[v6.6] KASAN: use-after-free Read in sysv_new_block

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 5fa4793a2d2d Linux 6.6.119
git tree: linux-6.6.y
console output: https://syzkaller.appspot.com/x/log.txt?x=14636074580000
kernel config: https://syzkaller.appspot.com/x/.config?x=691a6769a86ac817
dashboard link: https://syzkaller.appspot.com/bug?extid=51d9b5fc591851f378d3
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/63699875f1dd/disk-5fa4793a.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/8506652fcb6f/vmlinux-5fa4793a.xz
kernel image: https://storage.googleapis.com/syzbot-assets/1b30ceed1710/bzImage-5fa4793a.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+51d9b5...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in sysv_new_block+0x735/0x8d0 fs/sysv/balloc.c:113
Read of size 4 at addr ffff8880571b30c8 by task syz.4.57/6321

CPU: 1 PID: 6321 Comm: syz.4.57 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: 0033:0x7f1327f8f749
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:00007f1328ea8038 EFLAGS: 00000246 ORIG_RAX: 0000000000000058
RAX: ffffffffffffffda RBX: 00007f13281e5fa0 RCX: 00007f1327f8f749
RDX: 0000000000000000 RSI: 0000200000000240 RDI: 0000200000000200
RBP: 00007f1328013f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f13281e6038 R14: 00007f13281e5fa0 R15: 00007ffc12ade7c8
</TASK>

The buggy address belongs to the physical page:
page:ffffea00015c6cc0 refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x571b3
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 6331, tgid 6330 (syz.1.59), ts 121715460511, free_ts 123009898553
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
do_unlinkat+0x37b/0x570 fs/namei.c:4406
__do_sys_unlink fs/namei.c:4447 [inline]
__se_sys_unlink fs/namei.c:4445 [inline]
__x64_sys_unlink+0x49/0x50 fs/namei.c:4445
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

Memory state around the buggy address:
ffff8880571b2f80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff8880571b3000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff8880571b3080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
^
ffff8880571b3100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff8880571b3180: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


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