Comment 0 for bug 1789638

Kernel: 4.15.0-1021-azure

How to reproduce:

stress-ng --sysfs 0 -t 120

One gets the following:

[ 22.451885] BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
[ 22.455286] IP: read_avail_show+0x1c/0x40
[ 22.455286] PGD 800000042d59e067 P4D 800000042d59e067 PUD 42eb8c067 PMD 0
[ 22.455286] Oops: 0000 [#1] SMP PTI
[ 22.455286] Modules linked in: nf_conntrack_ipv4 nf_defrag_ipv4 xt_owner xt_conntrack nf_conntrack iptable_security ip_tables x_tables serio_raw joydev hv_balloon ib_iser iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi autofs4 btrfs zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear hid_generic crct10dif_pclmul crc32_pclmul ghash_clmulni_intel pcbc aesni_intel aes_x86_64 crypto_simd glue_helper cryptd hyperv_fb hid_hyperv pata_acpi cfbfillrect hyperv_keyboard cfbimgblt hid cfbcopyarea hv_netvsc hv_utils
[ 22.455286] CPU: 1 PID: 1670 Comm: cat Not tainted 4.15.0-1021-azure #21~16.04.1-Ubuntu
[ 22.455286] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090007 06/02/2017
[ 22.455286] RIP: 0010:read_avail_show+0x1c/0x40
[ 22.455286] RSP: 0018:ffffafa4c4eafdb0 EFLAGS: 00010286
[ 22.455286] RAX: 0000000000000000 RBX: ffff9db36c93e880 RCX: ffff9db36f136908
[ 22.860062] RDX: 0000000000000000 RSI: ffff9db364548000 RDI: ffff9db364548000
[ 22.888042] RBP: ffffafa4c4eafdb0 R08: ffff9db364548000 R09: ffff9db36c049840
[ 22.920041] R10: ffff9db364548000 R11: 0000000000000000 R12: ffffffff92ae9440
[ 22.948058] R13: ffff9db36c22d200 R14: 0000000000000001 R15: ffff9db36c93e880
[ 22.972043] FS: 00007f67eeec6700(0000) GS:ffff9db37fd00000(0000) knlGS:0000000000000000
[ 23.004046] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 23.024016] CR2: 0000000000000004 CR3: 000000042c37a003 CR4: 00000000001606e0
[ 23.048014] Call Trace:
[ 23.060019] vmbus_chan_attr_show+0x21/0x30
[ 23.076018] sysfs_kf_seq_show+0xa2/0x130
[ 23.088030] kernfs_seq_show+0x27/0x30
[ 23.100020] seq_read+0xb7/0x480
[ 23.112014] kernfs_fop_read+0x111/0x190
[ 23.128017] ? security_file_permission+0xa1/0xc0
[ 23.144013] __vfs_read+0x1b/0x40
[ 23.156019] vfs_read+0x93/0x130
[ 23.168013] SyS_read+0x55/0xc0
[ 23.180021] do_syscall_64+0x73/0x130
[ 23.192014] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[ 23.212022] RIP: 0033:0x7f67ee9d8260
[ 23.224016] RSP: 002b:00007fffdc193ff8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[ 23.252022] RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007f67ee9d8260
[ 23.276019] RDX: 0000000000020000 RSI: 00007f67eed0c000 RDI: 0000000000000003
[ 23.300020] RBP: 0000000000020000 R08: ffffffffffffffff R09: 0000000000000000
[ 23.328025] R10: 000000000000037b R11: 0000000000000246 R12: 00007f67eed0c000
[ 23.352036] R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000020000
[ 23.376678] Code: fb 3a 17 00 48 98 5d c3 0f 1f 80 00 00 00 00 0f 1f 44 00 00 55 48 8b 87 38 01 00 00 49 89 f0 8b 97 48 01 00 00 4c 89 c7 48 89 e5 <8b> 48 04 8b 00 29 ca 89 c6 29 ce 01 c2 39 c1 0f 46 d6 48 c7 c6
[ 23.444022] RIP: read_avail_show+0x1c/0x40 RSP: ffffafa4c4eafdb0
[ 23.468021] CR2: 0000000000000004
[ 23.481135] ---[ end trace 348a4b7d5a6747d1 ]---

Cornered this down to just reading:

cat /sys/devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A03:00/device:07/VMBUS:01/99221fa0-24ad-11e2-be98-001aa01bbf6e/channels/4/read_avail