syzbot

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BUG: KASAN: use-after-free in ext4_ext_insert_extent+0x41b6/0x4af0 fs/ext4/extents.c:2086
Read of size 4 at addr ffff8880767e94c8 by task syz.0.749/12162

CPU: 1 UID: 0 PID: 12162 Comm: syz.0.749 Not tainted 6.15.0-syzkaller-13473-gc0c9379f235d #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:408 [inline]
 print_report+0xd2/0x2b0 mm/kasan/report.c:521
 kasan_report+0x118/0x150 mm/kasan/report.c:634
 ext4_ext_insert_extent+0x41b6/0x4af0 fs/ext4/extents.c:2086
 ext4_ext_map_blocks+0x1792/0x6ac0 fs/ext4/extents.c:4404
 ext4_map_create_blocks fs/ext4/inode.c:609 [inline]
 ext4_map_blocks+0x931/0x18d0 fs/ext4/inode.c:813
 _ext4_get_block+0x200/0x4c0 fs/ext4/inode.c:892
 ext4_get_block_unwritten+0x2e/0x100 fs/ext4/inode.c:925
 ext4_block_write_begin+0x6f8/0x14b0 fs/ext4/inode.c:1178
 ext4_write_begin+0xa4f/0x1680 fs/ext4/ext4_jbd2.h:-1
 ext4_da_write_begin+0x449/0xd20 fs/ext4/inode.c:3057
 generic_perform_write+0x2c4/0x910 mm/filemap.c:4112
 ext4_buffered_write_iter+0xce/0x3a0 fs/ext4/file.c:299
 ext4_file_write_iter+0x298/0x1bc0 fs/ext4/file.c:-1
 do_iter_readv_writev+0x56e/0x7f0 fs/read_write.c:-1
 vfs_writev+0x31a/0x960 fs/read_write.c:1057
 do_writev+0x14d/0x2d0 fs/read_write.c:1103
 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
RIP: 0033:0x7f82f038e929
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 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:00007f82f11b8038 EFLAGS: 00000246 ORIG_RAX: 0000000000000014
RAX: ffffffffffffffda RBX: 00007f82f05b5fa0 RCX: 00007f82f038e929
RDX: 0000000000000001 RSI: 0000200000000140 RDI: 0000000000000005
RBP: 00007f82f0410b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f82f05b5fa0 R15: 00007ffca1c04d68
 </TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x638 pfn:0x767e9
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000000000 dead000000000100 dead000000000122 0000000000000000
raw: 0000000000000638 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as freed
page last allocated via order 0, migratetype Movable, gfp_mask 0x140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), pid 7501, tgid 7500 (syz.2.179), ts 194378279520, free_ts 198529268639
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1704
 prep_new_page mm/page_alloc.c:1712 [inline]
 get_page_from_freelist+0x21d5/0x22b0 mm/page_alloc.c:3669
 __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:4959
 alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2419
 folio_alloc_mpol_noprof+0x39/0x70 mm/mempolicy.c:2438
 shmem_alloc_folio mm/shmem.c:1851 [inline]
 shmem_alloc_and_add_folio+0x447/0xf60 mm/shmem.c:1890
 shmem_get_folio_gfp+0x59d/0x1660 mm/shmem.c:2532
 shmem_get_folio mm/shmem.c:2638 [inline]
 shmem_write_begin+0xf7/0x2b0 mm/shmem.c:3288
 generic_perform_write+0x2c4/0x910 mm/filemap.c:4112
 shmem_file_write_iter+0xf8/0x120 mm/shmem.c:3463
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x54b/0xa90 fs/read_write.c:686
 ksys_write+0x145/0x250 fs/read_write.c:738
 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
page last free pid 5834 tgid 5834 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1248 [inline]
 free_unref_folios+0xcd2/0x1570 mm/page_alloc.c:2763
 folios_put_refs+0x559/0x640 mm/swap.c:992
 folio_batch_release include/linux/pagevec.h:101 [inline]
 shmem_undo_range+0x49e/0x14b0 mm/shmem.c:1125
 shmem_truncate_range mm/shmem.c:1237 [inline]
 shmem_evict_inode+0x272/0xa70 mm/shmem.c:1365
 evict+0x501/0x9c0 fs/inode.c:810
 __dentry_kill+0x209/0x660 fs/dcache.c:669
 dput+0x19f/0x2b0 fs/dcache.c:911
 __fput+0x68e/0xa70 fs/file_table.c:473
 task_work_run+0x1d1/0x260 kernel/task_work.c:227
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 exit_to_user_mode_loop+0xec/0x110 kernel/entry/common.c:114
 exit_to_user_mode_prepare include/linux/entry-common.h:330 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:414 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:449 [inline]
 do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
 ffff8880767e9380: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8880767e9400: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff8880767e9480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                              ^
 ffff8880767e9500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8880767e9580: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
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