[PATCH v3] HID: multitouch: use kzalloc for haptic data to fix use-after-free

[Aldo Ariel Panzardo]

mt_probe() allocates td->haptic with devm_kzalloc(), tying its
lifetime to the HID device's driver unbind.  hid_haptic_init() then
stores the pointer in ff->private and installs hid_haptic_destroy()
as the force-feedback destroy callback.

When the HID device is removed while a process still holds an evdev
fd, devres frees td->haptic at unbind time.  hid_haptic_destroy()
runs later from input_dev_release() and dereferences freed memory.
The input core then calls kfree(ff->private) on the same pointer,
double-freeing it.

The existing get_device()/put_device() pair in init/destroy pins the
struct hid_device but does not keep its devres allocations alive,
since devres runs at driver unbind, not at the final device kref put.

Replace devm_kzalloc() with plain kzalloc() so the haptic struct
survives driver unbind.  The input core's input_ff_destroy() already
calls kfree(ff->private) after the destroy callback, so
hid_haptic_destroy() must not free the struct itself -- it only needs
to tear down the sub-allocations it owns.

On the non-haptic path in mt_probe(), replace devm_kfree() with
kfree().  On the error paths before hid_hw_start(), free the struct
explicitly since ownership has not yet transferred to the input core.
On hid_hw_start() failure, only free the struct when the haptic
subsystem was not initialized (td->is_haptic_touchpad is false);
otherwise the input core's teardown already freed it.

Fixes: 8d0bf7908b5a ("HID: multitouch: add haptic multitouch support")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@xxxxxxxxx>
---
v3: drop the kfree(haptic) added to hid_haptic_destroy() in v1/v2 --
    input_ff_destroy() already calls kfree(ff->private) after the
    destroy callback, so the explicit kfree was a guaranteed double
    free (found by Sashiko AI review).  Also handle the hid_hw_start()
    failure path: free td->haptic only when the haptic subsystem was
    not initialized (!td->is_haptic_touchpad), since otherwise the
    input core's teardown already freed it.
v2: do not free td->haptic on hid_hw_start() failure (found by
    Sashiko AI review).
v1: https://lore.kernel.org/linux-input/20261009031207.3233206-1-qwe.aldo@xxxxxxxxx/

 drivers/hid/hid-multitouch.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
index 4e19a0c4d..9cbe61832 100644
--- a/drivers/hid/hid-multitouch.c
+++ b/drivers/hid/hid-multitouch.c
@@ -2132,7 +2132,7 @@ static int mt_probe(struct hid_device *hdev, const struct hid_device_id *id)
 		dev_err(&hdev->dev, "cannot allocate multitouch data\n");
 		return -ENOMEM;
 	}
-	td->haptic = devm_kzalloc(&hdev->dev, sizeof(*(td->haptic)), GFP_KERNEL);
+	td->haptic = kzalloc(sizeof(*(td->haptic)), GFP_KERNEL);
 	if (!td->haptic)
 		return -ENOMEM;
 
@@ -2181,12 +2181,14 @@ static int mt_probe(struct hid_device *hdev, const struct hid_device_id *id)
 
 	ret = hid_parse(hdev);
 	if (ret != 0)
-		return ret;
+		goto err_free_haptic;
 
 	if (mtclass->name == MT_CLS_APPLE_TOUCHBAR &&
 	    !hid_find_field(hdev, HID_INPUT_REPORT,
-			    HID_DG_TOUCHPAD, HID_DG_TRANSDUCER_INDEX))
-		return -ENODEV;
+			    HID_DG_TOUCHPAD, HID_DG_TRANSDUCER_INDEX)) {
+		ret = -ENODEV;
+		goto err_free_haptic;
+	}
 
 	if (mtclass->quirks & MT_QUIRK_FIX_CONST_CONTACT_ID)
 		mt_fix_const_fields(hdev, HID_DG_CONTACTID);
@@ -2195,8 +2197,11 @@ static int mt_probe(struct hid_device *hdev, const struct hid_device_id *id)
 		hdev->quirks |= HID_QUIRK_NOGET;
 
 	ret = hid_hw_start(hdev, HID_CONNECT_DEFAULT);
-	if (ret)
+	if (ret) {
+		if (!td->is_haptic_touchpad)
+			kfree(td->haptic);
 		return ret;
+	}
 
 	ret = sysfs_create_group(&hdev->dev.kobj, &mt_attribute_group);
 	if (ret)
@@ -2206,9 +2211,13 @@ static int mt_probe(struct hid_device *hdev, const struct hid_device_id *id)
 	mt_set_modes(hdev, HID_LATENCY_NORMAL, TOUCHPAD_REPORT_ALL);
 
 	if (!td->is_haptic_touchpad)
-		devm_kfree(&hdev->dev, td->haptic);
+		kfree(td->haptic);
 
 	return 0;
+
+err_free_haptic:
+	kfree(td->haptic);
+	return ret;
 }
 
 static int mt_suspend(struct hid_device *hdev, pm_message_t state)
-- 
2.43.0