Re: [BUG net-next] arch/x86/kernel/cpu/bugs.c:2935: "Unpatched return thunk in use. This should not happen!" [STACKTRACE]

Mirsad Todorovac
2024-03-26T19:15:28Z

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On 3/26/24 11:16, Borislav Petkov wrote:
> On Wed, Mar 20, 2024 at 02:28:57AM +0100, Mirsad Todorovac wrote:
>> Please find the kernel .config attached.
>
> Thanks, that's one huuuge kernel you're building. :)
>
>> I got another one of these "Unpatched thunk" and it seems connected
>> with selftest/kvm.
>>
>> But running selftests/kvm one by one did not trigger the bug.
>
> Which commands are you exactly running?
>
> I'll try to reproduce here.

I think I have a reproducer here on the latest torvalds vanilla tree (on Ubuntu 22.04 LTS box):

root# tools/testing/selftests/kvm/x86_64/nx_huge_pages_test.sh
Running test with CAP_SYS_BOOT enabled
Running as root, skipping nx_huge_pages_test with CAP_SYS_BOOT disabled
root# git describe
v6.9-rc1-5-g928a87efa423
root#

> Thx.

Not at all.

The stacktrace for the bug triggered by the above command was:

kernel: [ 101.973612] ------------[ cut here ]------------
kernel: [ 101.973615] Unpatched return thunk in use. This should not happen!
kernel: [ 101.973618] WARNING: CPU: 1 PID: 3827 at arch/x86/kernel/cpu/bugs.c:2935 __warn_thunk (./arch/x86/kernel/cpu/bugs.c:2935 (discriminator 3))
kernel: [ 101.973625] Modules linked in: xfrm_user nf_tables nfnetlink nvme_fabrics binfmt_misc snd_hda_codec_realtek snd_hda_codec_generic snd_hda_scodec_component snd_hda_codec_hdmi snd_hda_intel snd_intel_dspcfg snd_intel_sdw_acpi snd_hda_codec intel_rapl_msr amd_atl snd_hda_core intel_rapl_common nls_iso8859_1 snd_hwdep snd_pcm edac_mce_amd amdgpu crct10dif_pclmul polyval_clmulni snd_seq_midi polyval_generic snd_seq_midi_event ghash_clmulni_intel sha512_ssse3 snd_rawmidi sha256_ssse3 amdxcp sha1_ssse3 drm_exec aesni_intel snd_seq gpu_sched crypto_simd drm_buddy cryptd drm_suballoc_helper drm_ttm_helper snd_seq_device joydev input_leds rapl ttm snd_timer wmi_bmof drm_display_helper cec snd drm_kms_helper k10temp ccp i2c_algo_bit soundcore mac_hid tcp_bbr msr parport_pc ppdev lp parport drm efi_pstore ip_tables x_tables autofs4 btrfs blake2b_generic xor raid6_pq hid_generic nvme r8169 xhci_pci ahci nvme_core crc32_pclmul i2c_piix4 xhci_pci_renesas nvme_auth realtek libahci video wmi gpio_amdpt
kernel: [ 101.973685] CPU: 1 PID: 3827 Comm: nx_huge_pages_t Not tainted 6.9.0-rc1-torv-00005-g928a87efa423-dirty #36
kernel: [ 101.973687] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
kernel: [ 101.973688] RIP: 0010:__warn_thunk (./arch/x86/kernel/cpu/bugs.c:2935 (discriminator 3))
kernel: [ 101.973691] Code: 62 c5 1d 01 83 e3 01 74 0e 48 8b 5d f8 c9 31 f6 31 ff e9 be 98 3b 01 48 c7 c7 98 21 c1 bc c6 05 22 26 8d 02 01 e8 90 aa 07 00 <0f> 0b 48 8b 5d f8 c9 31 f6 31 ff e9 9b 98 3b 01 90 90 90 90 90 90
All code
========
0: 62 c5 1d 01 83 (bad)
5: e3 01 jrcxz 0x8
7: 74 0e je 0x17
9: 48 8b 5d f8 mov -0x8(%rbp),%rbx
d: c9 leave
e: 31 f6 xor %esi,%esi
10: 31 ff xor %edi,%edi
12: e9 be 98 3b 01 jmp 0x13b98d5
17: 48 c7 c7 98 21 c1 bc mov $0xffffffffbcc12198,%rdi
1e: c6 05 22 26 8d 02 01 movb $0x1,0x28d2622(%rip) # 0x28d2647
25: e8 90 aa 07 00 call 0x7aaba
2a:* 0f 0b ud2 <-- trapping instruction
2c: 48 8b 5d f8 mov -0x8(%rbp),%rbx
30: c9 leave
31: 31 f6 xor %esi,%esi
33: 31 ff xor %edi,%edi
35: e9 9b 98 3b 01 jmp 0x13b98d5
3a: 90 nop
3b: 90 nop
3c: 90 nop
3d: 90 nop
3e: 90 nop
3f: 90 nop

Code starting with the faulting instruction
===========================================
0: 0f 0b ud2
2: 48 8b 5d f8 mov -0x8(%rbp),%rbx
6: c9 leave
7: 31 f6 xor %esi,%esi
9: 31 ff xor %edi,%edi
b: e9 9b 98 3b 01 jmp 0x13b98ab
10: 90 nop
11: 90 nop
12: 90 nop
13: 90 nop
14: 90 nop
15: 90 nop
kernel: [ 101.973692] RSP: 0018:ffffbbd90580fc90 EFLAGS: 00010046
kernel: [ 101.973694] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
kernel: [ 101.973695] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
kernel: [ 101.973696] RBP: ffffbbd90580fc98 R08: 0000000000000000 R09: 0000000000000000
kernel: [ 101.973697] R10: 0000000000000000 R11: 0000000000000000 R12: ffff9964e4b7d4f0
kernel: [ 101.973698] R13: 0000000000000000 R14: 0000000000000000 R15: ffff9964e4b7dc70
kernel: [ 101.973699] FS: 0000720b95372740(0000) GS:ffff9973d7a80000(0000) knlGS:0000000000000000
kernel: [ 101.973700] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kernel: [ 101.973701] CR2: 0000000000000000 CR3: 00000001aea6c000 CR4: 0000000000f50ef0
kernel: [ 101.973703] PKRU: 55555554
kernel: [ 101.973703] Call Trace:
kernel: [ 101.973704] <TASK>
kernel: [ 101.973706] ? show_regs (./arch/x86/kernel/dumpstack.c:479)
kernel: [ 101.973709] ? __warn_thunk (./arch/x86/kernel/cpu/bugs.c:2935 (discriminator 3))
kernel: [ 101.973711] ? __warn (./kernel/panic.c:694)
kernel: [ 101.973713] ? __warn_thunk (./arch/x86/kernel/cpu/bugs.c:2935 (discriminator 3))
kernel: [ 101.973715] ? report_bug (./lib/bug.c:201 ./lib/bug.c:219)
kernel: [ 101.973718] ? irq_work_queue (./kernel/irq_work.c:119)
kernel: [ 101.973722] ? handle_bug (./arch/x86/kernel/traps.c:218)
kernel: [ 101.973725] ? exc_invalid_op (./arch/x86/kernel/traps.c:260 (discriminator 1))
kernel: [ 101.973727] ? asm_exc_invalid_op (././arch/x86/include/asm/idtentry.h:621)
kernel: [ 101.973731] ? __warn_thunk (./arch/x86/kernel/cpu/bugs.c:2935 (discriminator 3))
kernel: [ 101.973734] warn_thunk_thunk (./arch/x86/entry/entry.S:48)
kernel: [ 101.973738] svm_vcpu_enter_exit (././include/linux/kvm_host.h:547 ./arch/x86/kvm/svm/svm.c:4115)
kernel: [ 101.973740] svm_vcpu_run (././arch/x86/include/asm/cpufeature.h:171 ./arch/x86/kvm/svm/svm.c:4186)
kernel: [ 101.973744] kvm_arch_vcpu_ioctl_run (./arch/x86/kvm/x86.c:11008 ./arch/x86/kvm/x86.c:11211 ./arch/x86/kvm/x86.c:11437)
kernel: [ 101.973747] ? srso_alias_return_thunk (./arch/x86/lib/retpoline.S:181)
kernel: [ 101.973750] ? srso_alias_return_thunk (./arch/x86/lib/retpoline.S:181)
kernel: [ 101.973752] ? kvm_vm_stats_read (./arch/x86/kvm/../../../virt/kvm/kvm_main.c:5066)
kernel: [ 101.973755] kvm_vcpu_ioctl (./arch/x86/kvm/../../../virt/kvm/kvm_main.c:4464)
kernel: [ 101.973757] ? srso_alias_return_thunk (./arch/x86/lib/retpoline.S:181)
kernel: [ 101.973759] ? trace_hardirqs_on_prepare (./kernel/trace/trace_preemptirq.c:47 ./kernel/trace/trace_preemptirq.c:42)
kernel: [ 101.973761] ? srso_alias_return_thunk (./arch/x86/lib/retpoline.S:181)
kernel: [ 101.973763] ? syscall_exit_to_user_mode (./kernel/entry/common.c:221)
kernel: [ 101.973765] ? srso_alias_return_thunk (./arch/x86/lib/retpoline.S:181)
kernel: [ 101.973767] ? do_syscall_64 (././arch/x86/include/asm/cpufeature.h:171 ./arch/x86/entry/common.c:98)
kernel: [ 101.973770] __x64_sys_ioctl (./fs/ioctl.c:51 ./fs/ioctl.c:904 ./fs/ioctl.c:890 ./fs/ioctl.c:890)
kernel: [ 101.973773] do_syscall_64 (./arch/x86/entry/common.c:52 ./arch/x86/entry/common.c:83)
kernel: [ 101.973775] ? srso_alias_return_thunk (./arch/x86/lib/retpoline.S:181)
kernel: [ 101.973777] ? irqentry_exit (./kernel/entry/common.c:367)
kernel: [ 101.973778] ? srso_alias_return_thunk (./arch/x86/lib/retpoline.S:181)
kernel: [ 101.973780] entry_SYSCALL_64_after_hwframe (./arch/x86/entry/entry_64.S:129)
kernel: [ 101.973782] RIP: 0033:0x720b9511a94f
kernel: [ 101.973798] Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41> 89 c0 3d 00 f0 ff ff 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
All code
========
0: 00 48 89 add %cl,-0x77(%rax)
3: 44 24 18 rex.R and $0x18,%al
6: 31 c0 xor %eax,%eax
8: 48 8d 44 24 60 lea 0x60(%rsp),%rax
d: c7 04 24 10 00 00 00 movl $0x10,(%rsp)
14: 48 89 44 24 08 mov %rax,0x8(%rsp)
19: 48 8d 44 24 20 lea 0x20(%rsp),%rax
1e: 48 89 44 24 10 mov %rax,0x10(%rsp)
23: b8 10 00 00 00 mov $0x10,%eax
28: 0f 05 syscall
2a:* 41 89 c0 mov %eax,%r8d <-- trapping instruction
2d: 3d 00 f0 ff ff cmp $0xfffff000,%eax
32: 77 1f ja 0x53
34: 48 8b 44 24 18 mov 0x18(%rsp),%rax
39: 64 fs
3a: 48 rex.W
3b: 2b .byte 0x2b
3c: 04 25 add $0x25,%al
3e: 28 00 sub %al,(%rax)

Code starting with the faulting instruction
===========================================
0: 41 89 c0 mov %eax,%r8d
3: 3d 00 f0 ff ff cmp $0xfffff000,%eax
8: 77 1f ja 0x29
a: 48 8b 44 24 18 mov 0x18(%rsp),%rax
f: 64 fs
10: 48 rex.W
11: 2b .byte 0x2b
12: 04 25 add $0x25,%al
14: 28 00 sub %al,(%rax)
kernel: [ 101.973799] RSP: 002b:00007ffd786b9ca0 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
kernel: [ 101.973801] RAX: ffffffffffffffda RBX: 0000000000600000 RCX: 0000720b9511a94f
kernel: [ 101.973802] RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005
kernel: [ 101.973803] RBP: 0000720b953726c0 R08: 000000000041b228 R09: 0000000000000000
kernel: [ 101.973804] R10: 0000720b951d8882 R11: 0000000000000246 R12: 000000000c9b18c0
kernel: [ 101.973805] R13: 000000000c9b18c0 R14: 0000000000000000 R15: 0000000000000064
kernel: [ 101.973809] </TASK>
kernel: [ 101.973810] ---[ end trace 0000000000000000 ]---

NOTE: Cc:-ed author of the reproducer for these results.
NOTE 2: The stacktrace is only displayed once, repeating the reproducer doesn't work until the next reboot.

Sending the latest config as well attached:

Best regards,
Mirsad Todorovac

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