[syzbot] [net?] possible deadlock in ptp_clock_adjtime

[syzbot <syzbot+28ddd7a3988eea351eb3@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    d7b8f8e20813 Linux 6.16-rc5
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12b3728c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c963f5de3d9eab7d
dashboard link: https://syzkaller.appspot.com/bug?extid=28ddd7a3988eea351eb3
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b35e485ae779/disk-d7b8f8e2.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/8478fc45f5f6/vmlinux-d7b8f8e2.xz
kernel image: https://storage.googleapis.com/syzbot-assets/86a991e3c539/bzImage-d7b8f8e2.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+28ddd7a3988eea351eb3@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.16.0-rc5-syzkaller #0 Tainted: G     U             
------------------------------------------------------
syz.0.4267/20512 is trying to acquire lock:
ffff88802ffba868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at: ptp_vclock_in_use drivers/ptp/ptp_private.h:113 [inline]
ffff88802ffba868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at: ptp_vclock_in_use drivers/ptp/ptp_private.h:99 [inline]
ffff88802ffba868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at: ptp_clock_freerun drivers/ptp/ptp_private.h:130 [inline]
ffff88802ffba868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at: ptp_clock_freerun drivers/ptp/ptp_private.h:125 [inline]
ffff88802ffba868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at: ptp_clock_adjtime+0x527/0x760 drivers/ptp/ptp_clock.c:125

but task is already holding lock:
ffff88802ffba130 (&clk->rwsem){++++}-{4:4}, at: get_posix_clock kernel/time/posix-clock.c:25 [inline]
ffff88802ffba130 (&clk->rwsem){++++}-{4:4}, at: get_clock_desc+0x125/0x240 kernel/time/posix-clock.c:209

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (&clk->rwsem){++++}-{4:4}:
       down_write+0x92/0x200 kernel/locking/rwsem.c:1577
       posix_clock_unregister+0x4d/0xd0 kernel/time/posix-clock.c:184
       ptp_clock_unregister+0x14f/0x250 drivers/ptp/ptp_clock.c:432
       ptp_vclock_unregister+0x11a/0x160 drivers/ptp/ptp_vclock.c:228
       unregister_vclock+0x108/0x1a0 drivers/ptp/ptp_sysfs.c:177
       device_for_each_child_reverse+0x133/0x1a0 drivers/base/core.c:4051
       n_vclocks_store+0x4b6/0x6d0 drivers/ptp/ptp_sysfs.c:241
       dev_attr_store+0x58/0x80 drivers/base/core.c:2440
       sysfs_kf_write+0xf2/0x150 fs/sysfs/file.c:145
       kernfs_fop_write_iter+0x354/0x510 fs/kernfs/file.c:334
       new_sync_write fs/read_write.c:593 [inline]
       vfs_write+0x6c4/0x1150 fs/read_write.c:686
       ksys_write+0x12a/0x250 fs/read_write.c:738
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0x490 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&ptp->n_vclocks_mux){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3168 [inline]
       check_prevs_add kernel/locking/lockdep.c:3287 [inline]
       validate_chain kernel/locking/lockdep.c:3911 [inline]
       __lock_acquire+0x126f/0x1c90 kernel/locking/lockdep.c:5240
       lock_acquire kernel/locking/lockdep.c:5871 [inline]
       lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5828
       __mutex_lock_common kernel/locking/mutex.c:602 [inline]
       __mutex_lock+0x199/0xb90 kernel/locking/mutex.c:747
       ptp_vclock_in_use drivers/ptp/ptp_private.h:113 [inline]
       ptp_vclock_in_use drivers/ptp/ptp_private.h:99 [inline]
       ptp_clock_freerun drivers/ptp/ptp_private.h:130 [inline]
       ptp_clock_freerun drivers/ptp/ptp_private.h:125 [inline]
       ptp_clock_adjtime+0x527/0x760 drivers/ptp/ptp_clock.c:125
       pc_clock_adjtime+0x115/0x1e0 kernel/time/posix-clock.c:239
       do_clock_adjtime kernel/time/posix-timers.c:1162 [inline]
       __do_sys_clock_adjtime+0x172/0x290 kernel/time/posix-timers.c:1174
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0x490 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(&clk->rwsem);
                               lock(&ptp->n_vclocks_mux);
                               lock(&clk->rwsem);
  lock(&ptp->n_vclocks_mux);

 *** DEADLOCK ***

1 lock held by syz.0.4267/20512:
 #0: ffff88802ffba130 (&clk->rwsem){++++}-{4:4}, at: get_posix_clock kernel/time/posix-clock.c:25 [inline]
 #0: ffff88802ffba130 (&clk->rwsem){++++}-{4:4}, at: get_clock_desc+0x125/0x240 kernel/time/posix-clock.c:209

stack backtrace:
CPU: 0 UID: 0 PID: 20512 Comm: syz.0.4267 Tainted: G     U              6.16.0-rc5-syzkaller #0 PREEMPT(full) 
Tainted: [U]=USER
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 print_circular_bug+0x275/0x350 kernel/locking/lockdep.c:2046
 check_noncircular+0x14c/0x170 kernel/locking/lockdep.c:2178
 check_prev_add kernel/locking/lockdep.c:3168 [inline]
 check_prevs_add kernel/locking/lockdep.c:3287 [inline]
 validate_chain kernel/locking/lockdep.c:3911 [inline]
 __lock_acquire+0x126f/0x1c90 kernel/locking/lockdep.c:5240
 lock_acquire kernel/locking/lockdep.c:5871 [inline]
 lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5828
 __mutex_lock_common kernel/locking/mutex.c:602 [inline]
 __mutex_lock+0x199/0xb90 kernel/locking/mutex.c:747
 ptp_vclock_in_use drivers/ptp/ptp_private.h:113 [inline]
 ptp_vclock_in_use drivers/ptp/ptp_private.h:99 [inline]
 ptp_clock_freerun drivers/ptp/ptp_private.h:130 [inline]
 ptp_clock_freerun drivers/ptp/ptp_private.h:125 [inline]
 ptp_clock_adjtime+0x527/0x760 drivers/ptp/ptp_clock.c:125
 pc_clock_adjtime+0x115/0x1e0 kernel/time/posix-clock.c:239
 do_clock_adjtime kernel/time/posix-timers.c:1162 [inline]
 __do_sys_clock_adjtime+0x172/0x290 kernel/time/posix-timers.c:1174
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0x490 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ffbd478e929
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:00007ffbd5676038 EFLAGS: 00000246 ORIG_RAX: 0000000000000131
RAX: ffffffffffffffda RBX: 00007ffbd49b6080 RCX: 00007ffbd478e929
RDX: 0000000000000000 RSI: 0000000000000000 RDI: fffffffffffffffb
RBP: 00007ffbd4810b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007ffbd49b6080 R15: 00007fff4cf8c228
 </TASK>


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