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BUG: KASAN: slab-use-after-free in bch2_copygc+0x105d/0x4510
Read of size 8 at addr ffff888120b22060 by task bch-copygc/loop/15723

CPU: 1 UID: 0 PID: 15723 Comm: bch-copygc/loop Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250
 print_report+0xca/0x240
 kasan_report+0x118/0x150
 kasan_check_range+0x2b0/0x2c0
 bch2_copygc+0x105d/0x4510
 bch2_copygc_thread+0x97a/0xe00
 kthread+0x711/0x8a0
 ret_from_fork+0x3fc/0x770
 ret_from_fork_asm+0x1a/0x30
 </TASK>

Allocated by task 15723:
 kasan_save_track+0x3e/0x80
 __kasan_kmalloc+0x93/0xb0
 __kvmalloc_node_noprof+0x30d/0x5f0
 bch2_bucket_bitmap_set+0x9a/0x1d0
 check_bucket_backpointer_mismatch+0x1bdf/0x23a0
 bch2_check_bucket_backpointer_mismatch+0x36c/0x690
 __bch2_move_data_phys+0x17a7/0x1c50
 bch2_evacuate_bucket+0x228/0x3a0
 bch2_copygc+0x3be3/0x4510
 bch2_copygc_thread+0x97a/0xe00
 kthread+0x711/0x8a0
 ret_from_fork+0x3fc/0x770
 ret_from_fork_asm+0x1a/0x30

Freed by task 15712:
 kasan_save_track+0x3e/0x80
 kasan_save_free_info+0x46/0x50
 __kasan_slab_free+0x5b/0x80
 kfree+0x18e/0x440
 bch2_check_extents_to_backpointers+0x2840/0x2bc0
 __bch2_run_recovery_passes+0x3bd/0x1060
 bch2_run_recovery_passes+0x184/0x210
 bch2_fs_recovery+0x2690/0x3a50
 bch2_fs_start+0xaaf/0xda0
 bch2_fs_get_tree+0xb39/0x1520
 vfs_get_tree+0x92/0x2b0
 do_new_mount+0x2a2/0x9e0
 __se_sys_mount+0x317/0x410
 do_syscall_64+0xfa/0x3b0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff888120b22060
 which belongs to the cache kmalloc-16 of size 16
The buggy address is located 0 bytes inside of
 freed 16-byte region [ffff888120b22060, ffff888120b22070)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff888120b228a0 pfn:0x120b22
flags: 0x57ff00000000000(node=1|zone=2|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 057ff00000000000 ffff88801a441640 dead000000000100 dead000000000122
raw: ffff888120b228a0 000000008080007f 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 0xd2c40(GFP_NOFS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 12986, tgid 12985 (syz.4.1455), ts 358524099601, free_ts 358510070093
 register_dummy_stack+0x89/0xe0
 init_page_owner+0x2e/0x620
 page_ext_init+0x511/0x550
 mm_core_init+0x56/0x70
page last free pid 13005 tgid 13004 stack trace:
 __free_frozen_pages+0xbc4/0xd30
 tlb_finish_mmu+0x112/0x1d0
 exit_mmap+0x44c/0xb50
 __mmput+0x118/0x420
 exit_mm+0x1da/0x2c0
 do_exit+0x648/0x2300
 do_group_exit+0x21c/0x2d0
 get_signal+0x1286/0x1340
 arch_do_signal_or_restart+0x9a/0x750
 exit_to_user_mode_loop+0x75/0x110
 do_syscall_64+0x2bd/0x3b0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
 ffff888120b21f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff888120b21f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff888120b22000: 00 00 fc fc 00 00 fc fc fa fb fc fc fa fb fc fc
                                                       ^
 ffff888120b22080: 00 00 fc fc 00 00 fc fc 00 07 fc fc 00 07 fc fc
 ffff888120b22100: 00 00 fc fc 00 00 fc fc 00 00 fc fc 00 05 fc fc
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