[PATCH] HID: cp2112: don't dereference hdev->hidraw unconditionally

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

cp2112_probe() builds the I2C adapter name from the hidraw minor:

	snprintf(dev->adap.name, sizeof(dev->adap.name),
		 "CP2112 SMBus Bridge on hidraw%d",
		 ((struct hidraw *)hdev->hidraw)->minor);

hdev->hidraw is set only when hidraw_connect() succeeds; on failure it
stays NULL and reading ->minor faults.  hidraw_connect() fails when all
HIDRAW_MAX_DEVICES minors are in use, or on allocation failure.

hid_hw_start() does not report that.  hid_connect() has

	if ((connect_mask & HID_CONNECT_HIDRAW) && !hidraw_connect(hdev))
		hdev->claimed |= HID_CLAIMED_HIDRAW;
	...
	if (!hdev->claimed && !hdev->driver->raw_event) {
		hid_err(hdev, "device has no listeners, quitting\n");
		return -ENODEV;
	}

When hidraw_connect() fails, claimed stays 0 -- but that bail-out also
requires the driver to have no ->raw_event.  cp2112 sets
.raw_event = cp2112_raw_event, so the -ENODEV path is skipped,
hid_connect() returns 0, cp2112_probe() passes its "if (ret)" check, and
the dereference faults.

The minor is only used for the adapter name, so fall back to the device
name instead of failing the probe.

Found by coverage-guided fuzzing of the HID subsystem through /dev/uhid
(syzkaller with extended descriptions), arm64 with KASAN:

Unable to handle kernel paging request at virtual address dfff800000000000
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
pc : cp2112_probe+0x794/0xef0 drivers/hid/hid-cp2112.c:1333
Call trace:
 cp2112_probe
 __hid_device_probe
 hid_device_probe
 really_probe
 device_add
 hid_add_device
 uhid_device_add_worker
 process_one_work

hid-led.c and hid-u2fzero.c read hdev->hidraw the same way, but neither
sets ->raw_event, so for them hid_connect() does return -ENODEV and their
error checks catch it.  cp2112 is the only one of the three where the
bail-out is defeated.

Fixes: 42cb6b35b9e6 ("HID: cp2112: use proper hidraw name with minor number")
Cc: stable@vger.kernel.org
Signed-off-by: Jinmo Yang <jinmo44.yang@gmail.com>
---
This is the minimal, backportable fix.  Dmitry's series

  [PATCH 0/5] HID: hidraw: Encapsulate struct hidraw and make hidraw.h private
  https://lore.kernel.org/linux-input/20261014051326.3784498-1-dmitry.torokhov@gmail.com/

solves the same thing more thoroughly in mainline: 2/5 adds
hidraw_instance(), which performs this check in one place, 3/5 converts
cp2112 (and hid-led and hid-u2fzero) to it, and 5/5 makes the raw cast
impossible by moving hidraw.h out of include/linux.  That is the better
end state and I am not proposing an alternative to it.

I am sending this separately only because that series moves headers and
touches include/linux/hid.h, so it is not a stable candidate, while the
dereference itself has been reachable since v4.12.  If you would rather
tag 2/5 and 3/5 for stable instead, please drop this patch -- that works
equally well for me and I will not resend it.

Either way the oops is real and unprivileged; the trace above is from
/dev/uhid with no special capabilities.  I can test whichever way you go.

 drivers/hid/hid-cp2112.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/hid/hid-cp2112.c b/drivers/hid/hid-cp2112.c
index 04379db93571..8883023d432d 100644
--- a/drivers/hid/hid-cp2112.c
+++ b/drivers/hid/hid-cp2112.c
@@ -1330,9 +1330,13 @@ static int cp2112_probe(struct hid_device *hdev, const struct hid_device_id *id)
 	dev->adap.algo		= &smbus_algorithm;
 	dev->adap.algo_data	= dev;
 	dev->adap.dev.parent	= &hdev->dev;
-	snprintf(dev->adap.name, sizeof(dev->adap.name),
-		 "CP2112 SMBus Bridge on hidraw%d",
-		 ((struct hidraw *)hdev->hidraw)->minor);
+	if (hdev->claimed & HID_CLAIMED_HIDRAW)
+		snprintf(dev->adap.name, sizeof(dev->adap.name),
+			 "CP2112 SMBus Bridge on hidraw%d",
+			 ((struct hidraw *)hdev->hidraw)->minor);
+	else
+		snprintf(dev->adap.name, sizeof(dev->adap.name),
+			 "CP2112 SMBus Bridge on %s", dev_name(&hdev->dev));
 	dev->hwversion = buf[2];
 	init_waitqueue_head(&dev->wait);
 
-- 
2.43.0