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BUG: KASAN: use-after-free in ext4_ext_binsearch fs/ext4/extents.c:841 [inline]
BUG: KASAN: use-after-free in ext4_find_extent+0xae6/0xcc0 fs/ext4/extents.c:956
Read of size 4 at addr ffff888055bfd02c by task kworker/u4:24/3950

CPU: 0 UID: 0 PID: 3950 Comm: kworker/u4:24 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
Workqueue: writeback wb_workfn (flush-7:0)
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 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
 ext4_ext_binsearch fs/ext4/extents.c:841 [inline]
 ext4_find_extent+0xae6/0xcc0 fs/ext4/extents.c:956
 ext4_ext_map_blocks+0x278/0x69c0 fs/ext4/extents.c:4208
 ext4_map_create_blocks fs/ext4/inode.c:613 [inline]
 ext4_map_blocks+0x82c/0x16f0 fs/ext4/inode.c:816
 mpage_map_one_extent fs/ext4/inode.c:2380 [inline]
 mpage_map_and_submit_extent fs/ext4/inode.c:2474 [inline]
 ext4_do_writepages+0x222f/0x4500 fs/ext4/inode.c:2932
 ext4_writepages+0x203/0x350 fs/ext4/inode.c:3026
 do_writepages+0x32e/0x550 mm/page-writeback.c:2598
 __writeback_single_inode+0x133/0x1240 fs/fs-writeback.c:1737
 writeback_sb_inodes+0x93a/0x1870 fs/fs-writeback.c:2030
 __writeback_inodes_wb+0x111/0x240 fs/fs-writeback.c:2107
 wb_writeback+0x43f/0xaa0 fs/fs-writeback.c:2218
 wb_check_start_all fs/fs-writeback.c:2344 [inline]
 wb_do_writeback fs/fs-writeback.c:2370 [inline]
 wb_workfn+0x8ee/0xed0 fs/fs-writeback.c:2403
 process_one_work kernel/workqueue.c:3257 [inline]
 process_scheduled_works+0xad1/0x1770 kernel/workqueue.c:3340
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3421
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x510/0xa50 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
 </TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x55bfd
flags: 0x4fff00000000000(node=1|zone=1|lastcpupid=0x7ff)
raw: 04fff00000000000 ffffea000156ff88 ffffea000156ff08 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner info is not present (never set?)

Memory state around the buggy address:
 ffff888055bfcf00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888055bfcf80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888055bfd000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                  ^
 ffff888055bfd080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888055bfd100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
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