[PATCH] HID: hidraw: fix use-after-free and double free on disconnect

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

BUG: KASAN: slab-use-after-free in _raw_spin_lock_irqsave+0xf8/0x1c8
Write of size 4 at addr ffff0000ce4ffb38 by task wacom_uaf/261

Call trace:
 _raw_spin_lock_irqsave+0xf8/0x1c8
 hidraw_report_event+0x64/0x2a8
 hid_report_raw_event+0x488/0x1228
 __hid_input_report+0x418/0x5b8
 hid_safe_input_report+0x68/0x90
 uhid_char_write+0xb04/0xfb8
 vfs_write+0x2e4/0xac0

Allocated by task 131:
 hidraw_connect+0x54/0x390
 hid_connect+0xb80/0x13d8
 hid_hw_start+0xc4/0x150
 wacom_parse_and_register+0x2ae4/0x4d40
 wacom_probe+0x724/0xb18

Freed by task 68:
 kfree+0x264/0x4c8
 drop_ref+0x268/0x340
 hidraw_disconnect+0x60/0x88
 hid_disconnect+0x17c/0x1c8
 hid_hw_stop+0x70/0x100
 wacom_mode_change_work+0x11c/0x6b0

The buggy address belongs to the object at ffff0000ce4ffb00
 which belongs to the cache kmalloc-96 of size 96
The buggy address is located 56 bytes inside of
 freed 96-byte region [ffff0000ce4ffb00, ffff0000ce4ffb60)

Reproduced on mainline 7.2.0-g66498c75b4f8 (arm64, KASAN) through
/dev/uhid writes alone.  Offset 56 is struct hidraw::list_lock.

hidraw_report_event() dereferences hid->hidraw with nothing keeping the
object alive, and hidraw_disconnect() frees it without clearing the
pointer:

	void hidraw_disconnect(struct hid_device *hid)
	{
		struct hidraw *hidraw = hid->hidraw;

		down_write(&minors_rwsem);
		drop_ref(hidraw, 1);		/* kfree() when !open */
		up_write(&minors_rwsem);
	}			/* hid->hidraw left dangling */

hid_disconnect() also clears the HID_CLAIMED_HIDRAW bit that guards the
call only after the free, so a report that passed the guard walks into
freed memory and the first thing it does there is take a spinlock.

driver_input_lock normally keeps the two apart -- hid_device_remove()
holds it across the callback and __hid_input_report() takes it -- but
hid_hw_stop() carries no such requirement and drivers call it from
workqueue context holding nothing, as wacom_mode_change_work() does.
minors_rwsem cannot be used on the reader side: hidraw_report_event()
also runs from softirq on real transports.

The same function also takes the pointer outside the lock that frees
it.  hid_disconnect() tests hdev->claimed & HID_CLAIMED_HIDRAW with
nothing excluding a second caller and clears claimed only at the end,
so two overlapping teardowns can both reach drop_ref() with the same
pointer.  I have one captured double free of this object with that
shape:

  BUG kmalloc-96: Object already free
   drop_ref+0x70/0x108
   hidraw_disconnect+0x3c/0x60
   hid_disconnect+0x88/0xb0
   hid_hw_stop+0x30/0x70
   wacom_remove+0x30/0x100

I could not isolate that branch to reproduce it on demand -- three
other faults on the same wacom teardown path fire first -- so I am not
claiming this trace proves that window.  Taking the pointer under
minors_rwsem is correct regardless of how often it is hit.

Publish the object through RCU and claim it under the lock.
hidraw_connect() stores the pointer with a release, so the
initialisation is visible to a reader that picks it up without holding
anything; readers take the RCU read side and load the pointer once;
hidraw_disconnect() now reads it under minors_rwsem, returns if another
teardown already took it, and detaches it before dropping the last
reference; drop_ref() hands the object to kfree_rcu(), so a reader that
already loaded the pointer is covered by the grace period.

Same victim object and same faulting function as the bot-reported window
addressed by

  https://lore.kernel.org/linux-input/20260628005846.31248-1-alhouseenyousef@gmail.com/
  ("HID: synchronize input before cleaning up a failed probe")

but a different window: that patch re-acquires driver_input_lock in
__hid_device_probe()'s failure path and leaves hid_disconnect() alone.

Fixes: 86166b7bcda0 ("HID: add hidraw interface")
Cc: stable@vger.kernel.org
Signed-off-by: Jinmo Yang <jinmo44.yang@gmail.com>
---
Validated at runtime with a test-only patch that widens the window
between the pointer load and the spin_lock (mdelay(200) in
hidraw_report_event), so the report race becomes deterministic instead
of a few instructions wide.  arm64 QEMU/KVM guests, same reproducer,
4 vCPU each:

  kernel                            exposure        KASAN   signature
  ----------------------------------------------------------------------
  baseline + widening               4700 binds/180s   1     yes, t+9.07s
  WRITE_ONCE + NULL guard only      4656 binds/180s   1     yes
  RCU + widening                    5718 binds/600s   0     no
  RCU, no widening                  7070 binds/180s   0     no

The second row is worth stating: clearing the pointer and adding a NULL
check, without RCU, does not close this.  A reader that has already
loaded the pointer still walks the object after kfree().  That is why
this patch uses a grace period rather than just a NULL guard.

Those rounds cover the report race only.  I also tried to reproduce the
disconnect-vs-disconnect window on its own, on the KASAN=n,
CONFIG_SLUB_DEBUG=y tree the captured double free came from, with a
reproducer that attaches wacom pen and touch as siblings and races
their teardown: 24 boots, drop_ref() never reached.  In the four of
those boots that also tracked the in-flight hid_device pointers, no two
teardowns of the same device were ever inside hidraw_disconnect()
together -- teardowns of the two sibling devices did overlap there, so
the instrumentation was live.  The teardown race itself reproduces
every boot, but three other faults on that path -- a wacom timer on a
freed object, a NULL deref in hid_hw_stop() and a usercopy fault in
uhid_char_read() -- end the guest before the hidraw branch is reached,
and serialising the two hid_hw_stop() calls closes the path altogether.
So that hunk rests on the captured trace plus the lock discipline, not
on a round of its own, and I would rather say so than overstate it.

On the accessors: hid_device::hidraw is a void * with no __rcu
annotation, and four drivers outside hidraw.c dereference it directly
(hid-cp2112.c, hid-u2fzero.c, hid-led.c and hid-core.c), all from
probe/connect paths where the pointer is stable.  Annotating the field
would let this use rcu_dereference()/rcu_assign_pointer() but would
cascade into those four, so the plain accessors are used with the same
ordering rcu_assign_pointer() would give: smp_store_release() is what it
expands to for a non-NULL store, and WRITE_ONCE() is what it expands to
for a constant NULL.  I am happy to do the __rcu conversion as a
separate patch if you would rather have the annotated form.

Not attempted here: clearing hdev->claimed before the *_disconnect()
calls in hid_disconnect().  hid_report_raw_event() has

	if (hid->claimed != HID_CLAIMED_HIDRAW && report->maxfield)
		hid_process_report(hid, report, cdata, interrupt);

so zeroing claimed makes that condition true and opens hid_process_report()
during teardown instead.
diff --git a/drivers/hid/hidraw.c b/drivers/hid/hidraw.c
index 9129fabed181..40c4709d5e98 100644
--- a/drivers/hid/hidraw.c
+++ b/drivers/hid/hidraw.c
@@ -354,7 +354,7 @@ static void drop_ref(struct hidraw *hidraw, int exists_bit)
 	if (!hidraw->open) {
 		if (!hidraw->exist) {
 			hidraw_table[hidraw->minor] = NULL;
-			kfree(hidraw);
+			kfree_rcu(hidraw, rcu);
 		} else {
 			/* close device for last reader */
 			hid_hw_close(hidraw->hid);
@@ -569,11 +569,24 @@ static const struct file_operations hidraw_ops = {
 
 int hidraw_report_event(struct hid_device *hid, u8 *data, int len)
 {
-	struct hidraw *dev = hid->hidraw;
 	struct hidraw_list *list;
+	struct hidraw *dev;
 	int ret = 0;
 	unsigned long flags;
 
+	/*
+	 * The object is freed by hidraw_disconnect() with nothing excluding
+	 * this path: hid_hw_stop() carries no quiesce requirement and drivers
+	 * call it from workqueue context.  Hold the RCU read side so the
+	 * object cannot go away between the load and the unlock.
+	 */
+	rcu_read_lock();
+	dev = READ_ONCE(hid->hidraw);
+	if (!dev) {
+		rcu_read_unlock();
+		return 0;
+	}
+
 	spin_lock_irqsave(&dev->list_lock, flags);
 	list_for_each_entry(list, &dev->list, node) {
 		int new_head = (list->head + 1) & (HIDRAW_BUFFER_SIZE - 1);
@@ -592,6 +605,7 @@ int hidraw_report_event(struct hid_device *hid, u8 *data, int len)
 	spin_unlock_irqrestore(&dev->list_lock, flags);
 
 	wake_up_interruptible(&dev->wait);
+	rcu_read_unlock();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(hidraw_report_event);
@@ -644,7 +658,11 @@ int hidraw_connect(struct hid_device *hid)
 	dev->minor = minor;
 
 	dev->exist = 1;
-	hid->hidraw = dev;
+	/*
+	 * Publish with a release so the initialisation above is visible to
+	 * an RCU reader that picks the pointer up without taking any lock.
+	 */
+	smp_store_release(&hid->hidraw, dev);
 
 	up_write(&minors_rwsem);
 out:
@@ -655,10 +673,25 @@ EXPORT_SYMBOL_GPL(hidraw_connect);
 
 void hidraw_disconnect(struct hid_device *hid)
 {
-	struct hidraw *hidraw = hid->hidraw;
+	struct hidraw *hidraw;
 
 	down_write(&minors_rwsem);
 
+	/*
+	 * Claim the object under the lock and detach it before dropping the
+	 * last reference.  Taking it outside lets two concurrent teardowns
+	 * capture the same pointer and both call drop_ref() on it.  Readers
+	 * that have not loaded the pointer yet now see NULL; readers that
+	 * already did are kept safe by the RCU grace period taken by
+	 * kfree_rcu() in drop_ref().
+	 */
+	hidraw = hid->hidraw;
+	if (!hidraw) {
+		up_write(&minors_rwsem);
+		return;
+	}
+
+	WRITE_ONCE(hid->hidraw, NULL);
 	drop_ref(hidraw, 1);
 
 	up_write(&minors_rwsem);
diff --git a/include/linux/hidraw.h b/include/linux/hidraw.h
index 18fd30a288de..87c3beff7a36 100644
--- a/include/linux/hidraw.h
+++ b/include/linux/hidraw.h
@@ -17,6 +17,7 @@ struct hidraw {
 	struct device *dev;
 	spinlock_t list_lock;
 	struct list_head list;
+	struct rcu_head rcu;
 };
 
 struct hidraw_report {