Describe CVE-2021-47178.
What is CVE-2021-47178?
In the Linux kernel, the following vulnerability has been resolved:

scsi: target: core: Avoid smp_processor_id() in preemptible code

The BUG message "BUG: using smp_processor_id() in preemptible [00000000]
code" was observed for TCMU devices with kernel config DEBUG_PREEMPT.

The message was observed when blktests block/005 was run on TCMU devices
with fileio backend or user:zbc backend [1]. The commit 1130b499b4a7
("scsi: target: tcm_loop: Use LIO wq cmd submission helper") triggered the
symptom. The commit modified work queue to handle commands and changed
'current->nr_cpu_allowed' at smp_processor_id() call.

The message was also observed at system shutdown when TCMU devices were not
cleaned up [2]. The function smp_processor_id() was called in SCSI host
work queue for abort handling, and triggered the BUG message. This symptom
was observed regardless of the commit 1130b499b4a7 ("scsi: target:
tcm_loop: Use LIO wq cmd submission helper").

To avoid the preemptible code check at smp_processor_id(), get CPU ID with
raw_smp_processor_id() instead. The CPU ID is used for performance
improvement then thread move to other CPU will not affect the code.

[1]

[   56.468103] run blktests block/005 at 2021-05-12 14:16:38
[   57.369473] check_preemption_disabled: 85 callbacks suppressed
[   57.369480] BUG: using smp_processor_id() in preemptible [00000000] code: fio/1511
[   57.369506] BUG: using smp_processor_id() in preemptible [00000000] code: fio/1510
[   57.369512] BUG: using smp_processor_id() in preemptible [00000000] code: fio/1506
[   57.369552] caller is __target_init_cmd+0x157/0x170 [target_core_mod]
[   57.369606] CPU: 4 PID: 1506 Comm: fio Not tainted 5.13.0-rc1+ #34
[   57.369613] Hardware name: System manufacturer System Product Name/PRIME Z270-A, BIOS 1302 03/15/2018
[   57.369617] Call Trace:
[   57.369621] BUG: using smp_processor_id() in preemptible [00000000] code: fio/1507
[   57.369628]  dump_stack+0x6d/0x89
[   57.369642]  check_preemption_disabled+0xc8/0xd0
[   57.369628] caller is __target_init_cmd+0x157/0x170 [target_core_mod]
[   57.369655]  __target_init_cmd+0x157/0x170 [target_core_mod]
[   57.369695]  target_init_cmd+0x76/0x90 [target_core_mod]
[   57.369732]  tcm_loop_queuecommand+0x109/0x210 [tcm_loop]
[   57.369744]  scsi_queue_rq+0x38e/0xc40
[   57.369761]  __blk_mq_try_issue_directly+0x109/0x1c0
[   57.369779]  blk_mq_try_issue_directly+0x43/0x90
[   57.369790]  blk_mq_submit_bio+0x4e5/0x5d0
[   57.369812]  submit_bio_noacct+0x46e/0x4e0
[   57.369830]  __blkdev_direct_IO_simple+0x1a3/0x2d0
[   57.369859]  ? set_init_blocksize.isra.0+0x60/0x60
[   57.369880]  generic_file_read_iter+0x89/0x160
[   57.369898]  blkdev_read_iter+0x44/0x60
[   57.369906]  new_sync_read+0x102/0x170
[   57.369929]  vfs_read+0xd4/0x160
[   57.369941]  __x64_sys_pread64+0x6e/0xa0
[   57.369946]  ? lockdep_hardirqs_on+0x79/0x100
[   57.369958]  do_syscall_64+0x3a/0x70
[   57.369965]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[   57.369973] RIP: 0033:0x7f7ed4c1399f
[   57.369979] Code: 08 89 3c 24 48 89 4c 24 18 e8 7d f3 ff ff 4c 8b 54 24 18 48 8b 54 24 10 41 89 c0 48 8b 74 24 08 8b 3c 24 b8 11 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 31 44 89 c7 48 89 04 24 e8 cd f3 ff ff 48 8b
[   57.369983] RSP: 002b:00007ffd7918c580 EFLAGS: 00000293 ORIG_RAX: 0000000000000011
[   57.369990] RAX: ffffffffffffffda RBX: 00000000015b4540 RCX: 00007f7ed4c1399f
[   57.369993] RDX: 0000000000001000 RSI: 00000000015de000 RDI: 0000000000000009
[   57.369996] RBP: 00000000015b4540 R08: 0000000000000000 R09: 0000000000000001
[   57.369999] R10: 0000000000e5c000 R11: 0000000000000293 R12: 00007f7eb5269a70
[   57.370002] R13: 0000000000000000 R14: 0000000000001000 R15: 00000000015b4568
[   57.370031] CPU: 7 PID: 1507 Comm: fio Not tainted 5.13.0-rc1+ #34
[   57.370036] Hardware name: System manufacturer System Product Name/PRIME Z270-A, BIOS 1302 03/15/2018
[   57.370039] Call Trace:
[   57.370045]  dump_stack+0x6d/0x89
[   57.370056]  ch
---truncated---.