Re: [syzbot] [ext4?] KASAN: use-after-free Read in ext4_ext_insert_extent

syzbot
2025-09-30T21:17:04Z

KASAN: use-after-free Read in ext4_ext_correct_indexes

==================================================================
BUG: KASAN: use-after-free in ext4_ext_correct_indexes+0x72/0x5b0 fs/ext4/extents.c:1712
Read of size 8 at addr ffff888078dca350 by task syz.2.45/6689

CPU: 0 UID: 0 PID: 6689 Comm: syz.2.45 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
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_correct_indexes+0x72/0x5b0 fs/ext4/extents.c:1712
ext4_ext_insert_extent+0x1fdd/0x4af0 fs/ext4/extents.c:2188
ext4_ext_map_blocks+0x1bbe/0x3880 fs/ext4/extents.c:4410
ext4_map_create_blocks fs/ext4/inode.c:609 [inline]
ext4_map_blocks+0x860/0x1740 fs/ext4/inode.c:811
_ext4_get_block+0x200/0x4c0 fs/ext4/inode.c:910
ext4_get_block_unwritten+0x2e/0x100 fs/ext4/inode.c:943
ext4_block_write_begin+0x990/0x1710 fs/ext4/inode.c:1198
ext4_write_begin+0xc04/0x19a0 fs/ext4/ext4_jbd2.h:-1
ext4_da_write_begin+0x445/0xda0 fs/ext4/inode.c:3129
generic_perform_write+0x2c2/0x900 mm/filemap.c:4175
ext4_buffered_write_iter+0xce/0x3a0 fs/ext4/file.c:299
ext4_file_write_iter+0x298/0x1bc0 fs/ext4/file.c:-1
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0x5c6/0xb30 fs/read_write.c:686
ksys_pwrite64 fs/read_write.c:793 [inline]
__do_sys_pwrite64 fs/read_write.c:801 [inline]
__se_sys_pwrite64 fs/read_write.c:798 [inline]
__x64_sys_pwrite64+0x193/0x220 fs/read_write.c:798
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: 0033:0x7f9e7b78e969
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:00007f9e7a9fe038 EFLAGS: 00000246 ORIG_RAX: 0000000000000012
RAX: ffffffffffffffda RBX: 00007f9e7b9b5fa0 RCX: 00007f9e7b78e969
RDX: 000000000000fdef RSI: 0000200000000140 RDI: 0000000000000004
RBP: 00007f9e7b810ab1 R08: 0000000000000000 R09: 0000000000000000
R10: 000000000000fecc R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f9e7b9b5fa0 R15: 00007fff3dade2a8
</TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff888078dcadc0 pfn:0x78dca
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000000000 ffffea00016119c8 ffffea00018708c8 0000000000000000
raw: ffff888078dcadc0 ffff888079136c60 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as freed
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x440dc0(GFP_KERNEL_ACCOUNT|__GFP_ZERO|__GFP_COMP), pid 6328, tgid 6328 (syz-executor), ts 132604632127, free_ts 133183234936
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1851
prep_new_page mm/page_alloc.c:1859 [inline]
get_page_from_freelist+0x21d5/0x22b0 mm/page_alloc.c:3858
__alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5148
alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2416
alloc_frozen_pages_noprof mm/mempolicy.c:2487 [inline]
alloc_pages_noprof+0xa9/0x190 mm/mempolicy.c:2507
pagetable_alloc_noprof include/linux/mm.h:2881 [inline]
__pte_alloc_one_noprof include/asm-generic/pgalloc.h:75 [inline]
pte_alloc_one+0x21/0x170 arch/x86/mm/pgtable.c:18
__pte_alloc+0x25/0x1a0 mm/memory.c:452
copy_pte_range mm/memory.c:1107 [inline]
copy_pmd_range+0x6a91/0x71d0 mm/memory.c:1261
copy_pud_range mm/memory.c:1298 [inline]
copy_p4d_range mm/memory.c:1322 [inline]
copy_page_range+0xc14/0x1270 mm/memory.c:1410
dup_mmap+0xf57/0x1ac0 mm/mmap.c:1834
dup_mm kernel/fork.c:1485 [inline]
copy_mm+0x13c/0x4b0 kernel/fork.c:1537
copy_process+0x1706/0x3c00 kernel/fork.c:2179
kernel_clone+0x224/0x7c0 kernel/fork.c:2609
__do_sys_clone kernel/fork.c:2752 [inline]
__se_sys_clone kernel/fork.c:2736 [inline]
__x64_sys_clone+0x18b/0x1e0 kernel/fork.c:2736
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
page last free pid 920 tgid 920 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
free_pages_prepare mm/page_alloc.c:1395 [inline]
__free_frozen_pages+0xbb1/0xd20 mm/page_alloc.c:2895
pagetable_free include/linux/mm.h:2898 [inline]
pagetable_dtor_free include/linux/mm.h:2996 [inline]
__tlb_remove_table+0x2d2/0x3b0 include/asm-generic/tlb.h:220
__tlb_remove_table_free mm/mmu_gather.c:227 [inline]
tlb_remove_table_rcu+0x85/0x100 mm/mmu_gather.c:290
rcu_do_batch kernel/rcu/tree.c:2605 [inline]
rcu_core+0xca8/0x1770 kernel/rcu/tree.c:2861
handle_softirqs+0x286/0x870 kernel/softirq.c:579
__do_softirq kernel/softirq.c:613 [inline]
invoke_softirq kernel/softirq.c:453 [inline]
__irq_exit_rcu+0xca/0x1f0 kernel/softirq.c:680
irq_exit_rcu+0x9/0x30 kernel/softirq.c:696
instr_sysvec_call_function_single arch/x86/kernel/smp.c:266 [inline]
sysvec_call_function_single+0xa3/0xc0 arch/x86/kernel/smp.c:266
asm_sysvec_call_function_single+0x1a/0x20 arch/x86/include/asm/idtentry.h:709

Memory state around the buggy address:
ffff888078dca200: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888078dca280: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888078dca300: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
^
ffff888078dca380: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888078dca400: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================

Tested on:

commit: 755fa5b4 Merge tag 'cgroup-for-6.18' of git://git.kern..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14fea42c580000
kernel config: https://syzkaller.appspot.com/x/.config?x=9c0c1e13e3c8731f
dashboard link: https://syzkaller.appspot.com/bug?extid=9db318d6167044609878
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=16fb6942580000