[PATCH 2/4] HID: wacom: Fix Use-After-Free in wacom_intuos_pad

[Lee Jones <lee@kernel.org>]

wacom_intuos_pad() accesses wacom->shared->touch_input locklessly
inside the interrupt handler context.  If the Touch sibling device
is disconnected, wacom_remove_shared_data() clears 'touch_input'
outside any lock, creating a Time-of-Check to Time-of-Use (TOCTOU)
race condition where a preempted reader in interrupt context
dereferences the freed pointer, leading to a Use-After-Free.

Resolve this by introducing RCU protection for the touch_input
pointer:

 - Annotate 'touch_input' in wacom_shared struct with __rcu
 - Wrap all lockless readers in wacom_wac.c with guard(rcu)() and
   rcu_dereference() using a unified wacom_report_touch_mute()
   helper
 - Update writers in wacom_sys.c using rcu_assign_pointer()
 - Call synchronize_rcu() in wacom_remove_shared_data() to ensure
   all active RCU readers have finished before the input device is
   freed

Also wrap wacom_set_shared_values() and touch/pen assignments in
wacom_add_shared_data() inside the wacom_udev_list_lock to serialize
concurrent probe assignments, and verify that 'shared->touch == hdev'
before setting touch_input to prevent concurrent sibling probe state
desynchronization.

Fixes: 961794a00eab ("Input: wacom - add reporting of SW_MUTE_DEVICE events")
Signed-off-by: Lee Jones <lee@kernel.org>
---

v1 -> v2: Split and use RCU as per Dmitry's review
v2 -> v3: Sashiko fixes
v3 -> v4: Dmitry's review [redundant check and guard()]
v4 -> v5: Jason's review [split, remove "awkward if"]

 drivers/hid/wacom_sys.c | 35 +++++++++++++++++++++++------------
 drivers/hid/wacom_wac.c | 36 ++++++++++++++++++------------------
 drivers/hid/wacom_wac.h |  2 +-
 3 files changed, 42 insertions(+), 31 deletions(-)

diff --git a/drivers/hid/wacom_sys.c b/drivers/hid/wacom_sys.c
index 60b380efdfb7..86895f13dbae 100644
--- a/drivers/hid/wacom_sys.c
+++ b/drivers/hid/wacom_sys.c
@@ -877,10 +877,16 @@ static void wacom_remove_shared_data(void *res)
 		data = container_of(wacom_wac->shared, struct wacom_hdev_data,
 				    shared);
 
-		if (wacom_wac->shared->touch == wacom->hdev)
-			wacom_wac->shared->touch = NULL;
-		else if (wacom_wac->shared->pen == wacom->hdev)
-			wacom_wac->shared->pen = NULL;
+		scoped_guard(mutex, &wacom_udev_list_lock) {
+			if (wacom_wac->shared->touch == wacom->hdev) {
+				wacom_wac->shared->touch = NULL;
+				rcu_assign_pointer(wacom_wac->shared->touch_input, NULL);
+			} else if (wacom_wac->shared->pen == wacom->hdev) {
+				wacom_wac->shared->pen = NULL;
+			}
+		}
+
+		synchronize_rcu();
 
 		kref_put(&data->kref, wacom_release_shared_data);
 		wacom_wac->shared = NULL;
@@ -909,6 +915,11 @@ static int wacom_add_shared_data(struct hid_device *hdev)
 		list_add_tail(&data->list, &wacom_udev_list);
 	}
 
+	if (wacom_wac->features.device_type & WACOM_DEVICETYPE_TOUCH)
+		data->shared.touch = hdev;
+	else if (wacom_wac->features.device_type & WACOM_DEVICETYPE_PEN)
+		data->shared.pen = hdev;
+
 	mutex_unlock(&wacom_udev_list_lock);
 
 	wacom_wac->shared = &data->shared;
@@ -917,11 +928,6 @@ static int wacom_add_shared_data(struct hid_device *hdev)
 	if (retval)
 		return retval;
 
-	if (wacom_wac->features.device_type & WACOM_DEVICETYPE_TOUCH)
-		wacom_wac->shared->touch = hdev;
-	else if (wacom_wac->features.device_type & WACOM_DEVICETYPE_PEN)
-		wacom_wac->shared->pen = hdev;
-
 	return retval;
 }
 
@@ -2345,9 +2351,15 @@ static void wacom_release_resources(struct wacom *wacom)
 
 static void wacom_set_shared_values(struct wacom_wac *wacom_wac)
 {
+	struct wacom *wacom = container_of(wacom_wac, struct wacom, wacom_wac);
+
+	guard(mutex)(&wacom_udev_list_lock);
+
 	if (wacom_wac->features.device_type & WACOM_DEVICETYPE_TOUCH) {
-		wacom_wac->shared->type = wacom_wac->features.type;
-		wacom_wac->shared->touch_input = wacom_wac->touch_input;
+		if (wacom_wac->shared->touch == wacom->hdev) {
+			wacom_wac->shared->type = wacom_wac->features.type;
+			rcu_assign_pointer(wacom_wac->shared->touch_input, wacom_wac->touch_input);
+		}
 	}
 
 	if (wacom_wac->has_mute_touch_switch) {
@@ -2360,7 +2372,6 @@ static void wacom_set_shared_values(struct wacom_wac *wacom_wac)
 		if (wacom_wac->is_soft_touch_switch)
 			wacom_wac->shared->is_touch_on = true;
 	}
-
 }
 
 static int wacom_parse_and_register(struct wacom *wacom, bool wireless)
diff --git a/drivers/hid/wacom_wac.c b/drivers/hid/wacom_wac.c
index a31325be50f8..f3429a2c8f36 100644
--- a/drivers/hid/wacom_wac.c
+++ b/drivers/hid/wacom_wac.c
@@ -510,6 +510,18 @@ static void wacom_intuos_schedule_prox_event(struct wacom_wac *wacom_wac)
 	}
 }
 
+static void wacom_report_touch_mute(struct wacom_wac *wacom_wac, bool mute)
+{
+	struct input_dev *touch_input;
+
+	guard(rcu)();
+	touch_input = rcu_dereference(wacom_wac->shared->touch_input);
+	if (touch_input) {
+		input_report_switch(touch_input, SW_MUTE_DEVICE, mute);
+		input_sync(touch_input);
+	}
+}
+
 static int wacom_intuos_pad(struct wacom_wac *wacom)
 {
 	struct wacom_features *features = &wacom->features;
@@ -650,12 +662,8 @@ static int wacom_intuos_pad(struct wacom_wac *wacom)
 	input_report_key(input, KEY_CONTROLPANEL, menu);
 	input_report_key(input, KEY_INFO, info);
 
-	if (wacom->shared && wacom->shared->touch_input) {
-		input_report_switch(wacom->shared->touch_input,
-				    SW_MUTE_DEVICE,
-				    !wacom->shared->is_touch_on);
-		input_sync(wacom->shared->touch_input);
-	}
+	if (wacom->shared)
+		wacom_report_touch_mute(wacom, !wacom->shared->is_touch_on);
 
 	input_report_abs(input, ABS_RX, strip1);
 	input_report_abs(input, ABS_RY, strip2);
@@ -2153,7 +2161,7 @@ static void wacom_wac_pad_event(struct hid_device *hdev, struct hid_field *field
 	 */
 	if ((equivalent_usage == WACOM_HID_WD_MUTE_DEVICE) ||
 	   (equivalent_usage == WACOM_HID_WD_TOUCHONOFF)) {
-		if (wacom_wac->shared->touch_input) {
+		if (wacom_wac->shared) {
 			bool *is_touch_on = &wacom_wac->shared->is_touch_on;
 
 			if (equivalent_usage == WACOM_HID_WD_MUTE_DEVICE && value)
@@ -2161,9 +2169,7 @@ static void wacom_wac_pad_event(struct hid_device *hdev, struct hid_field *field
 			else if (equivalent_usage == WACOM_HID_WD_TOUCHONOFF)
 				*is_touch_on = value;
 
-			input_report_switch(wacom_wac->shared->touch_input,
-					    SW_MUTE_DEVICE, !(*is_touch_on));
-			input_sync(wacom_wac->shared->touch_input);
+			wacom_report_touch_mute(wacom_wac, !(*is_touch_on));
 		}
 		return;
 	}
@@ -3381,11 +3387,8 @@ static int wacom_wireless_irq(struct wacom_wac *wacom, size_t len)
 
 		if ((wacom->shared->type == INTUOSHT ||
 		    wacom->shared->type == INTUOSHT2) &&
-		    wacom->shared->touch_input &&
 		    wacom->shared->touch_max) {
-			input_report_switch(wacom->shared->touch_input,
-					SW_MUTE_DEVICE, data[5] & 0x40);
-			input_sync(wacom->shared->touch_input);
+			wacom_report_touch_mute(wacom, data[5] & 0x40);
 		}
 
 		pid = get_unaligned_be16(&data[6]);
@@ -3420,11 +3423,8 @@ static int wacom_status_irq(struct wacom_wac *wacom_wac, size_t len)
 
 	if ((features->type == INTUOSHT ||
 	    features->type == INTUOSHT2) &&
-	    wacom_wac->shared->touch_input &&
 	    features->touch_max) {
-		input_report_switch(wacom_wac->shared->touch_input,
-				    SW_MUTE_DEVICE, data[8] & 0x40);
-		input_sync(wacom_wac->shared->touch_input);
+		wacom_report_touch_mute(wacom_wac, data[8] & 0x40);
 	}
 
 	if (data[9] & 0x02) { /* wireless module is attached */
diff --git a/drivers/hid/wacom_wac.h b/drivers/hid/wacom_wac.h
index 126bec6e5c0c..a8bbba4a6f37 100644
--- a/drivers/hid/wacom_wac.h
+++ b/drivers/hid/wacom_wac.h
@@ -285,7 +285,7 @@ struct wacom_shared {
 	/* for wireless device to access USB interfaces */
 	unsigned touch_max;
 	int type;
-	struct input_dev *touch_input;
+	struct input_dev __rcu *touch_input;
 	struct hid_device *pen;
 	struct hid_device *touch;
 	bool has_mute_touch_switch;
-- 
2.55.0.rc2.803.g1fd1e6609c-goog