u64s 9 type backpointer 0:8912896:0 len 0 ver 0: bucket=0:34:0 btree=extents level=0 data_type=user suboffset=0 len=8 gen=0 pos=4099:8:U32_MAX
  u64s 5 type deleted 4099:8:U32_MAX len 0 ver 0, fixing
bcachefs (loop0): backpointer doesn't match extent it points to:
  u64s 9 type backpointer 0:8921088:0 len 0 ver 0: bucket=0:34:8 btree=extents level=0 data_type=user suboffset=0 len=8 gen=0 pos=536870913:24:U32_MAX
  u64s 5 type deleted 536870913:24:U32_MAX len 0 ver 0, fixing
 done
bcachefs (loop0): check_inodes... done
bcachefs (loop0): check_extents... done
bcachefs (loop0): check_dirents...
bcachefs (loop0): hash table key at wrong offset: btree dirents inode 4096 offset 189491840996961599, hashed to 3599253987125639218
  u64s 7 type dirent 4096:189491840996961599:U32_MAX len 0 ver 0: file0 -> 4098 type dir, fixing
bcachefs (loop0): hash table key at wrong offset: btree dirents inode 4096 offset 1896155912177158345, hashed to 2522370371696746673
  u64s 7 type dirent 4096:1896155912177158345:U32_MAX len 0 ver 0: file3 -> 536870913 type reg, fixing
bcachefs (loop0): dirent points to missing inode:
  u64s 7 type dirent 4096:2522370371696746673:U32_MAX len 0 ver 0: file3 -> 536870913 type reg, fixing
bcachefs (loop0): hash table key at wrong offset: btree dirents inode 4096 offset 2695648408715017799, hashed to 5115730495017562328
  u64s 7 type dirent 4096:2695648408715017799:U32_MAX len 0 ver 0: file2 -> 536870913 type reg, fixing
bcachefs (loop0): dirent points to missing inode:
  u64s 7 type dirent 4096:3599253987125639218:U32_MAX len 0 ver 0: file0 -> 4098 type dir, fixing
bcachefs (loop0): hash table key at wrong offset: btree dirents inode 4096 offset 4330382808765825483, hashed to 5521592781784957642
  u64s 7 type dirent 4096:4330382808765825483:U32_MAX len 0 ver 0: file1 -> 536870912 type reg, fixing
bcachefs (loop0): dirent points to missing inode:
  u64s 7 type dirent 4096:5115730495017562328:U32_MAX len 0 ver 0: file2 -> 536870913 type reg, fixing
bcachefs (loop0): dirent points to missing inode:
  u64s 7 type dirent 4096:5521592781784957642:U32_MAX len 0 ver 0: file1 -> 536870912 type reg, fixing
bcachefs (loop0): hash table key at wrong offset: btree dirents inode 4096 offset 8130059955150870709, hashed to 2932249477474183151
  u64s 8 type dirent 4096:8130059955150870709:U32_MAX len 0 ver 0: lost+found -> 4097 type dir, fixing
bcachefs (loop0): hash table key at wrong offset: btree dirents inode 4096 offset 9097378837824744618, hashed to 6782548715641329734
  u64s 8 type dirent 4096:9097378837824744618:U32_MAX len 0 ver 0: file.cold -> 536870914 type reg, fixing
bcachefs (loop0): directory 4096:4294967295 with wrong i_nlink: got 0, should be 1, fixing
bcachefs (loop0): hash table key at wrong offset: btree dirents inode 4098 offset 5675548428000973578, hashed to 3401079042334272574
  u64s 7 type dirent 4098:5675548428000973578:U32_MAX len 0 ver 0: file1 -> 4100 type lnk, fixing
bcachefs (loop0): hash table key at wrong offset: btree dirents inode 4098 offset 8977922886548783724, hashed to 8216491066183496210
  u64s 7 type dirent 4098:8977922886548783724:U32_MAX len 0 ver 0: file0 -> 4099 type reg, fixing
 done
bcachefs (loop0): resume_logged_ops... done
bcachefs (loop0): delete_dead_inodes... done
bcachefs (loop0): Fixed errors, running fsck a second time to verify fs is clean
bcachefs (loop0): check_lrus... done
bcachefs (loop0): check_backpointers_to_extents... done
bcachefs (loop0): check_inodes... done
bcachefs (loop0): check_extents... done
bcachefs (loop0): check_dirents...
bcachefs (loop0): dirent points to missing inode:
  u64s 8 type dirent 4096:2932249477474183151:U32_MAX len 0 ver 0: lost+found -> 4097 type dir, fixing
bcachefs (loop0): dirent points to missing inode:
  u64s 8 type dirent 4096:6782548715641329734:U32_MAX len 0 ver 0: file.cold -> 536870914 type reg, fixing
bcachefs (loop0): dirent points to missing inode:
  u64s 7 type dirent 4098:3401079042334272574:U32_MAX len 0 ver 0: file1 -> 4100 type lnk, fixing
==================================================================
BUG: KASAN: use-after-free in check_dirent fs/bcachefs/fsck.c:2205 [inline]
BUG: KASAN: use-after-free in bch2_check_dirents+0x2aac/0x3b90 fs/bcachefs/fsck.c:2230
Read of size 1 at addr ffff888056680120 by task syz.0.0/5328

CPU: 0 UID: 0 PID: 5328 Comm: syz.0.0 Not tainted 6.15.0-rc1-syzkaller-00246-g900241a5cc15 #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 lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:408 [inline]
 print_report+0x16e/0x5b0 mm/kasan/report.c:521
 kasan_report+0x143/0x180 mm/kasan/report.c:634
 check_dirent fs/bcachefs/fsck.c:2205 [inline]
 bch2_check_dirents+0x2aac/0x3b90 fs/bcachefs/fsck.c:2230
 bch2_run_recovery_pass+0xf0/0x1e0 fs/bcachefs/recovery_passes.c:226
 bch2_run_recovery_passes+0x2ad/0xa90 fs/bcachefs/recovery_passes.c:285
 bch2_fs_recovery+0x2c65/0x3e20 fs/bcachefs/recovery.c:973
 bch2_fs_start+0x310/0x620 fs/bcachefs/super.c:1059
 bch2_fs_get_tree+0x113e/0x18f0 fs/bcachefs/fs.c:2253
 vfs_get_tree+0x90/0x2b0 fs/super.c:1759
 do_new_mount+0x2cf/0xb70 fs/namespace.c:3879
 do_mount fs/namespace.c:4219 [inline]
 __do_sys_mount fs/namespace.c:4430 [inline]
 __se_sys_mount+0x38c/0x400 fs/namespace.c:4407
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fd08958e90a
Code: d8 64 89 02 48 c7 c0 ff ff ff ff eb a6 e8 de 1a 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fd08a422e68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007fd08a422ef0 RCX: 00007fd08958e90a
RDX: 0000200000000000 RSI: 0000200000000040 RDI: 00007fd08a422eb0
RBP: 0000200000000000 R08: 00007fd08a422ef0 R09: 0000000001000001
R10: 0000000001000001 R11: 0000000000000246 R12: 0000200000000040
R13: 00007fd08a422eb0 R14: 0000000000005977 R15: 0000200000000540
 </TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x56680
flags: 0x4fff00000000000(node=1|zone=1|lastcpupid=0x7ff)
page_type: f0(buddy)
raw: 04fff00000000000 ffff88805ffd6f08 ffff88805ffd6f08 0000000000000000
raw: 0000000000000000 0000000000000005 00000000f0000000 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as freed
page last allocated via order 5, migratetype Unmovable, gfp_mask 0x52800(GFP_NOWAIT|__GFP_NORETRY|__GFP_COMP), pid 5328, tgid 5327 (syz.0.0), ts 83743172498, free_ts 83858722822
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x1f4/0x240 mm/page_alloc.c:1717
 prep_new_page mm/page_alloc.c:1725 [inline]
 get_page_from_freelist+0x352b/0x36c0 mm/page_alloc.c:3652
 __alloc_frozen_pages_noprof+0x211/0x5b0 mm/page_alloc.c:4934
 __alloc_pages_noprof+0xa/0x30 mm/page_alloc.c:4968
 __alloc_pages_node_noprof include/linux/gfp.h:284 [inline]
 alloc_pages_node_noprof include/linux/gfp.h:311 [inline]
 ___kmalloc_large_node+0x92/0x210 mm/slub.c:4275
 __kmalloc_large_node_noprof+0x1a/0x80 mm/slub.c:4303
 __do_kmalloc_node mm/slub.c:4319 [inline]
 __kvmalloc_node_noprof+0x7c/0x5a0 mm/slub.c:5016
 btree_bounce_alloc fs/bcachefs/btree_io.c:124 [inline]
 btree_node_sort+0x67c/0x1870 fs/bcachefs/btree_io.c:323
 bch2_btree_post_write_cleanup+0x11a/0xaa0 fs/bcachefs/btree_io.c:2508
 bch2_btree_node_write_trans+0x18a/0x7a0 fs/bcachefs/btree_io.c:2577
 btree_node_write_if_need fs/bcachefs/btree_io.h:157 [inline]
 __btree_node_flush+0x3a1/0x470 fs/bcachefs/btree_trans_commit.c:253
 bch2_btree_node_flush0+0x27/0x40 fs/bcachefs/btree_trans_commit.c:262
 journal_flush_pins+0x89b/0xe40 fs/bcachefs/journal_reclaim.c:589
 journal_flush_pins_or_still_flushing fs/bcachefs/journal_reclaim.c:859 [inline]
 journal_flush_done+0x107/0x820 fs/bcachefs/journal_reclaim.c:877
 bch2_journal_flush_pins+0x19f/0x3c0 fs/bcachefs/journal_reclaim.c:909
 bch2_journal_flush_all_pins fs/bcachefs/journal_reclaim.h:76 [inline]
 bch2_fs_recovery+0x2af1/0x3e20 fs/bcachefs/recovery.c:957
page last free pid 5328 tgid 5327 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1262 [inline]
 __free_pages_ok+0xb1d/0xda0 mm/page_alloc.c:1437
 __folio_put+0x2b5/0x360 mm/swap.c:112
 folio_put include/linux/mm.h:1563 [inline]
 free_large_kmalloc+0x143/0x1e0 mm/slub.c:4771
 kfree+0x216/0x430 mm/slub.c:4839
 btree_bounce_free fs/bcachefs/btree_io.c:112 [inline]
 btree_node_sort+0x1124/0x1870 fs/bcachefs/btree_io.c:380
 bch2_btree_post_write_cleanup+0x11a/0xaa0 fs/bcachefs/btree_io.c:2508
 bch2_btree_node_prep_for_write+0x35a/0x670 fs/bcachefs/btree_trans_commit.c:93
 bch2_trans_lock_write+0x66f/0xb60 fs/bcachefs/btree_trans_commit.c:129
 do_bch2_trans_commit fs/bcachefs/btree_trans_commit.c:840 [inline]
 __bch2_trans_commit+0x3252/0x9da0 fs/bcachefs/btree_trans_commit.c:1050
 bch2_trans_commit fs/bcachefs/btree_update.h:195 [inline]
 check_dirent fs/bcachefs/fsck.c:2200 [inline]
 bch2_check_dirents+0x2801/0x3b90 fs/bcachefs/fsck.c:2230
 bch2_run_recovery_pass+0xf0/0x1e0 fs/bcachefs/recovery_passes.c:226
 bch2_run_recovery_passes+0x2ad/0xa90 fs/bcachefs/recovery_passes.c:285
 bch2_fs_recovery+0x2c65/0x3e20 fs/bcachefs/recovery.c:973
 bch2_fs_start+0x310/0x620 fs/bcachefs/super.c:1059
 bch2_fs_get_tree+0x113e/0x18f0 fs/bcachefs/fs.c:2253
 vfs_get_tree+0x90/0x2b0 fs/super.c:1759

Memory state around the buggy address:
 ffff888056680000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888056680080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888056680100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                               ^
 ffff888056680180: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888056680200: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================