Hi Dexuan, 

To clarify my question: What is the cause of the VMexit (ie. trap to the hyperv) in this scenario? 

It might be worth it to do an experiment and reverse the order of operations i.e. first start the hello program in a loop and THEN start the nested VM via KVM. Then it would be the nested KVM operation causing an expected VMexit which is not handled properly due to the running 32 bit program. 

Regards,K.

    On Wednesday, December 16, 2020, 5:52:20 PM PST, kaveh moezzi <email address hidden> wrote:  
 
  Thank you Dexuan. What does VMexit mean here please? Trying to see how the VM exit triggers a kernel panic, since we were just running a 32 bit hello world program, which should not cause a the host VM to "exit". 

Regards. 

    On Wednesday, December 16, 2020, 5:11:37 PM PST, Dexuan Cui <email address hidden> wrote:  
 
 Hyper-V team just identified a bug where the Hyper-V hypervisor can
truncate the host SYSENTER_ESP/EIP to 16 bits on VMexit for some reason.
A further investigation is ongoing.

-- 
You received this bug notification because you are subscribed to the bug
report.
https://bugs.launchpad.net/bugs/1904632

Title:
  Ubuntu 18.04 Azure VM host kernel panic

Status in linux-azure package in Ubuntu:
  New

Bug description:
  Running a container on an DV3 Standard_D8_v3 Azure host, as the
  container comes up, the Azure host VM kernel panics per the logs
  below.

  Isolated the issue to a process in the container which uses the
  virtual NICs available on the Azure host. The container also is
  running Ubuntu 18.04 based packages. The problem happens every single
  time the container is started, unless its NIC access process is not
  started.

  Has this sort of kernel panic on Azure been seen and what is the root
  cause and remedy please.

  Also the kernel logs on the Azure host show it vulnerable to the
  following CVE. There are other VMs and containers that can run on the
  Azure host without a kernel panic on it, but providing this info in
  case there is some tie-in to the panic.

  https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-3646

  Kernel panic from the Azure Host console:

  Microsoft.EnterpriseCloud.Monitoring.OmsAgentForLinux_1.13.33_e857c609-bc35-4b66-9a8b-e86fd8707e82.scope
  2020-11-17T00:50:11.537914Z INFO MonitorHandler ExtHandler Stopped tracking cgroup: Microsoft.EnterpriseCloud.Monitoring.OmsAgentForLinux-1.13.33, path: /sys/fs/cgroup/memory/system.slice/Microsoft.EnterpriseCloud.Monitoring.OmsAgentForLinux_1.13.33_e857c609-bc35-4b66-9a8b-e86fd8707e82.scope
  2020-11-17T00:50:23.291433Z INFO ExtHandler ExtHandler Checking for agent updates (family: Prod)
  2020-11-17T00:51:11.677191Z INFO ExtHandler ExtHandler [HEARTBEAT] Agent WALinuxAgent-2.2.52 is running as the goal state agent [DEBUG HeartbeatCounter: 7;HeartbeatId: 8A2DD5B7-02E5-46E2-9EDB-F8CCBA274479;DroppedPackets: 0;UpdateGSErrors: 0;AutoUpdate: 1]
  [11218.537937] PANIC: double fault, error_code: 0x0
  [11218.541423] Kernel panic - not syncing: Machine halted.
  [11218.541423] CPU: 0 PID: 9281 Comm: vmxt Not tainted 4.15.18+test #1
  [11218.541423] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090008  12/07/2018
  [11218.541423] Call Trace:
  [11218.541423]  <#DF>
  [11218.541423]  dump_stack+0x63/0x8b
  [11218.541423]  panic+0xe4/0x244
  [11218.541423]  df_debug+0x2d/0x30
  [11218.541423]  do_double_fault+0x9a/0x130
  [11218.541423]  double_fault+0x1e/0x30
  [11218.541423] RIP: 0010:0x1a80
  [11218.541423] RSP: 0018:0000000000002200 EFLAGS: 00010096
  [11218.541423] RAX: 0000000000000102 RBX: 00000000f7a40768 RCX: 000000000000002f
  [11218.541423] RDX: 00000000f7ee9970 RSI: 00000000f7a40700 RDI: 00000000f7c3a000
  [11218.541423] RBP: 00000000fffd6430 R08: 0000000000000000 R09: 0000000000000000
  [11218.541423] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
  [11218.541423] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
  [11218.541423]  </#DF>
  [11218.541423] Kernel Offset: 0x2a400000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
  [11218.541423] ---[ end Kernel panic - not syncing: Machine halted.
  [11218.636804] ------------[ cut here ]------------
  [11218.640802] sched: Unexpected reschedule of offline CPU#2!
  [11218.640802] WARNING: CPU: 0 PID: 9281 at arch/x86/kernel/smp.c:128 native_smp_send_reschedule+0x3f/0x50
  [11218.640802] Modules linked in: xt_nat xt_u32 vxlan ip6_udp_tunnel udp_tunnel veth nf_conntrack_netlink nfnetlink xfrm_user xfrm_algo xt_addrtype br_netfilter xt_CHECKSUM iptable_mangle ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat ipt_REJECT nf_reject_ipv4 xt_tcpudp bridge stp llc ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter aufs xt_owner iptable_security xt_conntrack overlay openvswitch nsh nf_conntrack_ipv6 nf_nat_ipv6 nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_defrag_ipv6 nf_nat nf_conntrack nls_iso8859_1 joydev input_leds mac_hid kvm_intel hv_balloon kvm serio_raw irqbypass intel_rapl_perf sch_fq_codel ib_iser rdma_cm iw_cm ib_cm ib_core iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi ip_tables x_tables autofs4 btrfs zstd_compress raid10 raid456 async_raid6_recov
  [11218.640802]  async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear hid_generic crct10dif_pclmul crc32_pclmul hid_hyperv ghash_clmulni_intel hv_utils hv_storvsc pcbc ptp hv_netvsc hid pps_core scsi_transport_fc hyperv_keyboard aesni_intel aes_x86_64 crypto_simd hyperv_fb floppy glue_helper cryptd psmouse hv_vmbus i2c_piix4 pata_acpi
  [11218.640802] CPU: 0 PID: 9281 Comm: vmxt Not tainted 4.15.18+test #1
  [11218.640802] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090008  12/07/2018
  [11218.640802] RIP: 0010:native_smp_send_reschedule+0x3f/0x50
  [11218.640802] RSP: 0018:ffff9446bfc03e08 EFLAGS: 00010082
  [11218.640802] RAX: 0000000000000000 RBX: 0000000000000002 RCX: 0000000000000006
  [11218.640802] RDX: 0000000000000007 RSI: 0000000000000082 RDI: ffff9446bfc16490
  [11218.640802] RBP: ffff9446bfc03e08 R08: 0000000000000000 R09: 0000000000001480
  [11218.640802] R10: 0000000000000549 R11: 0000000000000038 R12: ffff9446bfca2880
  [11218.640802] R13: 0000000000000000 R14: 000000010029a6b8 R15: ffff9446bfc1cd28
  [11218.640802] FS:  0000000000000000(0000) GS:ffff9446bfc00000(0063) knlGS:00000000f7a40700
  [11218.640802] CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
  [11218.640802] CR2: 00000000000021f8 CR3: 000000084c576004 CR4: 00000000003626f0
  [11218.640802] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  [11218.640802] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  [11218.640802] Call Trace:
  [11218.640802]  <IRQ>
  [11218.640802]  trigger_load_balance+0x12a/0x230
  [11218.640802]  scheduler_tick+0xae/0xd0
  [11218.640802]  ? tick_sched_do_timer+0x40/0x40
  [11218.640802]  update_process_times+0x47/0x60
  [11218.640802]  tick_sched_handle+0x2a/0x60
  [11218.640802]  tick_sched_timer+0x39/0x80
  [11218.640802]  __hrtimer_run_queues+0xe7/0x230
  [11218.640802]  hrtimer_interrupt+0xb1/0x200
  [11218.640802]  vmbus_isr+0x16c/0x2a0 [hv_vmbus]
  [11218.640802]  hyperv_vector_handler+0x3f/0x6e
  [11218.640802]  hyperv_callback_vector+0x84/0x90
  [11218.640802]  </IRQ>
  [11218.640802]  <#DF>
  [11218.640802] RIP: 0010:panic+0x1fe/0x244
  [11218.640802] RSP: 0018:fffffe0000007e90 EFLAGS: 00000286 ORIG_RAX: ffffffffffffff0c
  [11218.640802] RAX: 0000000000000034 RBX: fffffe0000007f00 RCX: 0000000000000006
  [11218.640802] RDX: 0000000000000000 RSI: 0000000000000096 RDI: ffff9446bfc16490
  [11218.640802] RBP: fffffe0000007f08 R08: 0000000000000000 R09: 000000000000147e
  [11218.640802] R10: 0000000000000000 R11: 0000000000000038 R12: 0000000000000000
  [11218.640802] R13: 0000000000000000 R14: 000000084c577804 R15: 0000000000000000
  [11218.640802]  df_debug+0x2d/0x30
  [11218.640802]  do_double_fault+0x9a/0x130
  [11218.640802]  double_fault+0x1e/0x30
  [11218.640802] RIP: 0010:0x1a80
  [11218.640802] RSP: 0018:0000000000002200 EFLAGS: 00010096
  [11218.640802] RAX: 0000000000000102 RBX: 00000000f7a40768 RCX: 000000000000002f
  [11218.640802] RDX: 00000000f7ee9970 RSI: 00000000f7a40700 RDI: 00000000f7c3a000
  [11218.640802] RBP: 00000000fffd6430 R08: 0000000000000000 R09: 0000000000000000
  [11218.640802] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
  [11218.640802] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
  [11218.640802]  </#DF>
  [11218.640802] Code: 92 c0 84 c0 74 17 48 8b 05 7f 71 15 01 be fd 00 00 00 48 8b 40 30 e8 31 b6 ba 00 5d c3 89 fe 48 c7 c7 10 e3 4b ac e8 b1 49 03 00 <0f> 0b 5d c3 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 
  [11218.640802] ---[ end trace 857d64f92b00ceb2 ]---
  [11221.669166] hyperv_fb: Unable to send packet via vmbus
  [11221.669167] hyperv_fb: Unable to send packet via vmbus
  [11221.669167] hyperv_fb: Unable to send packet via vmbus
  [11221.669167] hyperv_fb: Unable to send packet via vmbu

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux-azure/+bug/1904632/+subscriptions