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[Eric Desrochers <eric.desrochers@xxxxxxxxxxxxx>]

** Also affects: linux (Ubuntu Cosmic)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1791758

Title:
  ldisc crash on reopened tty

Status in linux package in Ubuntu:
  In Progress
Status in linux source package in Trusty:
  Confirmed
Status in linux source package in Xenial:
  Confirmed
Status in linux source package in Bionic:
  Confirmed
Status in linux source package in Cosmic:
  Confirmed

Bug description:
  [Impact]

  The following Oops was discovered by user:

  [684766.666639] BUG: unable to handle kernel paging request at 0000000000002268
  [684766.667642] IP: [<ffffffff814e2a5a>] n_tty_receive_buf_common+0x6a/0xae0
  [684766.668487] PGD 80000019574fe067 PUD 19574ff067 PMD 0
  [684766.669194] Oops: 0000 [#1] SMP
  [684766.669687] Modules linked in: xt_nat dccp_diag dccp tcp_diag udp_diag inet_diag unix_diag xt_connmark ipt_REJECT nf_reject_ipv4 nf_conntrack_netlink nfnetlink veth ip6table_filter ip6_tables xt_tcpmss xt_multiport xt_conntrack iptable_filter xt_CHECKSUM xt_tcpudp iptable_mangle xt_CT iptable_raw ipt_MASQUERADE nf_nat_masquerade_ipv4 xt_comment iptable_nat ip_tables x_tables target_core_mod configfs softdog scini(POE) ib_iser rdma_cm iw_cm ib_cm ib_sa ib_mad ib_core ib_addr iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi openvswitch(OE) nf_nat_ipv6 nf_nat_ipv4 nf_nat gre kvm_intel kvm irqbypass ttm crct10dif_pclmul drm_kms_helper crc32_pclmul ghash_clmulni_intel drm aesni_intel aes_x86_64 i2c_piix4 lrw gf128mul fb_sys_fops syscopyarea glue_helper sysfillrect ablk_helper cryptd sysimgblt joydev
  [684766.679406]  input_leds mac_hid serio_raw 8250_fintek br_netfilter bridge stp llc nf_conntrack_proto_gre nf_conntrack_ipv6 nf_defrag_ipv6 nf_conntrack_ipv4 nf_defrag_ipv4 nf_conntrack xfs raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 psmouse multipath floppy pata_acpi linear dm_multipath
  [684766.683585] CPU: 15 PID: 7470 Comm: kworker/u40:1 Tainted: P           OE   4.4.0-124-generic #148~14.04.1-Ubuntu
  [684766.684967] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
  [684766.686062] Workqueue: events_unbound flush_to_ldisc
  [684766.686703] task: ffff88165e5d8000 ti: ffff88170dc2c000 task.ti: ffff88170dc2c000
  [684766.687670] RIP: 0010:[<ffffffff814e2a5a>]  [<ffffffff814e2a5a>] n_tty_receive_buf_common+0x6a/0xae0
  [684766.688870] RSP: 0018:ffff88170dc2fd28  EFLAGS: 00010202
  [684766.689521] RAX: 0000000000000000 RBX: ffff88162c895000 RCX: 0000000000000001
  [684766.690488] RDX: 0000000000000000 RSI: ffff88162c895020 RDI: ffff8819c2d3d4d8
  [684766.691518] RBP: ffff88170dc2fdc0 R08: 0000000000000001 R09: ffffffff81ec2ba0
  [684766.692480] R10: 0000000000000004 R11: 0000000000000000 R12: ffff8819c2d3d400
  [684766.693423] R13: ffff8819c45b2670 R14: ffff8816a358c028 R15: ffff8819c2d3d400
  [684766.694390] FS:  0000000000000000(0000) GS:ffff8819d73c0000(0000) knlGS:0000000000000000
  [684766.695484] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [684766.696182] CR2: 0000000000002268 CR3: 0000001957520000 CR4: 0000000000360670
  [684766.697141] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  [684766.698114] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  [684766.699079] Stack:
  [684766.699412]  0000000000000000 ffff8819c2d3d4d8 0000000000000000 ffff8819c2d3d648
  [684766.700467]  ffff8819c2d3d620 ffff8819c9c10400 ffff88170dc2fd68 ffffffff8106312e
  [684766.701501]  ffff88170dc2fd78 0000000000000001 0000000000000000 ffff88162c895020
  [684766.702534] Call Trace:
  [684766.702905]  [<ffffffff8106312e>] ? kvm_sched_clock_read+0x1e/0x30
  [684766.703685]  [<ffffffff814e34e4>] n_tty_receive_buf2+0x14/0x20
  [684766.704505]  [<ffffffff814e5f05>] flush_to_ldisc+0xd5/0x120
  [684766.705269]  [<ffffffff81099506>] process_one_work+0x156/0x400
  [684766.706008]  [<ffffffff81099eea>] worker_thread+0x11a/0x480
  [684766.706686]  [<ffffffff81099dd0>] ? rescuer_thread+0x310/0x310
  [684766.707386]  [<ffffffff8109f3b8>] kthread+0xd8/0xf0
  [684766.707993]  [<ffffffff8109f2e0>] ? kthread_park+0x60/0x60
  [684766.708664]  [<ffffffff8181a9b5>] ret_from_fork+0x55/0x80
  [684766.709335]  [<ffffffff8109f2e0>] ? kthread_park+0x60/0x60
  [684766.709998] Code: 85 70 ff ff ff e8 97 5f 33 00 49 8d 87 20 02 00 00 c7 45 b4 00 00 00 00 48 89 45 88 49 8d 87 48 02 00 00 48 89 45 80 48 8b 45 b8 <48> 8b b0 68 22 00 00 48 8b 08 89 f0 29 c8 41 f6 87 30 01 00 00
  [684766.713290] RIP  [<ffffffff814e2a5a>] n_tty_receive_buf_common+0x6a/0xae0
  [684766.714105]  RSP <ffff88170dc2fd28>
  [684766.714609] CR2: 0000000000002268

  The issue happened in a VM
  KDUMP was configured, so a full Kernel crashdump was created

  User has Ubuntu Trusty, Kernel 4.4.0-124 on its VM

  [Test Case]

  * Deploy a Trusty KVM instance with a LTS Xenial kernel (v4.4 series)
  * SSH in frequently while system is under load, send commands before the prompt has returned.
  ----

  Check comment #5 for a summary about the upstream proposals to resolve
  this issue.

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