linux-kernel - Re: [syzbot] [ext4?] kernel BUG in ext4_es_cache

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Message-ID: <68d8ff2a.a70a0220.10c4b.001c.GAE@google.com>
Date: Sun, 28 Sep 2025 02:26:02 -0700
From: syzbot <syzbot+038b7bf43423e132b308@...kaller.appspotmail.com>
To: =syzbot@...kaller.appspotmail.com, kartikey406@...il.com, 
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [ext4?] kernel BUG in ext4_es_cache_extent (3)

Hello,

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

EXT4-fs (loop0): mounted filesystem 00000000-0000-0000-0000-000000000000 r/w without journal. Quota mode: none.
EXT4-fs error (device loop0): ext4_cache_extents:545: inode #15: comm syz.0.17: corrupted extent tree: lblk 0 < prev 66
==================================================================
BUG: KASAN: use-after-free in ext4_ext_binsearch fs/ext4/extents.c:848 [inline]
BUG: KASAN: use-after-free in ext4_find_extent+0x85c/0xa20 fs/ext4/extents.c:963
Read of size 4 at addr ffff888041c23e48 by task syz.0.17/5858

CPU: 0 UID: 0 PID: 5858 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
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:848 [inline]
 ext4_find_extent+0x85c/0xa20 fs/ext4/extents.c:963
 ext4_get_verity_descriptor_location fs/ext4/verity.c:292 [inline]
 ext4_get_verity_descriptor+0x102/0x590 fs/ext4/verity.c:346
 fsverity_get_descriptor+0x90/0x4b0 fs/verity/open.c:323
 ensure_verity_info fs/verity/open.c:363 [inline]
 __fsverity_file_open+0xd7/0x1f0 fs/verity/open.c:384
 fsverity_file_open include/linux/fsverity.h:300 [inline]
 ext4_file_open+0x32d/0xa40 fs/ext4/file.c:909
 do_dentry_open+0x950/0x13f0 fs/open.c:965
 vfs_open+0x3b/0x340 fs/open.c:1095
 do_open fs/namei.c:3887 [inline]
 path_openat+0x2ee5/0x3830 fs/namei.c:4046
 do_filp_open+0x1fa/0x410 fs/namei.c:4073
 do_sys_openat2+0x121/0x1c0 fs/open.c:1435
 do_sys_open fs/open.c:1450 [inline]
 __do_sys_openat fs/open.c:1466 [inline]
 __se_sys_openat fs/open.c:1461 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1461
 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: 0033:0x7f6cf3b8ebe9
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:00007f6cf4aaa038 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007f6cf3dc5fa0 RCX: 00007f6cf3b8ebe9
RDX: 0000000000000000 RSI: 0000200000000100 RDI: ffffffffffffff9c
RBP: 00007f6cf3c11e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6cf3dc6038 R14: 00007f6cf3dc5fa0 R15: 00007fff26487178
 </TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x55b5a552d pfn:0x41c23
flags: 0x4fff00000000000(node=1|zone=1|lastcpupid=0x7ff)
raw: 04fff00000000000 ffffea0001070948 ffffea0001070888 0000000000000000
raw: 000000055b5a552d 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 5236, tgid 5236 (dhcpcd), ts 65849526604, free_ts 140329459250
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1851
 compaction_alloc_noprof mm/compaction.c:1836 [inline]
 compaction_alloc+0x3099/0x34a0 mm/compaction.c:1847
 migrate_folio_unmap mm/migrate.c:1211 [inline]
 migrate_pages_batch+0x7e2/0x35c0 mm/migrate.c:1873
 migrate_pages_sync mm/migrate.c:1996 [inline]
 migrate_pages+0x1bcc/0x2930 mm/migrate.c:2105
 compact_zone+0x23e1/0x4ab0 mm/compaction.c:2647
 compact_node+0x1d2/0x280 mm/compaction.c:2916
 kcompactd+0xbc8/0x1290 mm/compaction.c:3214
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x439/0x7d0 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
page last free pid 30 tgid 30 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1395 [inline]
 __free_frozen_pages+0xbc4/0xd30 mm/page_alloc.c:2895
 __folio_put+0x21b/0x2c0 mm/swap.c:112
 migrate_folio_move mm/migrate.c:1409 [inline]
 migrate_folios_move mm/migrate.c:1719 [inline]
 migrate_pages_batch+0x227f/0x35c0 mm/migrate.c:1966
 migrate_pages_sync mm/migrate.c:1996 [inline]
 migrate_pages+0x1bcc/0x2930 mm/migrate.c:2105
 compact_zone+0x23e1/0x4ab0 mm/compaction.c:2647
 compact_node+0x1d2/0x280 mm/compaction.c:2916
 kcompactd+0xbc8/0x1290 mm/compaction.c:3214
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x439/0x7d0 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Memory state around the buggy address:
 ffff888041c23d00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888041c23d80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888041c23e00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                              ^
 ffff888041c23e80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888041c23f00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


Tested on:

commit:         51a24b7d Merge tag 'trace-tools-v6.17-rc5' of git://gi..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15d8f2e2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=bf99f2510ef92ba5
dashboard link: https://syzkaller.appspot.com/bug?extid=038b7bf43423e132b308
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=15ec3f12580000