Re: [syzbot] [bcachefs?] KASAN: slab-use-after-free Write in bch2_get_next_dev

[Kent Overstreet]

On Fri, Jul 18, 2025 at 09:09:35AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    024e09e444bd Add linux-next specific files for 20250717
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=107317d4580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=99c60e2181be9059
> dashboard link: https://syzkaller.appspot.com/bug?extid=2b98caf09c41174a9697
> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11ad7d8c580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/0a89a1e83302/disk-024e09e4.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/e2264946780a/vmlinux-024e09e4.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/d684a7204e1c/bzImage-024e09e4.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/5f7f631c7091/mount_2.gz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+2b98caf09c41174a9697@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ==================================================================
> BUG: KASAN: slab-use-after-free in instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
> BUG: KASAN: slab-use-after-free in atomic_long_inc_return include/linux/atomic/atomic-instrumented.h:3609 [inline]
> BUG: KASAN: slab-use-after-free in bch2_dev_get fs/bcachefs/sb-members.h:114 [inline]
> BUG: KASAN: slab-use-after-free in bch2_get_next_dev+0x38f/0x4e0 fs/bcachefs/sb-members.h:145
> Write of size 8 at addr ffff888029ae2040 by task kworker/u9:0/51
> 
> CPU: 1 UID: 0 PID: 51 Comm: kworker/u9:0 Not tainted 6.16.0-rc6-next-20250717-syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
> Workqueue: bcachefs_journal journal_write_done
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>  print_address_description mm/kasan/report.c:378 [inline]
>  print_report+0xca/0x230 mm/kasan/report.c:480
>  kasan_report+0x118/0x150 mm/kasan/report.c:593
>  check_region_inline mm/kasan/generic.c:-1 [inline]
>  kasan_check_range+0x2b0/0x2c0 mm/kasan/generic.c:189
>  instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
>  atomic_long_inc_return include/linux/atomic/atomic-instrumented.h:3609 [inline]
>  bch2_dev_get fs/bcachefs/sb-members.h:114 [inline]
>  bch2_get_next_dev+0x38f/0x4e0 fs/bcachefs/sb-members.h:145
>  bch2_do_discards+0x20/0x160 fs/bcachefs/alloc_background.c:1935
>  journal_write_done+0x11b9/0x13f0 fs/bcachefs/journal_io.c:1822
>  process_one_work kernel/workqueue.c:3239 [inline]
>  process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3322
>  worker_thread+0x8a0/0xda0 kernel/workqueue.c:3403
>  kthread+0x70e/0x8a0 kernel/kthread.c:463
>  ret_from_fork+0x3f9/0x770 arch/x86/kernel/process.c:148
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> 
> Allocated by task 7622:
>  kasan_save_stack mm/kasan/common.c:47 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
>  poison_kmalloc_redzone mm/kasan/common.c:377 [inline]
>  __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:394
>  kasan_kmalloc include/linux/kasan.h:260 [inline]
>  __kmalloc_cache_noprof+0x230/0x3d0 mm/slub.c:4396
>  kmalloc_noprof include/linux/slab.h:905 [inline]
>  kzalloc_noprof include/linux/slab.h:1039 [inline]
>  __bch2_dev_alloc+0xa7/0xc30 fs/bcachefs/super.c:1487
>  bch2_dev_alloc+0xcd/0x180 fs/bcachefs/super.c:1558
>  bch2_fs_alloc fs/bcachefs/super.c:1068 [inline]
>  bch2_fs_open+0x256f/0x2760 fs/bcachefs/super.c:2395
>  bch2_fs_get_tree+0x44f/0x1540 fs/bcachefs/fs.c:2440
>  vfs_get_tree+0x8f/0x2b0 fs/super.c:1815
>  do_new_mount+0x2a2/0x9e0 fs/namespace.c:3805
>  do_mount fs/namespace.c:4133 [inline]
>  __do_sys_mount fs/namespace.c:4344 [inline]
>  __se_sys_mount+0x317/0x410 fs/namespace.c:4321
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> Freed by task 7622:
>  kasan_save_stack mm/kasan/common.c:47 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
>  kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:576
>  poison_slab_object mm/kasan/common.c:247 [inline]
>  __kasan_slab_free+0x62/0x70 mm/kasan/common.c:264
>  kasan_slab_free include/linux/kasan.h:233 [inline]
>  slab_free_hook mm/slub.c:2417 [inline]
>  slab_free mm/slub.c:4680 [inline]
>  kfree+0x18e/0x440 mm/slub.c:4879
>  kobject_cleanup lib/kobject.c:689 [inline]
>  kobject_release lib/kobject.c:720 [inline]
>  kref_put include/linux/kref.h:65 [inline]
>  kobject_put+0x22b/0x480 lib/kobject.c:737
>  bch2_fs_free+0x4b3/0x5c0 fs/bcachefs/super.c:749
>  bch2_fs_get_tree+0xb76/0x1540 fs/bcachefs/fs.c:2572
>  vfs_get_tree+0x8f/0x2b0 fs/super.c:1815
>  do_new_mount+0x2a2/0x9e0 fs/namespace.c:3805
>  do_mount fs/namespace.c:4133 [inline]
>  __do_sys_mount fs/namespace.c:4344 [inline]
>  __se_sys_mount+0x317/0x410 fs/namespace.c:4321
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> Last potentially related work creation:
>  kasan_save_stack+0x3e/0x60 mm/kasan/common.c:47
>  kasan_record_aux_stack+0xbd/0xd0 mm/kasan/generic.c:548
>  insert_work+0x3d/0x330 kernel/workqueue.c:2187
>  __queue_work+0xcd2/0xfb0 kernel/workqueue.c:2342
>  queue_work_on+0x181/0x270 kernel/workqueue.c:2393
>  bch2_btree_node_read_done+0x159b/0x5560 fs/bcachefs/btree_io.c:1156
>  btree_node_read_work+0x4ee/0xfa0 fs/bcachefs/btree_io.c:1444
>  bch2_btree_node_read+0x83e/0x2aa0 fs/bcachefs/btree_io.c:1865
>  __bch2_btree_root_read fs/bcachefs/btree_io.c:1906 [inline]
>  bch2_btree_root_read+0x5e5/0x780 fs/bcachefs/btree_io.c:1928
>  read_btree_roots+0x2b7/0x850 fs/bcachefs/recovery.c:602
>  bch2_fs_recovery+0x2513/0x38a0 fs/bcachefs/recovery.c:985
>  bch2_fs_start+0xa0f/0xcc0 fs/bcachefs/super.c:1216
>  bch2_fs_get_tree+0xb39/0x1540 fs/bcachefs/fs.c:2456
>  vfs_get_tree+0x8f/0x2b0 fs/super.c:1815
>  do_new_mount+0x2a2/0x9e0 fs/namespace.c:3805
>  do_mount fs/namespace.c:4133 [inline]
>  __do_sys_mount fs/namespace.c:4344 [inline]
>  __se_sys_mount+0x317/0x410 fs/namespace.c:4321
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> The buggy address belongs to the object at ffff888029ae2000
>  which belongs to the cache kmalloc-4k of size 4096
> The buggy address is located 64 bytes inside of
>  freed 4096-byte region [ffff888029ae2000, ffff888029ae3000)
> 
> The buggy address belongs to the physical page:
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x29ae0
> head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
> page_type: f5(slab)
> raw: 00fff00000000040 ffff88801a442140 dead000000000122 0000000000000000
> raw: 0000000000000000 0000000000040004 00000000f5000000 0000000000000000
> head: 00fff00000000040 ffff88801a442140 dead000000000122 0000000000000000
> head: 0000000000000000 0000000000040004 00000000f5000000 0000000000000000
> head: 00fff00000000003 ffffea0000a6b801 00000000ffffffff 00000000ffffffff
> head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
> page dumped because: kasan: bad access detected
> page_owner tracks the page as allocated
> page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd2820(GFP_ATOMIC|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 59, tgid 59 (kworker/u8:4), ts 293379161383, free_ts 284295390486
>  set_page_owner include/linux/page_owner.h:32 [inline]
>  post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1851
>  prep_new_page mm/page_alloc.c:1859 [inline]
>  get_page_from_freelist+0x21e4/0x22c0 mm/page_alloc.c:3858
>  __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5148
>  alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2416
>  alloc_slab_page mm/slub.c:2487 [inline]
>  allocate_slab+0x8a/0x370 mm/slub.c:2655
>  new_slab mm/slub.c:2709 [inline]
>  ___slab_alloc+0xbeb/0x1410 mm/slub.c:3891
>  __slab_alloc mm/slub.c:3981 [inline]
>  __slab_alloc_node mm/slub.c:4056 [inline]
>  slab_alloc_node mm/slub.c:4217 [inline]
>  __do_kmalloc_node mm/slub.c:4364 [inline]
>  __kmalloc_node_track_caller_noprof+0x2f8/0x4e0 mm/slub.c:4384
>  kmalloc_reserve+0x136/0x290 net/core/skbuff.c:600
>  __alloc_skb+0x142/0x2d0 net/core/skbuff.c:669
>  alloc_skb include/linux/skbuff.h:1336 [inline]
>  nsim_dev_trap_skb_build drivers/net/netdevsim/dev.c:761 [inline]
>  nsim_dev_trap_report drivers/net/netdevsim/dev.c:818 [inline]
>  nsim_dev_trap_report_work+0x29a/0xb80 drivers/net/netdevsim/dev.c:864
>  process_one_work kernel/workqueue.c:3239 [inline]
>  process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3322
>  worker_thread+0x8a0/0xda0 kernel/workqueue.c:3403
>  kthread+0x70e/0x8a0 kernel/kthread.c:463
>  ret_from_fork+0x3f9/0x770 arch/x86/kernel/process.c:148
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> page last free pid 7530 tgid 7530 stack trace:
>  reset_page_owner include/linux/page_owner.h:25 [inline]
>  free_pages_prepare mm/page_alloc.c:1395 [inline]
>  __free_frozen_pages+0xbc4/0xd30 mm/page_alloc.c:2895
>  discard_slab mm/slub.c:2753 [inline]
>  __put_partials+0x156/0x1a0 mm/slub.c:3218
>  put_cpu_partial+0x17c/0x250 mm/slub.c:3293
>  __slab_free+0x2d5/0x3c0 mm/slub.c:4550
>  qlink_free mm/kasan/quarantine.c:163 [inline]
>  qlist_free_all+0x97/0x140 mm/kasan/quarantine.c:179
>  kasan_quarantine_reduce+0x148/0x160 mm/kasan/quarantine.c:286
>  __kasan_slab_alloc+0x22/0x80 mm/kasan/common.c:329
>  kasan_slab_alloc include/linux/kasan.h:250 [inline]
>  slab_post_alloc_hook mm/slub.c:4180 [inline]
>  slab_alloc_node mm/slub.c:4229 [inline]
>  __do_kmalloc_node mm/slub.c:4364 [inline]
>  __kmalloc_node_track_caller_noprof+0x216/0x4e0 mm/slub.c:4384
>  __do_krealloc mm/slub.c:4942 [inline]
>  krealloc_noprof+0x124/0x340 mm/slub.c:4995
>  bch2_printbuf_make_room+0x1f5/0x360 fs/bcachefs/printbuf.c:59
>  bch2_prt_printf+0x282/0x740 fs/bcachefs/printbuf.c:186
>  bch2_btree_path_to_text_short+0x148/0xde0 fs/bcachefs/btree_iter.c:1516
>  __bch2_trans_paths_to_text+0xe5/0x190 fs/bcachefs/btree_iter.c:1589
>  bch2_trans_update_max_paths+0x15f/0x390 fs/bcachefs/btree_iter.c:1625
>  btree_path_alloc+0x89a/0xa70 fs/bcachefs/btree_iter.c:1718
>  bch2_path_get+0x8bd/0x1540 fs/bcachefs/btree_iter.c:1774
> 
> Memory state around the buggy address:
>  ffff888029ae1f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>  ffff888029ae1f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >ffff888029ae2000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                                            ^
>  ffff888029ae2080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff888029ae2100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> 
> 
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx.
> 
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> 
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
> 
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.

#syz test: