Re: [syzbot] [ext4?] KASAN: use-after-free Read in ext4_find_extent (4)

[syzbot <syzbot+ee60e584b5c6bb229126@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
KASAN: use-after-free Read in ext4_find_extent

==================================================================
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 ffff88805f4ebd18 by task kworker/u8:3/50

CPU: 1 UID: 0 PID: 50 Comm: kworker/u8:3 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Workqueue: ext4-rsv-conversion ext4_end_io_rsv_work
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
 ext4_ext_binsearch fs/ext4/extents.c:841 [inline]
 ext4_find_extent+0xae6/0xcc0 fs/ext4/extents.c:956
 ext4_ext_map_blocks+0x288/0x6ac0 fs/ext4/extents.c:4208
 ext4_map_create_blocks fs/ext4/inode.c:609 [inline]
 ext4_map_blocks+0x860/0x1740 fs/ext4/inode.c:811
 ext4_convert_unwritten_extents+0x2ae/0x5d0 fs/ext4/extents.c:4976
 ext4_convert_unwritten_io_end_vec+0xff/0x170 fs/ext4/extents.c:5016
 ext4_end_io_end+0xc7/0x410 fs/ext4/page-io.c:199
 ext4_do_flush_completed_IO fs/ext4/page-io.c:290 [inline]
 ext4_end_io_rsv_work+0x262/0x330 fs/ext4/page-io.c:305
 process_one_work kernel/workqueue.c:3263 [inline]
 process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x4 pfn:0x5f4eb
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000000000 ffffea0001ab2448 ffffea0001b7a848 0000000000000000
raw: 0000000000000004 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 0x141cca(GFP_HIGHUSER_MOVABLE|__GFP_WRITE|__GFP_COMP), pid 6484, tgid 6483 (syz.0.20), ts 137862241539, free_ts 137891188282
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x234/0x290 mm/page_alloc.c:1845
 prep_new_page mm/page_alloc.c:1853 [inline]
 get_page_from_freelist+0x2356/0x2430 mm/page_alloc.c:3879
 __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5178
 alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2416
 alloc_frozen_pages_noprof mm/mempolicy.c:2487 [inline]
 alloc_pages_noprof+0xa9/0x190 mm/mempolicy.c:2507
 folio_alloc_noprof+0x1e/0x30 mm/mempolicy.c:2517
 filemap_alloc_folio_noprof+0xdf/0x470 mm/filemap.c:1020
 __filemap_get_folio+0x3f2/0xaf0 mm/filemap.c:2012
 write_begin_get_folio include/linux/pagemap.h:784 [inline]
 ext4_write_begin+0x4c8/0x19a0 fs/ext4/inode.c:1318
 ext4_da_write_begin+0x445/0xda0 fs/ext4/inode.c:3129
 generic_perform_write+0x2c5/0x900 mm/filemap.c:4254
 ext4_buffered_write_iter+0xce/0x3a0 fs/ext4/file.c:299
 ext4_file_write_iter+0x298/0x1bc0 fs/ext4/file.c:-1
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x5c9/0xb30 fs/read_write.c:686
 ksys_pwrite64 fs/read_write.c:793 [inline]
 __do_sys_pwrite64 fs/read_write.c:801 [inline]
 __se_sys_pwrite64 fs/read_write.c:798 [inline]
 __x64_sys_pwrite64+0x193/0x220 fs/read_write.c:798
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
page last free pid 6341 tgid 6341 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1394 [inline]
 free_unref_folios+0xe5d/0x15f0 mm/page_alloc.c:2958
 folios_put_refs+0x584/0x670 mm/swap.c:1002
 folio_batch_release include/linux/pagevec.h:101 [inline]
 truncate_inode_pages_range+0x346/0xda0 mm/truncate.c:408
 ext4_evict_inode+0x1aa/0xee0 fs/ext4/inode.c:185
 evict+0x504/0x9c0 fs/inode.c:810
 dispose_list fs/inode.c:852 [inline]
 evict_inodes+0x64c/0x6d0 fs/inode.c:906
 generic_shutdown_super+0x9a/0x2c0 fs/super.c:627
 kill_block_super+0x44/0x90 fs/super.c:1729
 ext4_kill_sb+0x68/0xb0 fs/ext4/super.c:7403
 deactivate_locked_super+0xbc/0x130 fs/super.c:473
 cleanup_mnt+0x425/0x4c0 fs/namespace.c:1318
 task_work_run+0x1d4/0x260 kernel/task_work.c:227
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 exit_to_user_mode_loop+0xe9/0x130 kernel/entry/common.c:43
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
 do_syscall_64+0x2bd/0xfa0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
 ffff88805f4ebc00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88805f4ebc80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff88805f4ebd00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                            ^
 ffff88805f4ebd80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88805f4ebe00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


Tested on:

commit:         23cb64fb Merge tag 'soc-fixes-6.18-3' of git://git.ker..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12608c2a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=6e611fe59206f39f
dashboard link: https://syzkaller.appspot.com/bug?extid=ee60e584b5c6bb229126
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=16dbca12580000