Re: [syzbot] [cgroups?] KASAN: slab-use-after-free Read in pressure_write

[syzbot <syzbot+33e571025d88efd1312c@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
possible deadlock in __kernfs_remove

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.1.18/6463 is trying to acquire lock:
ffff888032ccf968 (kn->active#59){++++}-{0:0}, at: __kernfs_remove+0x3cf/0x660 fs/kernfs/dir.c:1513

but task is already holding lock:
ffffffff8de0ad98 (cgroup_mutex){+.+.}-{4:4}, at: cgroup_lock include/linux/cgroup.h:394 [inline]
ffffffff8de0ad98 (cgroup_mutex){+.+.}-{4:4}, at: cgroup_kn_lock_live+0x13c/0x230 kernel/cgroup/cgroup.c:1732

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (cgroup_mutex){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/rtmutex_api.c:533 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:552
       cgroup_lock include/linux/cgroup.h:394 [inline]
       cgroup_lock_and_drain_offline+0x8d/0x4a0 kernel/cgroup/cgroup.c:3265
       cgroup_kn_lock_live+0x120/0x230 kernel/cgroup/cgroup.c:1730
       cgroup_subtree_control_write+0x4b3/0x10a0 kernel/cgroup/cgroup.c:3580
       cgroup_file_write+0x36f/0x790 kernel/cgroup/cgroup.c:4313
       kernfs_fop_write_iter+0x3b0/0x540 fs/kernfs/file.c:352
       new_sync_write fs/read_write.c:595 [inline]
       vfs_write+0x629/0xba0 fs/read_write.c:688
       ksys_write+0x156/0x270 fs/read_write.c:740
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (&of->mutex){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/rtmutex_api.c:533 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:552
       cgroup_file_release+0xdd/0x110 kernel/cgroup/cgroup.c:4283
       kernfs_release_file fs/kernfs/file.c:764 [inline]
       kernfs_fop_release+0x21d/0x450 fs/kernfs/file.c:779
       __fput+0x461/0xa90 fs/file_table.c:469
       fput_close_sync+0x11f/0x240 fs/file_table.c:574
       __do_sys_close fs/open.c:1509 [inline]
       __se_sys_close fs/open.c:1494 [inline]
       __x64_sys_close+0x7e/0x110 fs/open.c:1494
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&kernfs_locks->open_file_mutex[count]){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/rtmutex_api.c:533 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:552
       kernfs_open_file_mutex_lock fs/kernfs/file.c:56 [inline]
       kernfs_get_open_node fs/kernfs/file.c:538 [inline]
       kernfs_fop_open+0x6e6/0xcb0 fs/kernfs/file.c:718
       do_dentry_open+0x83d/0x13e0 fs/open.c:949
       vfs_open+0x3b/0x350 fs/open.c:1081
       do_open fs/namei.c:4677 [inline]
       path_openat+0x2e43/0x38a0 fs/namei.c:4836
       do_file_open+0x23e/0x4a0 fs/namei.c:4865
       do_sys_openat2+0x113/0x200 fs/open.c:1366
       do_sys_open fs/open.c:1372 [inline]
       __do_sys_openat fs/open.c:1388 [inline]
       __se_sys_openat fs/open.c:1383 [inline]
       __x64_sys_openat+0x138/0x170 fs/open.c:1383
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (kn->active#59){++++}-{0:0}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain kernel/locking/lockdep.c:3908 [inline]
       __lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
       lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
       kernfs_drain+0x284/0x600 fs/kernfs/dir.c:511
       __kernfs_remove+0x3cf/0x660 fs/kernfs/dir.c:1513
       kernfs_remove_by_name_ns+0xaf/0x130 fs/kernfs/dir.c:1722
       kernfs_remove_by_name include/linux/kernfs.h:633 [inline]
       cgroup_rm_file kernel/cgroup/cgroup.c:1758 [inline]
       cgroup_addrm_files+0x684/0xc30 kernel/cgroup/cgroup.c:4485
       cgroup_destroy_locked+0x321/0x630 kernel/cgroup/cgroup.c:6199
       cgroup_rmdir+0x3e8/0x710 kernel/cgroup/cgroup.c:6313
       kernfs_iop_rmdir+0x203/0x350 fs/kernfs/dir.c:1291
       vfs_rmdir+0x400/0x6f0 fs/namei.c:5344
       filename_rmdir+0x292/0x520 fs/namei.c:5399
       __do_sys_rmdir fs/namei.c:5422 [inline]
       __se_sys_rmdir+0x2e/0x140 fs/namei.c:5419
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  kn->active#59 --> &of->mutex --> cgroup_mutex

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(cgroup_mutex);
                               lock(&of->mutex);
                               lock(cgroup_mutex);
  lock(kn->active#59);

 *** DEADLOCK ***

4 locks held by syz.1.18/6463:
 #0: ffff88803ad2a480 (sb_writers#9){.+.+}-{0:0}, at: mnt_want_write+0x41/0x90 fs/namespace.c:493
 #1: ffff88804012b2f8 (&type->i_mutex_dir_key#6/1){+.+.}-{4:4}, at: inode_lock_nested include/linux/fs.h:1073 [inline]
 #1: ffff88804012b2f8 (&type->i_mutex_dir_key#6/1){+.+.}-{4:4}, at: __start_dirop fs/namei.c:2929 [inline]
 #1: ffff88804012b2f8 (&type->i_mutex_dir_key#6/1){+.+.}-{4:4}, at: start_dirop fs/namei.c:2940 [inline]
 #1: ffff88804012b2f8 (&type->i_mutex_dir_key#6/1){+.+.}-{4:4}, at: filename_rmdir+0x1cd/0x520 fs/namei.c:5392
 #2: ffff88805a98f4f8 (&type->i_mutex_dir_key#6){++++}-{4:4}, at: inode_lock include/linux/fs.h:1028 [inline]
 #2: ffff88805a98f4f8 (&type->i_mutex_dir_key#6){++++}-{4:4}, at: vfs_rmdir+0x109/0x6f0 fs/namei.c:5329
 #3: ffffffff8de0ad98 (cgroup_mutex){+.+.}-{4:4}, at: cgroup_lock include/linux/cgroup.h:394 [inline]
 #3: ffffffff8de0ad98 (cgroup_mutex){+.+.}-{4:4}, at: cgroup_kn_lock_live+0x13c/0x230 kernel/cgroup/cgroup.c:1732

stack backtrace:
CPU: 0 UID: 0 PID: 6463 Comm: syz.1.18 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2043
 check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain kernel/locking/lockdep.c:3908 [inline]
 __lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
 kernfs_drain+0x284/0x600 fs/kernfs/dir.c:511
 __kernfs_remove+0x3cf/0x660 fs/kernfs/dir.c:1513
 kernfs_remove_by_name_ns+0xaf/0x130 fs/kernfs/dir.c:1722
 kernfs_remove_by_name include/linux/kernfs.h:633 [inline]
 cgroup_rm_file kernel/cgroup/cgroup.c:1758 [inline]
 cgroup_addrm_files+0x684/0xc30 kernel/cgroup/cgroup.c:4485
 cgroup_destroy_locked+0x321/0x630 kernel/cgroup/cgroup.c:6199
 cgroup_rmdir+0x3e8/0x710 kernel/cgroup/cgroup.c:6313
 kernfs_iop_rmdir+0x203/0x350 fs/kernfs/dir.c:1291
 vfs_rmdir+0x400/0x6f0 fs/namei.c:5344
 filename_rmdir+0x292/0x520 fs/namei.c:5399
 __do_sys_rmdir fs/namei.c:5422 [inline]
 __se_sys_rmdir+0x2e/0x140 fs/namei.c:5419
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff79991c819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ff798f76028 EFLAGS: 00000246 ORIG_RAX: 0000000000000054
RAX: ffffffffffffffda RBX: 00007ff799b95fa0 RCX: 00007ff79991c819
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000200000000100
RBP: 00007ff7999b2c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ff799b96038 R14: 00007ff799b95fa0 R15: 00007ffce7cda7c8
 </TASK>


Tested on:

commit:         7f87a5ea Merge tag 'hid-for-linus-2026040801' of git:/..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10b05e06580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=45cb3c58fd963c27
dashboard link: https://syzkaller.appspot.com/bug?extid=33e571025d88efd1312c
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=155d9316580000