[PATCH] net/nfc: Fix A-B/B-A deadlock between nfc_unregister_device and rfkill_fop_write

[Yunseong Kim <ysk@kzalloc.com>]

A potential deadlock due to A-B/B-A deadlock exists between the NFC core
and the RFKill subsystem, involving the NFC device lock and the
rfkill_global_mutex.

This issue is particularly visible on PREEMPT_RT kernels, which can
report the following warning:

| rtmutex deadlock detected
| WARNING: CPU: 0 PID: 22729 at kernel/locking/rtmutex.c:1674 rt_mutex_handle_deadlock+0x68/0xec kernel/locking/rtmutex.c:-1
| Modules linked in:
| CPU: 0 UID: 0 PID: 22729 Comm: syz.7.2187 Kdump: loaded Not tainted 6.17.0-rc1-00001-g1149a5db27c8-dirty #55 PREEMPT_RT
| Hardware name: QEMU KVM Virtual Machine, BIOS 2025.02-8ubuntu1 06/11/2025
| pstate: 63400005 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
| pc : rt_mutex_handle_deadlock+0x68/0xec kernel/locking/rtmutex.c:-1
| lr : rt_mutex_handle_deadlock+0x40/0xec kernel/locking/rtmutex.c:1674
| sp : ffff8000967c7720
| x29: ffff8000967c7720 x28: 1fffe0001946d182 x27: dfff800000000000
| x26: 0000000000000001 x25: 0000000000000003 x24: 1fffe0001946d00b
| x23: 1fffe0001946d182 x22: ffff80008aec8940 x21: dfff800000000000
| x20: ffff0000ca368058 x19: ffff0000ca368c10 x18: ffff80008af6b6e0
| x17: 1fffe000590b8088 x16: ffff80008046cc08 x15: 0000000000000001
| x14: 1fffe000590ba990 x13: 0000000000000000 x12: 0000000000000000
| x11: ffff6000590ba991 x10: 0000000000000002 x9 : 0fe446e029bcfe00
| x8 : 0000000000000000 x7 : 0000000000000000 x6 : 000000000000003f
| x5 : 0000000000000001 x4 : 0000000000001000 x3 : ffff800080503efc
| x2 : 0000000000000001 x1 : 0000000000000001 x0 : 0000000000000001
| Call trace:
|  rt_mutex_handle_deadlock+0x68/0xec kernel/locking/rtmutex.c:-1 (P)
|  __rt_mutex_slowlock+0x1cc/0x480 kernel/locking/rtmutex.c:1734
|  __rt_mutex_slowlock_locked kernel/locking/rtmutex.c:1760 [inline]
|  rt_mutex_slowlock+0x140/0x21c kernel/locking/rtmutex.c:1800
|  __rt_mutex_lock kernel/locking/rtmutex.c:1815 [inline]
|  __mutex_lock_common kernel/locking/rtmutex_api.c:536 [inline]
|  mutex_lock+0xf0/0x10c kernel/locking/rtmutex_api.c:603
|  device_lock include/linux/device.h:911 [inline]
|  nfc_dev_down net/nfc/core.c:143 [inline]
|  nfc_rfkill_set_block+0x48/0x2a4 net/nfc/core.c:179
|  rfkill_set_block+0x184/0x364 net/rfkill/core.c:346
|  rfkill_fop_write+0x4dc/0x624 net/rfkill/core.c:1301
|  vfs_write+0x2b8/0xa30 fs/read_write.c:684
|  ksys_write+0x120/0x210 fs/read_write.c:738
|  __do_sys_write fs/read_write.c:749 [inline]
|  __se_sys_write fs/read_write.c:746 [inline]
|  __arm64_sys_write+0x7c/0x90 fs/read_write.c:746
|  __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
|  invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49
|  el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:132
|  do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
|  el0_svc+0x40/0x140 arch/arm64/kernel/entry-common.c:879
|  el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:898
|  el0t_64_sync+0x1ac/0x1b0 arch/arm64/kernel/entry.S:596

The scenario is as follows:

Task A (rfkill_fop_write):
  1. Acquires rfkill_global_mutex.
  2. Iterates devices and calls rfkill_set_block()
     -> nfc_rfkill_set_block()
     -> nfc_dev_down().
  3. Tries to acquire NFC device_lock.

Task B (nfc_unregister_device):
  1. Acquires NFC device_lock.
  2. Calls rfkill_unregister().
  3. Tries to acquire rfkill_global_mutex.

Task A waits for the device_lock held by Task B, while Task B waits for
the rfkill_global_mutex held by Task A.

To fix this, move the calls to rfkill_unregister() and rfkill_destroy()
outside the device_lock critical section in nfc_unregister_device().

We ensure this is safe by first acquiring the device_lock, setting the
shutting_down flag (which prevents races with nfc_dev_down()),
stashing the rfkill pointer in a local variable, nullifying the pointer
in the nfc_dev structure, and then releasing the device_lock before
calling the rfkill unregister functions. This breaks the lock inversion.

Signed-off-by: Yunseong Kim <ysk@kzalloc.com>
---
 net/nfc/core.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/nfc/core.c b/net/nfc/core.c
index ae1c842f9c64..c8dc6414514b 100644
--- a/net/nfc/core.c
+++ b/net/nfc/core.c
@@ -1154,6 +1154,7 @@ EXPORT_SYMBOL(nfc_register_device);
 void nfc_unregister_device(struct nfc_dev *dev)
 {
 	int rc;
+	struct rfkill *rfk = NULL;
 
 	pr_debug("dev_name=%s\n", dev_name(&dev->dev));
 
@@ -1163,14 +1164,18 @@ void nfc_unregister_device(struct nfc_dev *dev)
 			 "was removed\n", dev_name(&dev->dev));
 
 	device_lock(&dev->dev);
+	dev->shutting_down = true;
 	if (dev->rfkill) {
-		rfkill_unregister(dev->rfkill);
-		rfkill_destroy(dev->rfkill);
+		rfk = dev->rfkill;
 		dev->rfkill = NULL;
 	}
-	dev->shutting_down = true;
 	device_unlock(&dev->dev);
 
+	if (rfk) {
+		rfkill_unregister(rfk);
+		rfkill_destroy(rfk);
+	}
+
 	if (dev->ops->check_presence) {
 		timer_delete_sync(&dev->check_pres_timer);
 		cancel_work_sync(&dev->check_pres_work);
-- 
2.50.0