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KASAN: use-after-free Read in ext4_ext_insert_extent

Status: premoderation: reported syz repro on 2025/06/20 07:17
Bug presence: origin:lts
[Documentation on labels]
Reported-by: syzbot+0e29293a1af2ec0f0fcd@syzkaller.appspotmail.com
First crash: 330d, last: 24d
Fix bisection: failed (error log, bisect log)
  
▶ ▼ Bug presence (2)
Date Name Commit Repro Result
2025/06/20 lts (merge base) 1c700860e8bc syz [report] KASAN: use-after-free Read in ext4_ext_remove_space
2025/06/20 upstream (ToT) 75f5f23f8787 syz Didn't crash
▶ ▼ Similar bugs (4)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
linux-5.15 KASAN: use-after-free Read in ext4_ext_insert_extent 19 1 741d 741d 0/3 auto-obsoleted due to no activity on 2024/08/13 04:16
upstream KASAN: use-after-free Read in ext4_ext_insert_extent ext4 22 C done error 10 72d 356d 0/29 upstream: reported C repro on 2025/05/25 18:12
android-5-10 KASAN: use-after-free Read in ext4_ext_insert_extent 19 syz 5 27d 428d 0/2 premoderation: reported syz repro on 2025/03/14 06:10
upstream KASAN: slab-use-after-free Read in ext4_ext_insert_extent ext4 19 1 917d 913d 0/29 auto-obsoleted due to no activity on 2024/02/09 07:34
▶ ▼ Last patch testing requests (4)
Created Duration User Patch Repo Result
2026/04/02 02:35 5m retest repro android13-5.15-lts report log
2025/11/28 09:07 16m retest repro android13-5.15-lts report log
2025/09/18 06:30 10m retest repro android13-5.15-lts report log
2025/07/04 17:08 9m retest repro android13-5.15-lts report log

Sample crash report:
==================================================================
BUG: KASAN: use-after-free in ext4_ext_insert_extent+0x3e72/0x47c0 fs/ext4/extents.c:2077
Read of size 4 at addr ffff888134b6c124 by task syz.7.182/961

CPU: 0 PID: 961 Comm: syz.7.182 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call Trace:
 <TASK>
 __dump_stack+0x21/0x30 lib/dump_stack.c:88
 dump_stack_lvl+0x110/0x170 lib/dump_stack.c:106
 print_address_description+0x7f/0x2c0 mm/kasan/report.c:248
 __kasan_report mm/kasan/report.c:427 [inline]
 kasan_report+0xf1/0x140 mm/kasan/report.c:444
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report_generic.c:308
 ext4_ext_insert_extent+0x3e72/0x47c0 fs/ext4/extents.c:2077
 ext4_ext_map_blocks+0x186c/0x61e0 fs/ext4/extents.c:4368
 ext4_map_blocks+0x988/0x1b30 fs/ext4/inode.c:674
 _ext4_get_block+0x1ea/0x540 fs/ext4/inode.c:817
 ext4_get_block+0x39/0x50 fs/ext4/inode.c:834
 ext4_block_write_begin+0x563/0x1160 fs/ext4/inode.c:1102
 ext4_write_begin+0x65a/0x12e0 fs/ext4/ext4_jbd2.h:-1
 ext4_da_write_begin+0x321/0xbf0 fs/ext4/inode.c:2988
 generic_perform_write+0x2b7/0x690 mm/filemap.c:3870
 ext4_buffered_write_iter+0x4ed/0x670 fs/ext4/file.c:270
 ext4_file_write_iter+0x440/0x1540 fs/ext4/file.c:-1
 call_write_iter include/linux/fs.h:2212 [inline]
 new_sync_write fs/read_write.c:507 [inline]
 vfs_write+0x835/0xfd0 fs/read_write.c:594
 ksys_write+0x149/0x250 fs/read_write.c:647
 __do_sys_write fs/read_write.c:659 [inline]
 __se_sys_write fs/read_write.c:656 [inline]
 __x64_sys_write+0x7b/0x90 fs/read_write.c:656
 x64_sys_call+0x8ef/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7fbcbf1a2819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fbcbf005028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fbcbf41bfa0 RCX: 00007fbcbf1a2819
RDX: 000000000208e24b RSI: 0000200000000040 RDI: 0000000000000005
RBP: 00007fbcbf238c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fbcbf41c038 R14: 00007fbcbf41bfa0 R15: 00007ffc2522d2d8
 </TASK>

The buggy address belongs to the page:
page:ffffea0004d2db00 refcount:1 mapcount:1 mapping:0000000000000000 index:0x200000ab9 pfn:0x134b6c
memcg:ffff888100256780
anon flags: 0x4000000000080014(uptodate|lru|swapbacked|zone=1)
raw: 4000000000080014 ffffea00049fb988 ffffea0004d2db88 ffff8881310afe61
raw: 0000000200000ab9 0000000000000000 0000000100000000 ffff888100256780
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Movable, gfp_mask 0x100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO), pid 965, ts 118832303685, free_ts 118245185922
 set_page_owner include/linux/page_owner.h:33 [inline]
 post_alloc_hook+0x192/0x1b0 mm/page_alloc.c:2605
 prep_new_page+0x1c/0x110 mm/page_alloc.c:2611
 get_page_from_freelist+0x2d3a/0x2dc0 mm/page_alloc.c:4485
 __alloc_pages+0x1a2/0x460 mm/page_alloc.c:5822
 __alloc_pages_node include/linux/gfp.h:595 [inline]
 alloc_pages_node include/linux/gfp.h:609 [inline]
 alloc_pages include/linux/gfp.h:622 [inline]
 do_anonymous_page mm/memory.c:4069 [inline]
 handle_pte_fault+0xebf/0x2710 mm/memory.c:4909
 __handle_mm_fault mm/memory.c:-1 [inline]
 do_handle_mm_fault+0x1af9/0x1de0 mm/memory.c:5333
 handle_mm_fault include/linux/mm.h:1847 [inline]
 do_user_addr_fault+0x828/0x1250 arch/x86/mm/fault.c:1416
 handle_page_fault arch/x86/mm/fault.c:1507 [inline]
 exc_page_fault+0x51/0xb0 arch/x86/mm/fault.c:1560
 asm_exc_page_fault+0x27/0x30 arch/x86/include/asm/idtentry.h:606
page last free stack trace:
 reset_page_owner include/linux/page_owner.h:26 [inline]
 free_pages_prepare mm/page_alloc.c:1472 [inline]
 free_pcp_prepare mm/page_alloc.c:1544 [inline]
 free_unref_page_prepare+0x542/0x550 mm/page_alloc.c:3534
 free_unref_page_list+0x13a/0x9d0 mm/page_alloc.c:3671
 release_pages+0x1006/0x1060 mm/swap.c:1015
 free_pages_and_swap_cache+0x86/0xa0 mm/swap_state.c:320
 tlb_batch_pages_flush mm/mmu_gather.c:49 [inline]
 tlb_flush_mmu_free mm/mmu_gather.c:240 [inline]
 tlb_flush_mmu mm/mmu_gather.c:247 [inline]
 tlb_finish_mmu+0x17e/0x310 mm/mmu_gather.c:338
 exit_mmap+0x43b/0x8b0 mm/mmap.c:3248
 __mmput+0x93/0x320 kernel/fork.c:1180
 mmput+0x50/0x150 kernel/fork.c:1203
 exit_mm kernel/exit.c:554 [inline]
 do_exit+0x9f2/0x27e0 kernel/exit.c:876
 do_group_exit+0x141/0x310 kernel/exit.c:1003
 get_signal+0x66a/0x1480 kernel/signal.c:2907
 arch_do_signal_or_restart+0xdf/0x11c0 arch/x86/kernel/signal.c:867
 handle_signal_work kernel/entry/common.c:154 [inline]
 exit_to_user_mode_loop+0xa7/0xe0 kernel/entry/common.c:178
 exit_to_user_mode_prepare+0x87/0xd0 kernel/entry/common.c:214
 __syscall_exit_to_user_mode_work kernel/entry/common.c:296 [inline]
 syscall_exit_to_user_mode+0x1a/0x30 kernel/entry/common.c:307
 do_syscall_64+0x58/0xa0 arch/x86/entry/common.c:86

Memory state around the buggy address:
 ffff888134b6c000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff888134b6c080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff888134b6c100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
                               ^
 ffff888134b6c180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff888134b6c200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================
EXT4-fs error (device loop7): ext4_ext_split:1146: inode #19: comm syz.7.182: eh_entries 43459 != eh_max 6483!
EXT4-fs error (device loop7): ext4_free_blocks:6232: comm syz.7.182: Freeing blocks not in datazone - block = 188159815740580, count = 3133
EXT4-fs error (device loop7): ext4_free_blocks:6232: comm syz.7.182: Freeing blocks not in datazone - block = 79262541775280, count = 27429
EXT4-fs error (device loop7): ext4_free_blocks:6232: comm syz.7.182: Freeing blocks not in datazone - block = 143745115626309, count = 5725
EXT4-fs error (device loop7): ext4_free_blocks:6232: comm syz.7.182: Freeing blocks not in datazone - block = 188575442438265, count = 5143
EXT4-fs error (device loop7): ext4_free_blocks:6232: comm syz.7.182: Freeing blocks not in datazone - block = 166696707070455, count = 22167
EXT4-fs error (device loop7): ext4_free_blocks:6232: comm syz.7.182: Freeing blocks not in datazone - block = 93910049282428, count = 20071
EXT4-fs error (device loop7): ext4_free_blocks:6232: comm syz.7.182: Freeing blocks not in datazone - block = 164328757051125, count = 12308
EXT4-fs error (device loop7): ext4_free_blocks:6232: comm syz.7.182: Freeing blocks not in datazone - block = 72483775432341, count = 7831
EXT4-fs error (device loop7): ext4_free_blocks:6232: comm syz.7.182: Freeing blocks not in datazone - block = 77468822857835, count = 25078

Crashes (3):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/04/22 06:02 android13-5.15-lts 68d122e51b06 0b6ab7ec .config console log report syz / log [disk image] [vmlinux] [kernel image] [mounted in repro (clean fs)] ci2-android-5-15 KASAN: use-after-free Read in ext4_ext_insert_extent
2025/06/20 07:17 android13-5.15-lts e678c93d43cc ed3e87f7 .config console log report syz / log [disk image] [vmlinux] [kernel image] [mounted in repro (clean fs)] ci2-android-5-15 KASAN: use-after-free Read in ext4_ext_insert_extent
2026/03/10 15:16 android13-5.15-lts 60dba25bf158 4683d576 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-5-15 KASAN: use-after-free Read in ext4_ext_insert_extent
* Struck through repros no longer work on HEAD.