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KASAN: slab-use-after-free Read in bch2_copygc (2)

Status: moderation: reported on 2025/09/10 12:57
Subsystems: bcachefs
[Documentation on labels]
Reported-by: syzbot+6a24aca9726195b7023f@syzkaller.appspotmail.com
First crash: 5d21h, last: 3d09h
Similar bugs (3)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
upstream KASAN: slab-use-after-free Read in bch2_copygc bcachefs 19 C done 25 7d23h 65d 29/29 fixed on 2025/09/04 16:56
upstream KMSAN: uninit-value in bch2_copygc bcachefs 7 C 88 243d 307d 0/29 auto-obsoleted due to no activity on 2025/03/22 22:59
upstream KMSAN: uninit-value in bch2_copygc (2) bcachefs 7 1 168d 164d 28/29 fixed on 2025/06/10 16:19

Sample crash report:
bcachefs (loop0): Detected missing backpointers in bucket 34, now have 1/128 with missing
  running recovery pass check_extents_to_backpointers (17), currently at delete_dead_snapshots (26)
==================================================================
BUG: KASAN: slab-use-after-free in instrument_atomic_read include/linux/instrumented.h:68 [inline]
BUG: KASAN: slab-use-after-free in _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
BUG: KASAN: slab-use-after-free in bch2_bucket_bitmap_test fs/bcachefs/backpointers.h:194 [inline]
BUG: KASAN: slab-use-after-free in bch2_bucket_is_movable fs/bcachefs/movinggc.c:78 [inline]
BUG: KASAN: slab-use-after-free in bch2_copygc_get_buckets fs/bcachefs/movinggc.c:157 [inline]
BUG: KASAN: slab-use-after-free in bch2_copygc+0x105d/0x4510 fs/bcachefs/movinggc.c:221
Read of size 8 at addr ffff8880426a4dc0 by task bch-copygc/loop/5375

CPU: 0 UID: 0 PID: 5375 Comm: bch-copygc/loop Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
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/0x240 mm/kasan/report.c:482
 kasan_report+0x118/0x150 mm/kasan/report.c:595
 check_region_inline mm/kasan/generic.c:-1 [inline]
 kasan_check_range+0x2b0/0x2c0 mm/kasan/generic.c:189
 instrument_atomic_read include/linux/instrumented.h:68 [inline]
 _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
 bch2_bucket_bitmap_test fs/bcachefs/backpointers.h:194 [inline]
 bch2_bucket_is_movable fs/bcachefs/movinggc.c:78 [inline]
 bch2_copygc_get_buckets fs/bcachefs/movinggc.c:157 [inline]
 bch2_copygc+0x105d/0x4510 fs/bcachefs/movinggc.c:221
 bch2_copygc_thread+0x97a/0xe00 fs/bcachefs/movinggc.c:409
 kthread+0x70e/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

Allocated by task 5375:
 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:388 [inline]
 __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:405
 kasan_kmalloc include/linux/kasan.h:260 [inline]
 __do_kmalloc_node mm/slub.c:4376 [inline]
 __kvmalloc_node_noprof+0x30d/0x5f0 mm/slub.c:5067
 kvmalloc_array_node_noprof include/linux/slab.h:1065 [inline]
 bch2_bucket_bitmap_set+0x9a/0x1d0 fs/bcachefs/backpointers.c:1351
 check_bucket_backpointer_mismatch+0x1bdf/0x23a0 fs/bcachefs/backpointers.c:964
 check_bucket_backpointer_pos_mismatch fs/bcachefs/backpointers.c:1193 [inline]
 bch2_check_bucket_backpointer_mismatch+0x36c/0x690 fs/bcachefs/backpointers.c:1205
 __bch2_move_data_phys+0x17a7/0x1c50 fs/bcachefs/move.c:922
 bch2_evacuate_bucket+0x228/0x3a0 fs/bcachefs/move.c:1082
 bch2_copygc+0x3be3/0x4510 fs/bcachefs/movinggc.c:234
 bch2_copygc_thread+0x97a/0xe00 fs/bcachefs/movinggc.c:409
 kthread+0x70e/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

The buggy address belongs to the object at ffff8880426a4dc0
 which belongs to the cache kmalloc-16 of size 16
The buggy address is located 0 bytes inside of
 freed 16-byte region [ffff8880426a4dc0, ffff8880426a4dd0)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x426a4
flags: 0x4fff00000000000(node=1|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 04fff00000000000 ffff88801a441640 dead000000000122 0000000000000000
raw: 0000000000000000 0000000080800080 00000000f5000000 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x52cc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP), pid 5364, tgid 5363 (syz.0.0), ts 86357103551, free_ts 86322734116
 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:2492 [inline]
 allocate_slab+0x8a/0x370 mm/slub.c:2660
 new_slab mm/slub.c:2714 [inline]
 ___slab_alloc+0xbeb/0x1420 mm/slub.c:3901
 __slab_alloc mm/slub.c:3992 [inline]
 __slab_alloc_node mm/slub.c:4067 [inline]
 slab_alloc_node mm/slub.c:4228 [inline]
 __do_kmalloc_node mm/slub.c:4375 [inline]
 __kmalloc_noprof+0x305/0x4f0 mm/slub.c:4388
 kmalloc_noprof include/linux/slab.h:909 [inline]
 kmalloc_array_noprof include/linux/slab.h:948 [inline]
 __bch2_sb_replicas_v0_to_cpu_replicas+0x131/0xad0 fs/bcachefs/replicas.c:563
 bch2_sb_replicas_v0_validate+0x48/0xb0 fs/bcachefs/replicas.c:774
 bch2_sb_field_validate+0x140/0x280 fs/bcachefs/super-io.c:1375
 bch2_sb_validate+0x1569/0x1980 fs/bcachefs/super-io.c:560
 bch2_write_super+0xd68/0x2d30 fs/bcachefs/super-io.c:1089
 bch2_sb_recovery_pass_complete fs/bcachefs/recovery_passes.c:154 [inline]
 bch2_run_recovery_pass fs/bcachefs/recovery_passes.c:496 [inline]
 __bch2_run_recovery_passes+0x64f/0x1060 fs/bcachefs/recovery_passes.c:539
 bch2_run_recovery_passes+0x184/0x210 fs/bcachefs/recovery_passes.c:610
 bch2_fs_recovery+0x2690/0x3a50 fs/bcachefs/recovery.c:1016
 bch2_fs_start+0xaaf/0xda0 fs/bcachefs/super.c:1213
page last free pid 5364 tgid 5363 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
 rcu_do_batch kernel/rcu/tree.c:2605 [inline]
 rcu_core+0xcab/0x1770 kernel/rcu/tree.c:2861
 handle_softirqs+0x283/0x870 kernel/softirq.c:579
 __do_softirq kernel/softirq.c:613 [inline]
 invoke_softirq kernel/softirq.c:453 [inline]
 __irq_exit_rcu+0xca/0x1f0 kernel/softirq.c:680
 irq_exit_rcu+0x9/0x30 kernel/softirq.c:696
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1050 [inline]
 sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1050
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702

Memory state around the buggy address:
 ffff8880426a4c80: 00 00 fc fc 00 00 fc fc fa fb fc fc fa fb fc fc
 ffff8880426a4d00: fa fb fc fc fa fb fc fc 00 00 fc fc 00 00 fc fc
>ffff8880426a4d80: 00 00 fc fc 00 00 fc fc fb fb fc fc 00 00 fc fc
                                           ^
 ffff8880426a4e00: 00 00 fc fc fa fb fc fc fa fb fc fc 00 00 fc fc
 ffff8880426a4e80: fa fb fc fc fa fb fc fc 00 00 fc fc 00 00 fc fc
==================================================================

Crashes (3):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2025/09/09 00:24 upstream f777d1112ee5 d291dd2d .config console log report [disk image (non-bootable)] [vmlinux] [kernel image] ci-snapshot-upstream-root KASAN: slab-use-after-free Read in bch2_copygc
2025/09/07 01:21 upstream b236920731dd d291dd2d .config console log report [disk image (non-bootable)] [vmlinux] [kernel image] ci-snapshot-upstream-root KASAN: slab-use-after-free Read in bch2_copygc
2025/09/06 12:49 upstream d1d10cea0895 d291dd2d .config console log report [disk image (non-bootable)] [vmlinux] [kernel image] ci-snapshot-upstream-root KASAN: slab-use-after-free Read in bch2_copygc
* Struck through repros no longer work on HEAD.