, taken from another VM, non-AKS):

5.15.0-1075-azure/kernel/fs/netfs/netfs.ko (segfault exists):

Environment:

• azure-cli 2.60.0
• Kubernetes 1.28.x
• Linux 5.15.0-1075-azure
• aks ubuntu 22.04
  • AKSUbuntu-2204gen2containerd-202410.09.0
  • AKSUbuntu-2204gen2containerd-202412.10.0

Additional context
https://ubuntu.com/security/CVE-2023-52582

https://access.redhat.com/solutions/6993035

torvalds/linux@7e043a8

[kro-cat]

Wrong image, was using AKSUbuntu-2204gen2containerd-202410.09.0 and earlier at the time of these incidents, updated information in bug report.
The kernel version in this image is the same as in 202412.10.0, but since 202410.09.0 is a depreciated image (and due to the fact that I haven't experienced any issues with 202412.10.0 so far), I'd say its fine to close this after a while. This was a really unpredictable issue that happened around the holidays, so I'm not sure when - or if - it'll show up again.

[kro-cat]

(and due to the fact that I haven't experienced any issues with 202412.10.0 so far)

Just confirmed the same segfault this past weekend, which does make sense because the kernel wasn't patched in 202412.10.0 . Full segfault below. It's safe to assume this will continue to happen until the kernel is patched or a different version is used for aks' images.

[727545.334935] BUG: kernel NULL pointer dereference, address: 0000000000000402
[727545.346441] #PF: supervisor read access in kernel mode
[727545.351151] #PF: error_code(0x0000) - not-present page
[727545.356023] PGD 143d0e067 P4D 143d0e067 PUD 0 
[727545.360903] Oops: 0000 [#1] SMP NOPTI
[727545.364506] CPU: 6 PID: 4100821 Comm: php-fpm Tainted: G        W         5.15.0-1075-azure #84-Ubuntu
[727545.373163] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.1 08/23/2024
[727545.382146] RIP: 0010:netfs_rreq_unlock+0xf7/0x3c0 [netfs]
[727545.387111] Code: 7d a0 4c 89 fe 45 31 f6 e8 c6 cd 74 fa 4c 8b 45 90 48 85 c0 48 89 c7 0f 84 33 01 00 00 4d 8d 48 50 4c 89 fa 45 31 e4 4d 89 cf <48> 8b 0f 48 8b 47 20 48 2b 45 98 48 c1 e9 10 c1 e0 0c 83 e1 01 80
[727545.410806] RSP: 0018:ffff9e498ef07978 EFLAGS: 00010246
[727545.417332] RAX: 0000000000000402 RBX: ffff91342192e9c0 RCX: 0000000000000000
[727545.426170] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000402
[727545.432419] RBP: ffff9e498ef07a10 R08: ffff913351b58d80 R09: ffff913351b58dd0
[727545.440987] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[727545.447472] R13: 0000000000000000 R14: 0000000000000000 R15: ffff913351b58dd0
[727545.456435] FS:  00007f2e0c518b28(0000) GS:ffff913a9fd80000(0000) knlGS:0000000000000000
[727545.463387] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[727545.468512] CR2: 0000000000000402 CR3: 000000012a63e004 CR4: 0000000000370ee0
[727545.475027] Call Trace:
[727545.477858]  <TASK>
[727545.480403]  ? srso_alias_return_thunk+0x5/0x7f
[727545.484822]  ? show_trace_log_lvl+0x28e/0x2ea
[727545.488745]  ? show_trace_log_lvl+0x28e/0x2ea
[727545.492660]  ? netfs_rreq_assess+0xab/0x1f0 [netfs]
[727545.497592]  ? show_regs.part.0+0x23/0x29
[727545.501655]  ? __die_body.cold+0x8/0xd
[727545.505244]  ? __die+0x2b/0x37
[727545.508176]  ? page_fault_oops+0x13b/0x170
[727545.514960]  ? do_user_addr_fault+0x302/0x630
[727545.519674]  ? sched_clock+0x9/0x10
[727545.526255]  ? srso_alias_return_thunk+0x5/0x7f
[727545.531191]  ? sched_clock_cpu+0x12/0xf0
[727545.536925]  ? srso_alias_return_thunk+0x5/0x7f
[727545.542295]  ? exc_page_fault+0x71/0x160
[727545.546061]  ? asm_exc_page_fault+0x27/0x30
[727545.550350]  ? netfs_rreq_unlock+0xf7/0x3c0 [netfs]
[727545.558196]  ? netfs_rreq_unlock+0xda/0x3c0 [netfs]
[727545.563304]  ? srso_alias_return_thunk+0x5/0x7f
[727545.567718]  ? raw_spin_rq_unlock+0x10/0x30
[727545.571566]  ? srso_alias_return_thunk+0x5/0x7f
[727545.576258]  netfs_rreq_assess+0xab/0x1f0 [netfs]
[727545.581962]  netfs_readpage+0x182/0x3a0 [netfs]
[727545.586659]  ? init_wait_var_entry+0x60/0x60
[727545.591207]  ceph_readpage+0xb6/0x100 [ceph]
[727545.595320]  filemap_read_page+0x38/0x100
[727545.599160]  filemap_get_pages+0x2f5/0x3f0
[727545.603016]  filemap_read+0xbc/0x3e0
[727545.606314]  ? ceph_get_caps+0xed/0x5e0 [ceph]
[727545.610344]  ? srso_alias_return_thunk+0x5/0x7f
[727545.615160]  ? ceph_mdsc_release_request+0x176/0x190 [ceph]
[727545.620745]  ? srso_alias_return_thunk+0x5/0x7f
[727545.624881]  generic_file_read_iter+0xe2/0x150
[727545.629683]  ceph_read_iter+0x180/0x610 [ceph]
[727545.633770]  new_sync_read+0x10d/0x190
[727545.637152]  ? new_sync_read+0x10d/0x190
[727545.640604]  ? trace_pid_list_first+0x30/0x40
[727545.644967]  vfs_read+0x106/0x1a0
[727545.648021]  ksys_read+0x67/0xf0
[727545.650947]  __x64_sys_read+0x19/0x20
[727545.654315]  x64_sys_call+0x1dba/0x1fa0
[727545.659550]  do_syscall_64+0x56/0xb0
[727545.667126]  ? srso_alias_return_thunk+0x5/0x7f
[727545.671032]  ? irqentry_exit_to_user_mode+0x10/0x30
[727545.675424]  ? srso_alias_return_thunk+0x5/0x7f
[727545.681295]  ? irqentry_exit+0x1d/0x30
[727545.684690]  ? srso_alias_return_thunk+0x5/0x7f
[727545.688638]  ? sysvec_hyperv_stimer0+0x4e/0x90
[727545.692407]  entry_SYSCALL_64_after_hwframe+0x6c/0xd6
[727545.698693] RIP: 0033:0x7f2e0c4d5992
[727545.701825] Code: c3 8b 07 85 c0 75 24 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> e9 01 cc ff ff 41 54 b8 02 00 00 00 55 48 89 f5 be 00 88 08 00
[727545.720757] RSP: 002b:00007fff742458c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[727545.727047] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f2e0c4d5992
[727545.733026] RDX: 0000000000002000 RSI: 00007f2e0710c000 RDI: 0000000000000009
[727545.739078] RBP: 00007f2e0c518b28 R08: 0000000000000000 R09: 0000000000000000
[727545.745368] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f2e070b1620
[727545.754626] R13: 0000000000002000 R14: 00007f2e0710c000 R15: 00000000443cfe78
[727545.760784]  </TASK>
[727545.762882] Modules linked in: xt_nat bpfilter ceph fscache netfs nbd rbd libceph cls_bpf sch_ingress ip_set xt_CT xt_mark algif_hash af_alg veth xfrm_user xfrm_algo xt_socket nf_socket_ipv4 nf_socket_ipv6 ip6table_filter ip6table_raw ip6table_mangle ip6_tables iptable_filter iptable_raw iptable_mangle iptable_nat xt_MASQUERADE nft_chain_nat nf_nat xt_addrtype xt_comment br_netfilter bridge stp llc mptcp_diag xsk_diag tcp_diag udp_diag raw_diag inet_diag unix_diag af_packet_diag netlink_diag nf_conntrack_netlink xt_tcpudp nft_counter xt_owner xt_conntrack nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nft_compat nf_tables libcrc32c nfnetlink nvme_fabrics overlay mlx5_ib ib_uverbs ib_core mlx5_core mlxfw psample tls udf crc_itu_t sunrpc binfmt_misc nls_iso8859_1 joydev mac_hid hid_generic serio_raw crct10dif_pclmul hyperv_drm crc32_pclmul ghash_clmulni_intel drm_kms_helper cec sha256_ssse3 sha1_ssse3 rc_core aesni_intel fb_sys_fops crypto_simd syscopyarea cryptd sysfillrect sysimgblt
[727545.762976]  hid_hyperv hid hv_netvsc hyperv_fb hyperv_keyboard dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua sch_fq_codel drm efi_pstore i2c_core ip_tables x_tables autofs4
[727545.895557] CR2: 0000000000000402
[727545.899282] ---[ end trace 0a4a4f3724a4a3e1 ]---
[727546.037476] RIP: 0010:netfs_rreq_unlock+0xf7/0x3c0 [netfs]
[727546.042395] Code: 7d a0 4c 89 fe 45 31 f6 e8 c6 cd 74 fa 4c 8b 45 90 48 85 c0 48 89 c7 0f 84 33 01 00 00 4d 8d 48 50 4c 89 fa 45 31 e4 4d 89 cf <48> 8b 0f 48 8b 47 20 48 2b 45 98 48 c1 e9 10 c1 e0 0c 83 e1 01 80
[727546.058669] RSP: 0018:ffff9e498ef07978 EFLAGS: 00010246
[727546.065230] RAX: 0000000000000402 RBX: ffff91342192e9c0 RCX: 0000000000000000
[727546.075005] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000402
[727546.082181] RBP: ffff9e498ef07a10 R08: ffff913351b58d80 R09: ffff913351b58dd0
[727546.089031] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[727546.095385] R13: 0000000000000000 R14: 0000000000000000 R15: ffff913351b58dd0
[727546.101436] FS:  00007f2e0c518b28(0000) GS:ffff913a9fd80000(0000) knlGS:0000000000000000
[727546.108145] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[727546.116117] CR2: 0000000000000402 CR3: 000000012a63e004 CR4: 0000000000370ee0
[727546.122130] Kernel panic - not syncing: Fatal exception
[727546.148639] Kernel Offset: 0x39e00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)