[PATCH net v2] ptp: prevent possible ABBA deadlock in ptp_clock_freerun()

ABBA deadlock occurs in the following scenario:

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

To solve this with minimal patches, we should change ptp_clock_freerun()
to briefly release the read lock before calling ptp_vclock_in_use() and
then re-lock it when we're done.

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 <redacted>
---
v2: Add CC Vladimir
- Link to v1: https://lore.kernel.org/all/20250705145031.140571-1-aha310510@gmail.com/ (local)
---
 drivers/ptp/ptp_private.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/ptp/ptp_private.h b/drivers/ptp/ptp_private.h
index a6aad743c282..e2c37e968c88 100644
--- a/drivers/ptp/ptp_private.h
+++ b/drivers/ptp/ptp_private.h

@@ -124,10 +124,16 @@ static inline bool ptp_vclock_in_use(struct ptp_clock *ptp)
 /* Check if ptp clock shall be free running */
 static inline bool ptp_clock_freerun(struct ptp_clock *ptp)
 {
+	bool ret = false;
+
 	if (ptp->has_cycles)
-		return false;
+		return ret;
+
+	up_read(&ptp->clock.rwsem);
+	ret = ptp_vclock_in_use(ptp);
+	down_read(&ptp->clock.rwsem);
 
-	return ptp_vclock_in_use(ptp);
+	return ret;
 }
 
 extern const struct class ptp_class;
--