[PATCH 1/1] HID: wacom: serialize mode changes with device removal

[Jinmo Yang <jinmo44.yang@gmail.com>]

wacom_mode_change_work() takes both siblings' struct wacom out of
wacom_shared and tears down and rebuilds them, holding no reference to
either. wacom_remove() cancels only the work owned by the device being
removed, so a worker owned by the other sibling keeps using this device
after its remove callback returns, and hid_device_remove() then frees
struct wacom along with the driver's devres group.

Reproduced under KASAN on hid.git master (fe2ec83746e5) with a uhid
reproducer that binds a sibling pair, sends a mode-change report and then
closes one sibling's descriptor, 10 boots out of 10:

  BUG: KASAN: slab-use-after-free in wacom_parse_and_register+0x4fa8/0x58e0
  Read of size 8 at addr ffff88800be431f8 by task kworker/0:2/75
  Workqueue: events wacom_mode_change_work
   wacom_parse_and_register+0x4fa8/0x58e0
   wacom_mode_change_work+0x333/0x7e0
   process_one_work+0xa3b/0x19b0
  Allocated by task 11:  devm_kmalloc <- wacom_probe
  Freed by task 82:      devres_release_group <- hid_device_remove,
                         under uhid_char_release <- __x64_sys_close

The same object is also written: wacom_feature_mapping() stores
features->touch_max into it, four bytes at a fixed offset, with the value
taken from a feature report. Closing one sibling's /dev/uhid descriptor is
all it takes, with no privilege, and a reference on the hid_device would
not help: struct wacom is devm_kzalloc() on &hdev->dev.

Serialize mode-change workers with wacom_remove() using a driver-wide
mutex, cancelling the device's own work first so that a worker blocked on
the mutex cannot deadlock removal. wacom_add_shared_data() registers its
devres action inside the group wacom_parse_and_register() opens, so
wacom_remove() clears this device's shared pen or touch pointer under the
mutex and no later worker can pick it up.

The worker also snapshots both shared pointers and returns on a NULL
wacom_wac.shared, because releasing the first sibling's devres can drop
the last reference to the shared data.

Fixes: 4082da80f46a ("HID: wacom: generic: add mode change touch key")
Cc: stable@vger.kernel.org
Signed-off-by: Jinmo Yang <jinmo44.yang@gmail.com>
---
Applies to hid.git master (fe2ec83746e5). It does not apply to for-next;
see the cover letter.

 drivers/hid/wacom_sys.c | 36 +++++++++++++++++++++++++++---------
 1 file changed, 27 insertions(+), 9 deletions(-)

diff --git a/drivers/hid/wacom_sys.c b/drivers/hid/wacom_sys.c
index 40770affdbde..ecd7b38b1418 100644
--- a/drivers/hid/wacom_sys.c
+++ b/drivers/hid/wacom_sys.c
@@ -764,6 +764,7 @@ struct wacom_hdev_data {
 
 static LIST_HEAD(wacom_udev_list);
 static DEFINE_MUTEX(wacom_udev_list_lock);
+static DEFINE_MUTEX(wacom_mode_change_lock);
 
 static bool wacom_are_sibling(struct hid_device *hdev,
 		struct hid_device *sibling)
@@ -2783,22 +2784,32 @@ static void wacom_remote_work(struct work_struct *work)
 static void wacom_mode_change_work(struct work_struct *work)
 {
 	struct wacom *wacom = container_of(work, struct wacom, mode_change_work);
-	struct wacom_shared *shared = wacom->wacom_wac.shared;
+	struct wacom_shared *shared;
+	struct hid_device *pen;
+	struct hid_device *touch;
 	struct wacom *wacom1 = NULL;
 	struct wacom *wacom2 = NULL;
-	bool is_direct = wacom->wacom_wac.is_direct_mode;
+	bool is_direct;
 	int error = 0;
 
-	if (shared->pen) {
-		wacom1 = hid_get_drvdata(shared->pen);
+	mutex_lock(&wacom_mode_change_lock);
+	shared = wacom->wacom_wac.shared;
+	if (!shared)
+		goto out;
+	pen = shared->pen;
+	touch = shared->touch;
+	is_direct = wacom->wacom_wac.is_direct_mode;
+
+	if (pen) {
+		wacom1 = hid_get_drvdata(pen);
 		wacom_release_resources(wacom1);
 		hid_hw_stop(wacom1->hdev);
 		wacom1->wacom_wac.has_mode_change = true;
 		wacom1->wacom_wac.is_direct_mode = is_direct;
 	}
 
-	if (shared->touch) {
-		wacom2 = hid_get_drvdata(shared->touch);
+	if (touch) {
+		wacom2 = hid_get_drvdata(touch);
 		wacom_release_resources(wacom2);
 		hid_hw_stop(wacom2->hdev);
 		wacom2->wacom_wac.has_mode_change = true;
@@ -2808,16 +2819,17 @@ static void wacom_mode_change_work(struct work_struct *work)
 	if (wacom1) {
 		error = wacom_parse_and_register(wacom1, false);
 		if (error)
-			return;
+			goto out;
 	}
 
 	if (wacom2) {
 		error = wacom_parse_and_register(wacom2, false);
 		if (error)
-			return;
+			goto out;
 	}
 
-	return;
+out:
+	mutex_unlock(&wacom_mode_change_lock);
 }
 
 static int wacom_probe(struct hid_device *hdev,
@@ -2916,6 +2928,10 @@ static void wacom_remove(struct hid_device *hdev)
 	cancel_work_sync(&wacom->battery_work);
 	cancel_work_sync(&wacom->remote_work);
 	cancel_work_sync(&wacom->mode_change_work);
+
+	/* A sibling's mode-change work can also access this device. */
+	mutex_lock(&wacom_mode_change_lock);
+
 	timer_delete_sync(&wacom->idleprox_timer);
 	if (hdev->bus == BUS_BLUETOOTH)
 		device_remove_file(&hdev->dev, &dev_attr_speed);
@@ -2925,6 +2941,8 @@ static void wacom_remove(struct hid_device *hdev)
 
 	if (wacom->wacom_wac.features.type != REMOTE)
 		wacom_release_resources(wacom);
+
+	mutex_unlock(&wacom_mode_change_lock);
 }
 
 static int wacom_resume(struct hid_device *hdev)
-- 
2.53.0