Message-ID: <696b1661.a70a0220.34546f.00a3.GAE@google.com>
Date: Fri, 16 Jan 2026 20:56:01 -0800
From: syzbot <syzbot+8debf4b3f7c7391cd8eb@...kaller.appspotmail.com>
To: kartikey406@...il.com, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [xfs?] KASAN: use-after-free Read in hpfs_bplus_lookup

Hello,

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

=== CRASH WILL HAPPEN ON NEXT LINE IF UAF ===
=== external[120]: about to access ffff8880545bffe0 ===
=== CRASH WILL HAPPEN ON NEXT LINE IF UAF ===
=== external[121]: about to access ffff8880545bffec ===
=== CRASH WILL HAPPEN ON NEXT LINE IF UAF ===
=== external[122]: about to access ffff8880545bfff8 ===
=== CRASH WILL HAPPEN ON NEXT LINE IF UAF ===
=== external[123]: about to access ffff8880545c0004 ===
=== CRASH WILL HAPPEN ON NEXT LINE IF UAF ===
==================================================================
BUG: KASAN: use-after-free in hpfs_bplus_lookup+0x7d5/0xbd0 fs/hpfs/anode.c:68
Read of size 4 at addr ffff8880545c0004 by task syz.0.21/6731

CPU: 1 UID: 0 PID: 6731 Comm: syz.0.21 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xba/0x230 mm/kasan/report.c:482
 kasan_report+0x117/0x150 mm/kasan/report.c:595
 hpfs_bplus_lookup+0x7d5/0xbd0 fs/hpfs/anode.c:68
 hpfs_bmap+0x230/0x4e0 fs/hpfs/file.c:54
 hpfs_get_block+0xa8/0x6e0 fs/hpfs/file.c:87
 do_mpage_readpage+0x801/0x1980 fs/mpage.c:222
 mpage_readahead+0x407/0x830 fs/mpage.c:371
 read_pages+0x193/0x5a0 mm/readahead.c:163
 page_cache_ra_unbounded+0x68a/0x8c0 mm/readahead.c:302
 filemap_get_pages+0x47c/0x1e60 mm/filemap.c:2690
 filemap_read+0x44a/0x1240 mm/filemap.c:2800
 __kernel_read+0x50d/0x9c0 fs/read_write.c:530
 integrity_kernel_read+0x89/0xd0 security/integrity/iint.c:28
 ima_calc_file_hash_tfm security/integrity/ima/ima_crypto.c:480 [inline]
 ima_calc_file_shash security/integrity/ima/ima_crypto.c:511 [inline]
 ima_calc_file_hash+0x12cf/0x1800 security/integrity/ima/ima_crypto.c:568
 ima_collect_measurement+0x491/0x930 security/integrity/ima/ima_api.c:293
 process_measurement+0x12ec/0x1cc0 security/integrity/ima/ima_main.c:406
 ima_file_check+0xdf/0x130 security/integrity/ima/ima_main.c:663
 security_file_post_open+0xb3/0x260 security/security.c:2652
 do_open fs/namei.c:4639 [inline]
 path_openat+0x350e/0x3e70 fs/namei.c:4796
 do_filp_open+0x22d/0x490 fs/namei.c:4823
 do_sys_openat2+0x12f/0x220 fs/open.c:1430
 do_sys_open fs/open.c:1436 [inline]
 __do_sys_openat fs/open.c:1452 [inline]
 __se_sys_openat fs/open.c:1447 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1447
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7feb1639aef9
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:00007feb159fe028 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007feb16605fa0 RCX: 00007feb1639aef9
RDX: 0000000000000000 RSI: 0000200000004280 RDI: ffffffffffffff9c
RBP: 00007feb1642fee0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007feb16606038 R14: 00007feb16605fa0 R15: 00007fff6450b218
 </TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xb10 pfn:0x545c0
flags: 0x80000000000000(node=0|zone=1)
page_type: f0(buddy)
raw: 0080000000000000 ffff88813fffc380 ffffea0001525008 0000000000000000
raw: 0000000000000b10 0000000000000006 00000000f0000000 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 6731, tgid 6730 (syz.0.21), ts 168496747643, free_ts 169044969619
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x228/0x280 mm/page_alloc.c:1884
 prep_new_page mm/page_alloc.c:1892 [inline]
 get_page_from_freelist+0x28bb/0x2950 mm/page_alloc.c:3945
 __alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5240
 alloc_pages_mpol+0xd1/0x380 mm/mempolicy.c:2486
 folio_alloc_mpol_noprof+0x39/0xe0 mm/mempolicy.c:2505
 shmem_alloc_folio mm/shmem.c:1890 [inline]
 shmem_alloc_and_add_folio mm/shmem.c:1932 [inline]
 shmem_get_folio_gfp+0x644/0x1a80 mm/shmem.c:2556
 shmem_get_folio mm/shmem.c:2662 [inline]
 shmem_write_begin+0x166/0x320 mm/shmem.c:3315
 generic_perform_write+0x2af/0x8b0 mm/filemap.c:4314
 shmem_file_write_iter+0xfb/0x120 mm/shmem.c:3490
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x629/0xba0 fs/read_write.c:686
 ksys_write+0x156/0x270 fs/read_write.c:738
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
page last free pid 6731 tgid 6730 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1433 [inline]
 free_unref_folios+0xc13/0x17f0 mm/page_alloc.c:3030
 folios_put_refs+0x56f/0x680 mm/swap.c:1002
 folio_batch_release include/linux/pagevec.h:101 [inline]
 shmem_undo_range+0x529/0x1570 mm/shmem.c:1137
 shmem_truncate_range mm/shmem.c:1249 [inline]
 shmem_evict_inode+0x240/0x9e0 mm/shmem.c:1379
 evict+0x61e/0xb10 fs/inode.c:837
 __dentry_kill+0x209/0x660 fs/dcache.c:670
 finish_dput+0xc9/0x480 fs/dcache.c:879
 __fput+0x6a0/0xa80 fs/file_table.c:476
 task_work_run+0x1d9/0x270 kernel/task_work.c:233
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 __exit_to_user_mode_loop kernel/entry/common.c:44 [inline]
 exit_to_user_mode_loop+0xed/0x480 kernel/entry/common.c:75
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
 syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:159 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:194 [inline]
 do_syscall_64+0x2b7/0xf80 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
 ffff8880545bff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8880545bff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff8880545c0000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff8880545c0080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8880545c0100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


Tested on:

commit:         39d33893 Merge tag 'drm-fixes-2026-01-16' of https://g..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13f523fa580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c33bf4a3a0c7a4f1
dashboard link: https://syzkaller.appspot.com/bug?extid=8debf4b3f7c7391cd8eb
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1365239a580000

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