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KASAN: use-after-free Read in ext4_ext_remove_space

Status: upstream: reported C repro on 2025/07/20 13:48
Bug presence: origin:lts
[Documentation on labels]
Reported-by: syzbot+8ecc8500ff5712c63801@syzkaller.appspotmail.com
First crash: 261d, last: 7d14h
Fix bisection: failed (error log)
  
▶ ▼ Bug presence (2)
Date Name Commit Repro Result
2025/07/20 lts (merge base) 58485ff1a74f C [report] KASAN: use-after-free Read in ext4_ext_remove_space
2025/07/20 upstream (ToT) 5f054ef2e0f1 C Didn't crash
▶ ▼ Similar bugs (8)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
android-5-15 KASAN: out-of-bounds Read in ext4_ext_remove_space origin:upstream missing-backport 22 C error error 50 2d12h 977d 0/2 upstream: reported C repro on 2023/08/04 22:38
android-5-10 KASAN: out-of-bounds Read in ext4_ext_remove_space 22 C error 56 1d20h 948d 0/2 upstream: reported C repro on 2023/09/02 13:17
upstream KASAN: slab-use-after-free Read in ext4_ext_remove_space ext4 19 C error 73 15d 372d 26/29 upstream: reported C repro on 2025/03/31 11:15
linux-6.1 KASAN: out-of-bounds Read in ext4_ext_remove_space origin:upstream missing-backport 19 C done 35 6d13h 825d 0/3 upstream: reported C repro on 2024/01/03 15:27
linux-6.6 KASAN: use-after-free Read in ext4_ext_remove_space 19 1 35d 35d 0/2 upstream: reported on 2026/03/04 03:56
upstream KASAN: out-of-bounds Read in ext4_ext_remove_space ext4 17 C error done 8 824d 998d 25/29 fixed on 2024/02/16 19:40
android-6-1 KASAN: out-of-bounds Read in ext4_ext_remove_space missing-backport 17 C error error 6 535d 967d 0/2 auto-obsoleted due to no activity on 2025/01/27 17:01
linux-5.15 KASAN: out-of-bounds Read in ext4_ext_remove_space missing-backport origin:upstream 17 C done 5 41d 807d 0/3 upstream: reported C repro on 2024/01/21 16:07
▶ ▼ Last patch testing requests (8)
Created Duration User Patch Repo Result
2026/03/09 19:21 7m retest repro android14-6.1 report log
2026/03/09 19:21 9m retest repro android14-6.1 report log
2026/03/09 19:21 6m retest repro android14-6.1 report log
2026/03/09 19:21 1h06m retest repro android14-6.1 report log
2026/02/01 11:10 7m retest repro android14-6.1 report log
2026/02/01 11:10 6m retest repro android14-6.1 report log
2026/02/01 11:10 9m retest repro android14-6.1 report log
2025/12/24 21:47 12m retest repro android14-6.1 report log
▶ ▼ Fix bisection attempts (2)
Created Duration User Patch Repo Result
2025/11/18 21:12 0m bisect fix android14-6.1 error
2025/09/13 14:37 1h52m bisect fix android14-6.1 OK (0) job log log

Sample crash report:
EXT4-fs error (device loop1): ext4_mb_mark_diskspace_used:3850: comm syz.1.18: Allocating blocks 497-513 which overlap fs metadata
EXT4-fs (loop1): pa ffff888135d29888: logic 256, phys. 385, len 8
EXT4-fs error (device loop1): ext4_mb_release_inode_pa:4890: group 0, free 0, pa_free 1
==================================================================
BUG: KASAN: use-after-free in ext4_ext_rm_leaf fs/ext4/extents.c:2602 [inline]
BUG: KASAN: use-after-free in ext4_ext_remove_space+0x348b/0x40d0 fs/ext4/extents.c:2940
Read of size 4 at addr ffff888135a8f54c by task syz.1.18/445

CPU: 1 PID: 445 Comm: syz.1.18 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
 <TASK>
 __dump_stack+0x21/0x24 lib/dump_stack.c:88
 dump_stack_lvl+0x110/0x170 lib/dump_stack.c:106
 print_address_description+0x71/0x200 mm/kasan/report.c:316
 print_report+0x4a/0x60 mm/kasan/report.c:420
 kasan_report+0x122/0x150 mm/kasan/report.c:524
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report_generic.c:350
 ext4_ext_rm_leaf fs/ext4/extents.c:2602 [inline]
 ext4_ext_remove_space+0x348b/0x40d0 fs/ext4/extents.c:2940
 ext4_ext_truncate+0x200/0x320 fs/ext4/extents.c:4476
 ext4_truncate+0x9be/0xfb0 fs/ext4/inode.c:4297
 ext4_truncate_failed_write fs/ext4/truncate.h:22 [inline]
 ext4_write_begin+0xa6a/0xf90 fs/ext4/inode.c:1289
 ext4_da_write_begin+0x3e7/0x8c0 fs/ext4/inode.c:3001
 generic_perform_write+0x32d/0x740 mm/filemap.c:3905
 ext4_buffered_write_iter+0x367/0x630 fs/ext4/file.c:285
 ext4_file_write_iter+0x191/0x1500 fs/ext4/file.c:-1
 call_write_iter include/linux/fs.h:2282 [inline]
 new_sync_write fs/read_write.c:491 [inline]
 vfs_write+0x603/0xce0 fs/read_write.c:584
 ksys_pwrite64 fs/read_write.c:699 [inline]
 __do_sys_pwrite64 fs/read_write.c:709 [inline]
 __se_sys_pwrite64 fs/read_write.c:706 [inline]
 __x64_sys_pwrite64+0x197/0x220 fs/read_write.c:706
 x64_sys_call+0x36/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:19
 do_syscall_x64 arch/x86/entry/common.c:46 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
 entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7f697899c819
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:00007f69798bb028 EFLAGS: 00000246 ORIG_RAX: 0000000000000012
RAX: ffffffffffffffda RBX: 00007f6978c15fa0 RCX: 00007f697899c819
RDX: 00000000200000c1 RSI: 00002000000000c0 RDI: 0000000000000004
RBP: 00007f6978a32c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000009000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6978c16038 R14: 00007f6978c15fa0 R15: 00007ffc3046c0f8
 </TASK>

The buggy address belongs to the physical page:
page:ffffea0004d6a3c0 refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x135a8f
flags: 0x4000000000000000(zone=1)
raw: 4000000000000000 ffffea0004d61108 ffffea0004d6a388 0000000000000000
raw: 0000000000000001 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 418, tgid 417 (syz.4.24), ts 31863598532, free_ts 32213716959
 set_page_owner include/linux/page_owner.h:33 [inline]
 post_alloc_hook+0x1f5/0x210 mm/page_alloc.c:2670
 prep_new_page+0x1c/0x110 mm/page_alloc.c:2677
 get_page_from_freelist+0x2d12/0x2d80 mm/page_alloc.c:4583
 __alloc_pages+0x1fa/0x610 mm/page_alloc.c:5924
 __folio_alloc+0x12/0x40 mm/page_alloc.c:5957
 __folio_alloc_node include/linux/gfp.h:245 [inline]
 folio_alloc include/linux/gfp.h:274 [inline]
 shmem_alloc_folio mm/shmem.c:1597 [inline]
 shmem_alloc_and_acct_folio+0x6dd/0x8c0 mm/shmem.c:1622
 shmem_get_folio_gfp+0x119f/0x2230 mm/shmem.c:1951
 shmem_get_folio mm/shmem.c:2082 [inline]
 shmem_write_begin+0xea/0x2c0 mm/shmem.c:2567
 generic_perform_write+0x32d/0x740 mm/filemap.c:3905
 __generic_file_write_iter+0x17a/0x230 mm/filemap.c:4004
 generic_file_write_iter+0xae/0x310 mm/filemap.c:4033
 call_write_iter include/linux/fs.h:2282 [inline]
 new_sync_write fs/read_write.c:491 [inline]
 vfs_write+0x603/0xce0 fs/read_write.c:584
 ksys_write+0x149/0x250 fs/read_write.c:637
 __do_sys_write fs/read_write.c:649 [inline]
 __se_sys_write fs/read_write.c:646 [inline]
 __x64_sys_write+0x7b/0x90 fs/read_write.c:646
 x64_sys_call+0x27b/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/common.c:46 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
page last free stack trace:
 reset_page_owner include/linux/page_owner.h:26 [inline]
 free_pages_prepare mm/page_alloc.c:1578 [inline]
 free_pcp_prepare mm/page_alloc.c:1652 [inline]
 free_unref_page_prepare+0x742/0x750 mm/page_alloc.c:3618
 free_unref_page_list+0x117/0x8c0 mm/page_alloc.c:3769
 release_pages+0xaf2/0xb50 mm/swap.c:1048
 __pagevec_release+0x71/0xe0 mm/swap.c:1068
 pagevec_release include/linux/pagevec.h:71 [inline]
 folio_batch_release include/linux/pagevec.h:135 [inline]
 shmem_undo_range+0x574/0x1540 mm/shmem.c:951
 shmem_truncate_range mm/shmem.c:1067 [inline]
 shmem_evict_inode+0x255/0xa50 mm/shmem.c:1176
 evict+0x4d7/0x8f0 fs/inode.c:708
 iput_final fs/inode.c:1844 [inline]
 iput+0x620/0x670 fs/inode.c:1870
 dentry_unlink_inode+0x33d/0x3f0 fs/dcache.c:405
 __dentry_kill+0x460/0x670 fs/dcache.c:611
 dentry_kill+0xc0/0x2a0 fs/dcache.c:-1
 dput+0x42/0x80 fs/dcache.c:918
 __fput+0x5be/0x8f0 fs/file_table.c:328
 ____fput+0x15/0x20 fs/file_table.c:348
 task_work_run+0x1e1/0x250 kernel/task_work.c:203
 resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
 exit_to_user_mode_loop+0x9b/0xb0 kernel/entry/common.c:177

Memory state around the buggy address:
 ffff888135a8f400: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888135a8f480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888135a8f500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                              ^
 ffff888135a8f580: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888135a8f600: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================

Crashes (10):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/03/31 19:10 android14-6.1 f5931d57f029 aeea1c72 .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
2026/01/18 08:52 android14-6.1 f8a5ad1fcf13 20d37d28 .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
2026/01/17 11:11 android14-6.1 f8a5ad1fcf13 20d37d28 .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
2025/07/20 17:34 android14-6.1 145c7fad733f 7117feec .config strace log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
2025/07/20 16:23 android14-6.1 145c7fad733f 7117feec .config strace log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
2025/07/20 15:35 android14-6.1 145c7fad733f 7117feec .config strace log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
2025/07/20 14:43 android14-6.1 145c7fad733f 7117feec .config strace log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
2025/07/20 13:37 android14-6.1 145c7fad733f 7117feec .config strace log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
2026/02/23 18:57 android14-6.1 34c1a8e17200 6beca497 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
2025/07/20 12:27 android14-6.1 145c7fad733f 7117feec .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-6-1 KASAN: use-after-free Read in ext4_ext_remove_space
* Struck through repros no longer work on HEAD.