[PATCH] HID: core: serialize report_descriptor_read() against probe and remove

[Krish Gulati <krishgulati7@gmail.com>]

Since commit 54f32fd5dff9 ("HID: Make report_descriptor available for
all devices"), the "report_descriptor" sysfs attribute is part of the
device's attribute group, so it exists from device_add() until
device_del(). It reads hdev->rdesc, but that buffer only lives from
hid_open_report() until hid_close_report(), that is, for the duration
of a driver binding. For a driver with a report_fixup() callback,
rdesc is a private copy that hid_close_report() frees, so a read that
races with the driver being unbound accesses freed memory. syzbot
reports this as:

  BUG: KASAN: slab-use-after-free in report_descriptor_read+0xb5/0x100
  Read of size 24 at addr ffff888078d61f00 by task fido_id/7281
  ...
  Allocated by task 10:
   kmemdup_noprof+0x2b/0x70
   hid_open_report+0x1fa/0xf00
   lg_probe+0x29c/0x890
   hid_device_probe+0x46a/0x870
   ...

Because hid_close_report() also clears rdesc and rsize, the same missing
serialization can make the reader see a NULL rdesc with a non-zero
rsize.

Hold the device lock in report_descriptor_read(). The driver core
already holds it across probe and remove, so a read can no longer
overlap with them.

Reported-by: syzbot+bc537ca7a0efe33988eb@syzkaller.appspotmail.com
Tested-by: syzbot+bc537ca7a0efe33988eb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bc537ca7a0efe33988eb
Fixes: 54f32fd5dff9 ("HID: Make report_descriptor available for all devices")
Cc: stable@vger.kernel.org
Signed-off-by: Krish Gulati <krishgulati7@gmail.com>
---
 drivers/hid/hid-core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index a3ff0514f9cd..824557a02544 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -2270,6 +2270,8 @@ report_descriptor_read(struct file *filp, struct kobject *kobj,
 	struct device *dev = kobj_to_dev(kobj);
 	struct hid_device *hdev = to_hid_device(dev);
 
+	guard(device)(dev);
+
 	if (off >= hdev->rsize)
 		return 0;
 
-- 
2.56.0