KASAN: use-after-free Read in ext4_ext_try_to_merge_right

Status: auto-obsoleted due to no activity on 2026/10/13 14:42
Reported-by: syzbot+e43fa000c0203108ea6d@syzkaller.appspotmail.com
First crash: 204d, last: 107d
Fix bisection: fixed by (bisect log) :
commit 5ad6d994255e27a3254079dfb50ca861fc31f2d0
Author: Helen Koike <koike@igalia.com>
Date: Tue Mar 17 14:23:10 2026 +0000

  ext4: reject mount if bigalloc with s_first_data_block != 0

  
▶ ▼ Similar bugs (1)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
linux-6.1 KASAN: use-after-free Read in ext4_ext_try_to_merge_right 19 1 493d 493d 0/3 auto-obsoleted due to no activity on 2025/09/22 05:31
▶ ▼ Last patch testing requests (1)
Created Duration User Patch Repo Result
2026/04/13 20:34 7m retest repro android13-5.10-lts report log
▶ ▼ Fix bisection attempts (4)
Created Duration User Patch Repo Result
2026/08/05 23:10 6h14m bisect fix android13-5.10-lts OK (1) job log
2026/07/05 12:50 1h51m bisect fix android13-5.10-lts OK (0) job log log
2026/06/04 09:15 2h03m bisect fix android13-5.10-lts OK (0) job log log
2026/05/05 07:15 1h52m bisect fix android13-5.10-lts OK (0) job log log

Sample crash report:
==================================================================
BUG: KASAN: use-after-free in ext4_ext_try_to_merge_right+0x416/0x820 fs/ext4/extents.c:1819
Read of size 718788 at addr ffff888112b16460 by task syz.0.601/3407

CPU: 0 PID: 3407 Comm: syz.0.601 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
 __dump_stack+0x21/0x24 lib/dump_stack.c:77
 dump_stack_lvl+0x1a7/0x208 lib/dump_stack.c:118
 print_address_description+0x7f/0x2c0 mm/kasan/report.c:248
 __kasan_report mm/kasan/report.c:435 [inline]
 kasan_report+0xe2/0x130 mm/kasan/report.c:452
 check_region_inline mm/kasan/generic.c:-1 [inline]
 kasan_check_range+0x249/0x2a0 mm/kasan/generic.c:189
 memmove+0x2d/0x70 mm/kasan/shadow.c:54
 ext4_ext_try_to_merge_right+0x416/0x820 fs/ext4/extents.c:1819
 ext4_ext_shift_path_extents fs/ext4/extents.c:5125 [inline]
 ext4_ext_shift_extents+0xd24/0x14a0 fs/ext4/extents.c:5302
 ext4_collapse_range fs/ext4/extents.c:5436 [inline]
 ext4_fallocate+0x109c/0x1a70 fs/ext4/extents.c:4769
 vfs_fallocate+0x4b4/0x590 fs/open.c:310
 ksys_fallocate fs/open.c:333 [inline]
 __do_sys_fallocate fs/open.c:341 [inline]
 __se_sys_fallocate fs/open.c:339 [inline]
 __x64_sys_fallocate+0xc0/0x110 fs/open.c:339
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb
RIP: 0033:0x7f7618b28819
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:00007f761898b028 EFLAGS: 00000246 ORIG_RAX: 000000000000011d
RAX: ffffffffffffffda RBX: 00007f7618da1fa0 RCX: 00007f7618b28819
RDX: 0000000000004000 RSI: 0000000000000008 RDI: 0000000000000007
RBP: 00007f7618bbec91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000008000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7618da2038 R14: 00007f7618da1fa0 R15: 00007ffee766ac38

The buggy address belongs to the page:
page:ffffea00044ac580 refcount:2 mapcount:0 mapping:ffff888109199390 index:0x20 pfn:0x112b16
aops:def_blk_aops ino:0
flags: 0x400000000002203e(referenced|uptodate|dirty|lru|active|private|mappedtodisk)
raw: 400000000002203e ffffea0004820388 ffffea00046a4cc8 ffff888109199390
raw: 0000000000000020 ffff8881321855e8 00000002ffffffff ffff888100142000
page dumped because: kasan: bad access detected
page->mem_cgroup:ffff888100142000
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x112cc0(GFP_USER|__GFP_NOWARN|__GFP_NORETRY), pid 297, ts 116438699098, free_ts 116375815968
 set_page_owner include/linux/page_owner.h:35 [inline]
 post_alloc_hook mm/page_alloc.c:2456 [inline]
 prep_new_page+0x179/0x180 mm/page_alloc.c:2462
 get_page_from_freelist+0x223b/0x23d0 mm/page_alloc.c:4254
 __alloc_pages_nodemask+0x290/0x620 mm/page_alloc.c:5384
 __alloc_pages include/linux/gfp.h:544 [inline]
 __alloc_pages_node include/linux/gfp.h:557 [inline]
 alloc_pages_node include/linux/gfp.h:571 [inline]
 alloc_pages include/linux/gfp.h:590 [inline]
 __page_cache_alloc include/linux/pagemap.h:290 [inline]
 page_cache_ra_unbounded+0x2dc/0x7e0 mm/readahead.c:226
 do_page_cache_ra mm/readahead.c:277 [inline]
 force_page_cache_ra+0x3a9/0x400 mm/readahead.c:308
 page_cache_sync_ra+0x237/0x2a0 mm/readahead.c:582
 page_cache_sync_readahead include/linux/pagemap.h:837 [inline]
 generic_file_buffered_read+0x574/0x2770 mm/filemap.c:2250
 generic_file_read_iter+0x11e/0x5c0 mm/filemap.c:2568
 blkdev_read_iter+0x12f/0x160 fs/block_dev.c:1956
 call_read_iter include/linux/fs.h:2060 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x62a/0xa60 fs/read_write.c:496
 ksys_read+0x149/0x250 fs/read_write.c:634
 __do_sys_read fs/read_write.c:644 [inline]
 __se_sys_read fs/read_write.c:642 [inline]
 __x64_sys_read+0x7b/0x90 fs/read_write.c:642
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb
page last free stack trace:
 reset_page_owner include/linux/page_owner.h:28 [inline]
 free_pages_prepare mm/page_alloc.c:1349 [inline]
 free_pcp_prepare mm/page_alloc.c:1421 [inline]
 free_unref_page_prepare+0x2b7/0x2d0 mm/page_alloc.c:3336
 free_unref_page_list+0x129/0x9c0 mm/page_alloc.c:3443
 release_pages+0xe52/0xea0 mm/swap.c:1103
 free_pages_and_swap_cache+0x86/0xa0 mm/swap_state.c:356
 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:326
 exit_mmap+0x329/0x590 mm/mmap.c:3387
 __mmput+0x93/0x2f0 kernel/fork.c:1153
 mmput+0x4e/0x150 kernel/fork.c:1176
 exit_mm kernel/exit.c:539 [inline]
 do_exit+0x9ec/0x2500 kernel/exit.c:859
 do_group_exit+0x141/0x310 kernel/exit.c:986
 get_signal+0xf7d/0x12e0 kernel/signal.c:2793
 arch_do_signal_or_restart+0xe2/0x11d0 arch/x86/kernel/signal.c:805
 handle_signal_work kernel/entry/common.c:145 [inline]
 exit_to_user_mode_loop+0xa2/0xe0 kernel/entry/common.c:169
 exit_to_user_mode_prepare+0x76/0xa0 kernel/entry/common.c:199
 syscall_exit_to_user_mode+0x1d/0x40 kernel/entry/common.c:274
 do_syscall_64+0x3d/0x40 arch/x86/entry/common.c:56

Memory state around the buggy address:
 ffff888112b17f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff888112b17f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff888112b18000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff888112b18080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888112b18100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Crashes (2):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/03/30 20:09 android13-5.10-lts cc0ae2abdc73 dcaebc52 .config console log report syz / log C [disk image] [vmlinux] [kernel image] [mounted in repro (corrupt fs)] ci2-android-5-10 KASAN: use-after-free Read in ext4_ext_try_to_merge_right
2026/03/30 18:59 android13-5.10-lts cc0ae2abdc73 dcaebc52 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-5-10 KASAN: use-after-free Read in ext4_ext_try_to_merge_right
* Struck through repros no longer work on HEAD.