Re: [syzbot] [ext4?] KASAN: slab-use-after-free Read in __ext4_check_dir_entry

[syzbot <syzbot+5322c5c260eb44d209ed@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    083fc6d7fa0d Merge tag 'sched-urgent-2025-09-26' of git://..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=106a3d34580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f5b21423ca3f0a96
dashboard link: https://syzkaller.appspot.com/bug?extid=5322c5c260eb44d209ed
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17e032e2580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=146a3d34580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/198ae77e2418/disk-083fc6d7.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/3d3f065fd75c/vmlinux-083fc6d7.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e65812e9d7b0/bzImage-083fc6d7.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/275ff9023118/mount_0.gz
  fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=1749327c580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5322c5c260eb44d209ed@syzkaller.appspotmail.com

EXT4-fs warning (device loop2): dx_probe:801: inode #2: comm syz.2.1679: Unrecognised inode hash code 4
EXT4-fs warning (device loop2): dx_probe:934: inode #2: comm syz.2.1679: Corrupt directory, running e2fsck is recommended
==================================================================
BUG: KASAN: slab-use-after-free in __ext4_check_dir_entry+0x708/0x8a0 fs/ext4/dir.c:85
Read of size 2 at addr ffff888032f79003 by task syz.2.1679/10706

CPU: 0 UID: 0 PID: 10706 Comm: syz.2.1679 Not tainted syzkaller #0 PREEMPT_{RT,(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_check_dir_entry+0x708/0x8a0 fs/ext4/dir.c:85
 ext4_readdir+0x12a2/0x3b70 fs/ext4/dir.c:262
 iterate_dir+0x3a5/0x580 fs/readdir.c:108
 __do_sys_getdents64 fs/readdir.c:410 [inline]
 __se_sys_getdents64+0xe4/0x260 fs/readdir.c:396
 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: 0033:0x7f33499feec9
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:00007f334906e038 EFLAGS: 00000246 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 00007f3349c55fa0 RCX: 00007f33499feec9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000006
RBP: 00007f3349a81f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3349c56038 R14: 00007f3349c55fa0 R15: 00007ffebc0b5dc8
 </TASK>

Allocated by task 43:
 kasan_save_stack mm/kasan/common.c:47 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
 unpoison_slab_object mm/kasan/common.c:330 [inline]
 __kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:356
 kasan_slab_alloc include/linux/kasan.h:250 [inline]
 slab_post_alloc_hook mm/slub.c:4191 [inline]
 slab_alloc_node mm/slub.c:4240 [inline]
 kmem_cache_alloc_node_noprof+0x14e/0x330 mm/slub.c:4292
 __alloc_skb+0x112/0x2d0 net/core/skbuff.c:659
 alloc_skb include/linux/skbuff.h:1336 [inline]
 nsim_dev_trap_skb_build drivers/net/netdevsim/dev.c:763 [inline]
 nsim_dev_trap_report drivers/net/netdevsim/dev.c:820 [inline]
 nsim_dev_trap_report_work+0x29f/0xbc0 drivers/net/netdevsim/dev.c:866
 process_one_work kernel/workqueue.c:3236 [inline]
 process_scheduled_works+0xade/0x17b0 kernel/workqueue.c:3319
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3400
 kthread+0x70e/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x436/0x7d0 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Freed by task 43:
 kasan_save_stack mm/kasan/common.c:47 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
 kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:576
 poison_slab_object mm/kasan/common.c:243 [inline]
 __kasan_slab_free+0x5b/0x80 mm/kasan/common.c:275
 kasan_slab_free include/linux/kasan.h:233 [inline]
 slab_free_hook mm/slub.c:2422 [inline]
 slab_free mm/slub.c:4695 [inline]
 kmem_cache_free+0x195/0x510 mm/slub.c:4797
 nsim_dev_trap_report drivers/net/netdevsim/dev.c:836 [inline]
 nsim_dev_trap_report_work+0x7fa/0xbc0 drivers/net/netdevsim/dev.c:866
 process_one_work kernel/workqueue.c:3236 [inline]
 process_scheduled_works+0xade/0x17b0 kernel/workqueue.c:3319
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3400
 kthread+0x70e/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x436/0x7d0 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

The buggy address belongs to the object at ffff888032f79000
 which belongs to the cache skbuff_head_cache of size 240
The buggy address is located 3 bytes inside of
 freed 240-byte region [ffff888032f79000, ffff888032f790f0)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x32f79
flags: 0x80000000000000(node=0|zone=1)
page_type: f5(slab)
raw: 0080000000000000 ffff88801de968c0 ffffea000085e780 dead000000000004
raw: 0000000000000000 00000000000c000c 00000000f5000000 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x52cc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP), pid 5208, tgid 5208 (udevd), ts 27594846702, free_ts 27586335299
 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+0x2119/0x21b0 mm/page_alloc.c:3858
 __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5148
 alloc_pages_mpol+0xd1/0x380 mm/mempolicy.c:2416
 alloc_slab_page mm/slub.c:2492 [inline]
 allocate_slab+0x8a/0x370 mm/slub.c:2660
 new_slab mm/slub.c:2714 [inline]
 ___slab_alloc+0x8d1/0xdc0 mm/slub.c:3901
 __slab_alloc mm/slub.c:3992 [inline]
 __slab_alloc_node mm/slub.c:4067 [inline]
 slab_alloc_node mm/slub.c:4228 [inline]
 kmem_cache_alloc_node_noprof+0xf2/0x330 mm/slub.c:4292
 __alloc_skb+0x112/0x2d0 net/core/skbuff.c:659
 netlink_sendmsg+0x5c6/0xb30 net/netlink/af_netlink.c:1871
 sock_sendmsg_nosec net/socket.c:714 [inline]
 __sock_sendmsg+0x21c/0x270 net/socket.c:729
 ____sys_sendmsg+0x508/0x820 net/socket.c:2614
 ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2668
 __sys_sendmsg net/socket.c:2700 [inline]
 __do_sys_sendmsg net/socket.c:2705 [inline]
 __se_sys_sendmsg net/socket.c:2703 [inline]
 __x64_sys_sendmsg+0x1a1/0x260 net/socket.c:2703
 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 5216 tgid 5216 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1395 [inline]
 __free_frozen_pages+0xb59/0xce0 mm/page_alloc.c:2895
 __slab_free+0x2db/0x390 mm/slub.c:4606
 qlink_free mm/kasan/quarantine.c:163 [inline]
 qlist_free_all+0x97/0x140 mm/kasan/quarantine.c:179
 kasan_quarantine_reduce+0x148/0x160 mm/kasan/quarantine.c:286
 __kasan_slab_alloc+0x22/0x80 mm/kasan/common.c:340
 kasan_slab_alloc include/linux/kasan.h:250 [inline]
 slab_post_alloc_hook mm/slub.c:4191 [inline]
 slab_alloc_node mm/slub.c:4240 [inline]
 kmem_cache_alloc_noprof+0x143/0x310 mm/slub.c:4247
 getname_flags+0xb8/0x540 fs/namei.c:146
 getname include/linux/fs.h:2919 [inline]
 do_sys_openat2+0xbc/0x1c0 fs/open.c:1429
 do_sys_open fs/open.c:1450 [inline]
 __do_sys_openat fs/open.c:1466 [inline]
 __se_sys_openat fs/open.c:1461 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1461
 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

Memory state around the buggy address:
 ffff888032f78f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff888032f78f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff888032f79000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff888032f79080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fc fc
 ffff888032f79100: fc fc fc fc fc fc fc fc fa fb fb fb fb fb fb fb
==================================================================


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