Input: uinput - fix circular locking dependency with ff-core

A lockdep circular locking dependency warning can be triggered
reproducibly when using a force-feedback gamepad with uinput (for
example, playing ELDEN RING under Wine with a Flydigi Vader 5
controller):

  ff->mutex -> udev->mutex -> input_mutex -> dev->mutex -> ff->mutex

The cycle is caused by four lock acquisition paths:

1. ff upload: input_ff_upload() holds ff->mutex and calls
   uinput_dev_upload_effect() -> uinput_request_submit() ->
   uinput_request_send(), which acquires udev->mutex.

2. device create: uinput_ioctl_handler() holds udev->mutex and calls
   uinput_create_device() -> input_register_device(), which acquires
   input_mutex.

3. device register: input_register_device() holds input_mutex and
   calls kbd_connect() -> input_register_handle(), which acquires
   dev->mutex.

4. evdev release: evdev_release() calls input_flush_device() under
   dev->mutex, which calls input_ff_flush() acquiring ff->mutex.

Fix this by introducing a new state_lock spinlock to protect
udev->state and udev->dev access in uinput_request_send() instead of
acquiring udev->mutex.  The function only needs to atomically check
device state and queue an input event into the ring buffer via
uinput_dev_event() -- both operations are safe under a spinlock
(ktime_get_ts64() and wake_up_interruptible() do not sleep).  This
breaks the ff->mutex -> udev->mutex link since a spinlock is a leaf in
the lock ordering and cannot form cycles with mutexes.

To keep state transitions visible to uinput_request_send(), protect
writes to udev->state in uinput_create_device() and
uinput_destroy_device() with the same state_lock spinlock.

Additionally, move init_completion(&request->done) from
uinput_request_send() to uinput_request_submit() before
uinput_request_reserve_slot().  Once the slot is allocated,
uinput_flush_requests() may call complete() on it at any time from
the destroy path, so the completion must be initialised before the
request becomes visible.

Lock ordering after the fix:

  ff->mutex -> state_lock (spinlock, leaf)
  udev->mutex -> state_lock (spinlock, leaf)
  udev->mutex -> input_mutex -> dev->mutex -> ff->mutex (no back-edge)

Fixes: ff462551235d ("Input: uinput - switch to the new FF interface")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/CABXGCsMoxag+kEwHhb7KqhuyxfmGGd0P=tHZyb1uKE0pLr8Hkg@mail.gmail.com/
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Link: https://patch.msgid.link/20260407075031.38351-1-mikhail.v.gavrilov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

diff --git a/drivers/input/misc/uinput.c b/drivers/input/misc/uinput.c
index 13336a2fd49c8a20b496db59e4ee9678a9051401..a973e82205b5c8e18e6dd8575ff241f1d7dc4063 100644 (file)
--- a/drivers/input/misc/uinput.c
+++ b/drivers/input/misc/uinput.c
@@ -57,6 +57,7 @@ struct uinput_device {
        struct input_dev        *dev;
        struct mutex            mutex;
        enum uinput_state       state;
+       spinlock_t              state_lock;
        wait_queue_head_t       waitq;
        unsigned char           ready;
        unsigned char           head;
@@ -146,19 +147,15 @@ static void uinput_request_release_slot(struct uinput_device *udev,
 static int uinput_request_send(struct uinput_device *udev,
                               struct uinput_request *request)
 {
-       int retval;
+       int retval = 0;
 
-       retval = mutex_lock_interruptible(&udev->mutex);
-       if (retval)
-               return retval;
+       spin_lock(&udev->state_lock);
 
        if (udev->state != UIST_CREATED) {
                retval = -ENODEV;
                goto out;
        }
 
-       init_completion(&request->done);
-
        /*
         * Tell our userspace application about this new request
         * by queueing an input event.
@@ -166,7 +163,7 @@ static int uinput_request_send(struct uinput_device *udev,
        uinput_dev_event(udev->dev, EV_UINPUT, request->code, request->id);
 
  out:
-       mutex_unlock(&udev->mutex);
+       spin_unlock(&udev->state_lock);
        return retval;
 }
 
@@ -175,6 +172,13 @@ static int uinput_request_submit(struct uinput_device *udev,
 {
        int retval;
 
+       /*
+        * Initialize completion before allocating the request slot.
+        * Once the slot is allocated, uinput_flush_requests() may
+        * complete it at any time, so it must be initialized first.
+        */
+       init_completion(&request->done);
+
        retval = uinput_request_reserve_slot(udev, request);
        if (retval)
                return retval;
@@ -289,7 +293,14 @@ static void uinput_destroy_device(struct uinput_device *udev)
        struct input_dev *dev = udev->dev;
        enum uinput_state old_state = udev->state;
 
+       /*
+        * Update state under state_lock so that concurrent
+        * uinput_request_send() sees the state change before we
+        * flush pending requests and tear down the device.
+        */
+       spin_lock(&udev->state_lock);
        udev->state = UIST_NEW_DEVICE;
+       spin_unlock(&udev->state_lock);
 
        if (dev) {
                name = dev->name;
@@ -366,7 +377,9 @@ static int uinput_create_device(struct uinput_device *udev)
        if (error)
                goto fail2;
 
+       spin_lock(&udev->state_lock);
        udev->state = UIST_CREATED;
+       spin_unlock(&udev->state_lock);
 
        return 0;
 
@@ -384,6 +397,7 @@ static int uinput_open(struct inode *inode, struct file *file)
                return -ENOMEM;
 
        mutex_init(&newdev->mutex);
+       spin_lock_init(&newdev->state_lock);
        spin_lock_init(&newdev->requests_lock);
        init_waitqueue_head(&newdev->requests_waitq);
        init_waitqueue_head(&newdev->waitq);