possible deadlock in tcm_register_callback

Status: upstream: reported on 2026/08/10 00:13
Subsystems: usb
Labels: prio:low
[Documentation on labels]
Reported-by: syzbot+642d0d809d2c72babdd2@syzkaller.appspotmail.com
First crash: 6d08h, last: 6d08h
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ID Workflow Result Correct Bug Created Started Finished Revision
1d1a1213 assessment-security DenialOfService: ❌ Exploitable: ❌ FilesystemTrigger: ❌ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ❌ UserNamespace: ❌ VMGuestTrigger: ❌ VMHostTrigger: ❌ ❓ possible deadlock in tcm_register_callback 2026/08/09 06:42 2026/08/09 06:42 2026/08/09 07:32 5d63f80b
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[syzbot] [usb?] possible deadlock in tcm_register_callback 0 (1) 2026/08/10 00:13

Sample crash report:
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.2.189/7749 is trying to acquire lock:
ffff8880550c4118 (&opts->dep_lock){+.+.}-{4:4}, at: tcm_register_callback+0x1f/0x70 drivers/usb/gadget/function/f_tcm.c:2625

but task is already holding lock:
ffff8880533c7070 (&p->frag_sem){++++}-{4:4}, at: flush_write_buffer fs/configfs/file.c:205 [inline]
ffff8880533c7070 (&p->frag_sem){++++}-{4:4}, at: configfs_write_iter+0x218/0x4e0 fs/configfs/file.c:229

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&p->frag_sem){++++}-{4:4}:
       lock_acquire kernel/locking/lockdep.c:5868 [inline]
       lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
       down_write_killable+0x93/0x250 kernel/locking/rwsem.c:1643
       configfs_rmdir+0x3fa/0x950 fs/configfs/dir.c:1534
       vfs_rmdir fs/namei.c:5381 [inline]
       vfs_rmdir+0x328/0x8a0 fs/namei.c:5354
       filename_rmdir+0x31a/0x5c0 fs/namei.c:5438
       __do_sys_rmdir fs/namei.c:5461 [inline]
       __se_sys_rmdir fs/namei.c:5458 [inline]
       __x64_sys_rmdir+0x46/0x70 fs/namei.c:5458
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&sb->s_type->i_mutex_key#24){++++}-{4:4}:
       lock_acquire kernel/locking/lockdep.c:5868 [inline]
       lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
       down_write+0x8b/0x1f0 kernel/locking/rwsem.c:1631
       inode_lock include/linux/fs.h:1024 [inline]
       configfs_depend_item_unlocked+0xf3/0x6a0 fs/configfs/dir.c:1259
       usbg_make_tpg+0x261/0x620 drivers/usb/gadget/function/f_tcm.c:1686
       target_fabric_make_tpg+0xab/0x8c0 drivers/target/target_core_fabric_configfs.c:939
       configfs_mkdir+0x51e/0xb70 fs/configfs/dir.c:1360
       vfs_mkdir+0x361/0x850 fs/namei.c:5276
       filename_mkdirat+0x48b/0x5e0 fs/namei.c:5309
       __do_sys_mkdirat fs/namei.c:5330 [inline]
       __se_sys_mkdirat fs/namei.c:5327 [inline]
       __x64_sys_mkdirat+0x89/0xc0 fs/namei.c:5327
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&opts->dep_lock){+.+.}-{4:4}:
       check_prev_add+0xeb/0xe60 kernel/locking/lockdep.c:3165
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain kernel/locking/lockdep.c:3908 [inline]
       __lock_acquire+0x136c/0x1a40 kernel/locking/lockdep.c:5237
       lock_acquire kernel/locking/lockdep.c:5868 [inline]
       lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x1a4/0x1bd0 kernel/locking/mutex.c:821
       tcm_register_callback+0x1f/0x70 drivers/usb/gadget/function/f_tcm.c:2625
       usbg_attach drivers/usb/gadget/function/f_tcm.c:2649 [inline]
       usbg_enable_tpg+0x95/0x160 drivers/usb/gadget/function/f_tcm.c:1825
       target_tpg_enable+0x1fc/0x5d0 drivers/target/target_core_tpg.c:481
       target_fabric_tpg_base_enable_store+0x162/0x190 drivers/target/target_core_fabric_configfs.c:840
       flush_write_buffer fs/configfs/file.c:207 [inline]
       configfs_write_iter+0x302/0x4e0 fs/configfs/file.c:229
       new_sync_write fs/read_write.c:595 [inline]
       vfs_write+0x6ac/0x1050 fs/read_write.c:687
       ksys_write+0x12a/0x250 fs/read_write.c:739
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x115/0x870 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:
  &opts->dep_lock --> &sb->s_type->i_mutex_key#24 --> &p->frag_sem

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(&p->frag_sem);
                               lock(&sb->s_type->i_mutex_key#24);
                               lock(&p->frag_sem);
  lock(&opts->dep_lock);

 *** DEADLOCK ***

4 locks held by syz.2.189/7749:
 #0: ffff888021776ef0 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2aa/0x380 fs/file.c:1259
 #1: ffff88801e736450 (sb_writers#13){.+.+}-{0:0}, at: ksys_write+0x12a/0x250 fs/read_write.c:739
 #2: ffff88802bb2dc80 (&buffer->mutex){+.+.}-{4:4}, at: configfs_write_iter+0x76/0x4e0 fs/configfs/file.c:226
 #3: ffff8880533c7070 (&p->frag_sem){++++}-{4:4}, at: flush_write_buffer fs/configfs/file.c:205 [inline]
 #3: ffff8880533c7070 (&p->frag_sem){++++}-{4:4}, at: configfs_write_iter+0x218/0x4e0 fs/configfs/file.c:229

stack backtrace:
CPU: 0 UID: 0 PID: 7749 Comm: syz.2.189 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
 print_circular_bug.cold+0x178/0x1c7 kernel/locking/lockdep.c:2043
 check_noncircular+0x146/0x160 kernel/locking/lockdep.c:2175
 check_prev_add+0xeb/0xe60 kernel/locking/lockdep.c:3165
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain kernel/locking/lockdep.c:3908 [inline]
 __lock_acquire+0x136c/0x1a40 kernel/locking/lockdep.c:5237
 lock_acquire kernel/locking/lockdep.c:5868 [inline]
 lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
 __mutex_lock_common kernel/locking/mutex.c:646 [inline]
 __mutex_lock+0x1a4/0x1bd0 kernel/locking/mutex.c:821
 tcm_register_callback+0x1f/0x70 drivers/usb/gadget/function/f_tcm.c:2625
 usbg_attach drivers/usb/gadget/function/f_tcm.c:2649 [inline]
 usbg_enable_tpg+0x95/0x160 drivers/usb/gadget/function/f_tcm.c:1825
 target_tpg_enable+0x1fc/0x5d0 drivers/target/target_core_tpg.c:481
 target_fabric_tpg_base_enable_store+0x162/0x190 drivers/target/target_core_fabric_configfs.c:840
 flush_write_buffer fs/configfs/file.c:207 [inline]
 configfs_write_iter+0x302/0x4e0 fs/configfs/file.c:229
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1050 fs/read_write.c:687
 ksys_write+0x12a/0x250 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f72db79e019
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:00007f72dc5e0028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f72dba25fa0 RCX: 00007f72db79e019
RDX: 0000000000000001 RSI: 00002000000009c0 RDI: 0000000000000007
RBP: 00007f72db83500c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f72dba26038 R14: 00007f72dba25fa0 R15: 00007ffdfd368f88
 </TASK>
UDC core: g1: couldn't find an available UDC or it's busy

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/08/04 15:24 upstream 848acc8ffe1b d4abbeac .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-kasan-gce-selinux-root possible deadlock in tcm_register_callback
* Struck through repros no longer work on HEAD.