linux-kernel - Re: [syzbot] [ext4?] KASAN: use-after-free Read in ext4_ext_insert

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Message-ID: <68dc48d0.a70a0220.10c4b.0160.GAE@google.com>
Date: Tue, 30 Sep 2025 14:17:04 -0700
From: syzbot <syzbot+9db318d6167044609878@...kaller.appspotmail.com>
To: kartikey406@...il.com, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [ext4?] KASAN: use-after-free Read in ext4_ext_insert_extent

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
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