VFS: Found a Xenix FS (block size = 1024) on device loop0
syz.0.34: attempt to access beyond end of device
loop0: rw=2049, sector=6491536, nr_sectors = 2 limit=128
Buffer I/O error on dev loop0, logical block 3245768, lost async page write
==================================================================
BUG: KASAN: use-after-free in sysv_new_block+0x735/0x8d0 fs/sysv/balloc.c:113
Read of size 4 at addr ffff88806c6150c8 by task syz.0.34/6030

CPU: 0 PID: 6030 Comm: syz.0.34 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x16c/0x230 lib/dump_stack.c:106
 print_address_description mm/kasan/report.c:364 [inline]
 print_report+0xac/0x220 mm/kasan/report.c:468
 kasan_report+0x117/0x150 mm/kasan/report.c:581
 sysv_new_block+0x735/0x8d0 fs/sysv/balloc.c:113
 alloc_branch fs/sysv/itree.c:134 [inline]
 get_block+0x285/0x1550 fs/sysv/itree.c:253
 __block_write_begin_int+0x566/0x1ad0 fs/buffer.c:2124
 __block_write_begin fs/buffer.c:2173 [inline]
 block_write_begin+0x9a/0x1e0 fs/buffer.c:2234
 sysv_write_begin+0x35/0x70 fs/sysv/itree.c:488
 page_symlink+0x243/0x3c0 fs/namei.c:5238
 sysv_symlink+0xcc/0x160 fs/sysv/namei.c:86
 vfs_symlink+0x138/0x2b0 fs/namei.c:4481
 do_symlinkat+0x1b2/0x3f0 fs/namei.c:4507
 __do_sys_symlink fs/namei.c:4528 [inline]
 __se_sys_symlink fs/namei.c:4526 [inline]
 __x64_sys_symlink+0x7e/0x90 fs/namei.c:4526
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
 entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7fd6ea98f749
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:00007fd6eb8bb038 EFLAGS: 00000246 ORIG_RAX: 0000000000000058
RAX: ffffffffffffffda RBX: 00007fd6eabe5fa0 RCX: 00007fd6ea98f749
RDX: 0000000000000000 RSI: 0000200000000240 RDI: 0000200000000200
RBP: 00007fd6eaa13f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fd6eabe6038 R14: 00007fd6eabe5fa0 R15: 00007ffda8dbb3b8
 </TASK>

The buggy address belongs to the physical page:
page:ffffea0001b18540 refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x6c615
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
page_type: 0xffffffff()
raw: 00fff00000000000 dead000000000100 dead000000000122 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 5992, tgid 5991 (syz.2.28), ts 101795046619, free_ts 102709648364
 set_page_owner include/linux/page_owner.h:31 [inline]
 post_alloc_hook+0x1cd/0x210 mm/page_alloc.c:1554
 prep_new_page mm/page_alloc.c:1561 [inline]
 get_page_from_freelist+0x195c/0x19f0 mm/page_alloc.c:3191
 __alloc_pages+0x1e3/0x460 mm/page_alloc.c:4457
 __folio_alloc+0x10/0x20 mm/page_alloc.c:4489
 vma_alloc_folio+0x47a/0x8f0 mm/mempolicy.c:2242
 shmem_alloc_folio+0x179/0x230 mm/shmem.c:1679
 shmem_alloc_and_acct_folio+0x189/0x630 mm/shmem.c:1704
 shmem_get_folio_gfp+0xcde/0x2ac0 mm/shmem.c:2041
 shmem_get_folio mm/shmem.c:2164 [inline]
 shmem_write_begin+0xf2/0x420 mm/shmem.c:2706
 generic_perform_write+0x2fb/0x5b0 mm/filemap.c:4031
 shmem_file_write_iter+0xfb/0x120 mm/shmem.c:2883
 call_write_iter include/linux/fs.h:2018 [inline]
 new_sync_write fs/read_write.c:491 [inline]
 vfs_write+0x43b/0x940 fs/read_write.c:584
 ksys_write+0x147/0x250 fs/read_write.c:637
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
 entry_SYSCALL_64_after_hwframe+0x68/0xd2
page last free stack trace:
 reset_page_owner include/linux/page_owner.h:24 [inline]
 free_pages_prepare mm/page_alloc.c:1154 [inline]
 free_unref_page_prepare+0x7ce/0x8e0 mm/page_alloc.c:2336
 free_unref_page_list+0xbe/0x860 mm/page_alloc.c:2475
 release_pages+0x1fa0/0x2220 mm/swap.c:1022
 __folio_batch_release+0x71/0xe0 mm/swap.c:1042
 folio_batch_release include/linux/pagevec.h:83 [inline]
 shmem_undo_range+0x5d0/0x1a40 mm/shmem.c:1026
 shmem_truncate_range mm/shmem.c:1135 [inline]
 shmem_evict_inode+0x273/0xa70 mm/shmem.c:1264
 evict+0x486/0x870 fs/inode.c:705
 __dentry_kill+0x431/0x650 fs/dcache.c:611
 dentry_kill+0xb8/0x290 fs/dcache.c:-1
 dput+0xfe/0x1e0 fs/dcache.c:918
 __fput+0x5e5/0x970 fs/file_table.c:392
 task_work_run+0x1ce/0x250 kernel/task_work.c:239
 resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
 exit_to_user_mode_loop+0xe6/0x110 kernel/entry/common.c:177
 exit_to_user_mode_prepare+0xf6/0x180 kernel/entry/common.c:210
 __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
 syscall_exit_to_user_mode+0x1a/0x50 kernel/entry/common.c:302
 do_syscall_64+0x61/0xb0 arch/x86/entry/common.c:87

Memory state around the buggy address:
 ffff88806c614f80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88806c615000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff88806c615080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                              ^
 ffff88806c615100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88806c615180: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================