[PATCH net v4] ptp: prevent possible ABBA deadlock in ptp_clock_freerun()

[Jeongjun Park <aha310510@gmail.com>]

syzbot reported the following ABBA deadlock:

       CPU0                           CPU1
       ----                           ----
  n_vclocks_store()
    lock(&ptp->n_vclocks_mux) [1]
        (physical clock)
                                     pc_clock_adjtime()
                                       lock(&clk->rwsem) [2]
                                        (physical clock)
                                       ...
                                       ptp_clock_freerun()
                                         ptp_vclock_in_use()
                                           lock(&ptp->n_vclocks_mux) [3]
                                              (physical clock)
    ptp_clock_unregister()
      posix_clock_unregister()
        lock(&clk->rwsem) [4]
          (virtual clock)

Since ptp virtual clock is registered only under ptp physical clock, both
ptp_clock and posix_clock must be physical clocks for ptp_vclock_in_use()
to lock &ptp->n_vclocks_mux and check ptp->n_vclocks.

However, when unregistering vclocks in n_vclocks_store(), the locking
ptp->n_vclocks_mux is a physical clock lock, but clk->rwsem of
ptp_clock_unregister() called through device_for_each_child_reverse()
is a virtual clock lock.

Therefore, clk->rwsem used in CPU0 and clk->rwsem used in CPU1 are
different locks, but in lockdep, a false positive occurs because the
possibility of deadlock is determined through lock-class.

To solve this, lock subclass annotation must be added to the posix_clock
rwsem of the vclock.

Reported-by: syzbot+7cfb66a237c4a5fb22ad@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7cfb66a237c4a5fb22ad
Fixes: 73f37068d540 ("ptp: support ptp physical/virtual clocks conversion")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
---
v4: Remove unnecessary lock class annotation and CC "POSIX CLOCKS and TIMERS" maintainer
- Link to v3: https://lore.kernel.org/all/20250719124022.1536524-1-aha310510@gmail.com/
v3: Annotate lock subclass to prevent false positives of lockdep
- Link to v2: https://lore.kernel.org/all/20250718114958.1473199-1-aha310510@gmail.com/
v2: Add CC Vladimir
- Link to v1: https://lore.kernel.org/all/20250705145031.140571-1-aha310510@gmail.com/
---
 drivers/ptp/ptp_private.h | 5 +++++
 drivers/ptp/ptp_vclock.c  | 7 +++++++
 2 files changed, 12 insertions(+)

diff --git a/drivers/ptp/ptp_private.h b/drivers/ptp/ptp_private.h
index a6aad743c282..b352df4cd3f9 100644
--- a/drivers/ptp/ptp_private.h
+++ b/drivers/ptp/ptp_private.h
@@ -24,6 +24,11 @@
 #define PTP_DEFAULT_MAX_VCLOCKS 20
 #define PTP_MAX_CHANNELS 2048
 
+enum {
+	PTP_LOCK_PHYSICAL = 0,
+	PTP_LOCK_VIRTUAL,
+};
+
 struct timestamp_event_queue {
 	struct ptp_extts_event buf[PTP_MAX_TIMESTAMPS];
 	int head;
diff --git a/drivers/ptp/ptp_vclock.c b/drivers/ptp/ptp_vclock.c
index 7febfdcbde8b..8ed4b8598924 100644
--- a/drivers/ptp/ptp_vclock.c
+++ b/drivers/ptp/ptp_vclock.c
@@ -154,6 +154,11 @@ static long ptp_vclock_refresh(struct ptp_clock_info *ptp)
 	return PTP_VCLOCK_REFRESH_INTERVAL;
 }
 
+static void ptp_vclock_set_subclass(struct ptp_clock *ptp)
+{
+	lockdep_set_subclass(&ptp->clock.rwsem, PTP_LOCK_VIRTUAL);
+}
+
 static const struct ptp_clock_info ptp_vclock_info = {
 	.owner		= THIS_MODULE,
 	.name		= "ptp virtual clock",
@@ -213,6 +218,8 @@ struct ptp_vclock *ptp_vclock_register(struct ptp_clock *pclock)
 		return NULL;
 	}
 
+	ptp_vclock_set_subclass(vclock->clock);
+
 	timecounter_init(&vclock->tc, &vclock->cc, 0);
 	ptp_schedule_worker(vclock->clock, PTP_VCLOCK_REFRESH_INTERVAL);
 
--