HID: debug: fix race condition with between rdesc_show() and device removal

There is a race condition that could happen if hid_debug_rdesc_show()
is running while hdev is in the process of going away (device removal,
system suspend, etc) which could result in NULL pointer dereference:

	 BUG: unable to handle kernel paging request at 0000000783316040
	 CPU: 1 PID: 1512 Comm: getevent Tainted: G     U     O 4.19.20-quilt-2e5dc0ac-00029-gc455a447dd55 MIPS#1
	 RIP: 0010:hid_dump_device+0x9b/0x160
	 Call Trace:
	  hid_debug_rdesc_show+0x72/0x1d0
	  seq_read+0xe0/0x410
	  full_proxy_read+0x5f/0x90
	  __vfs_read+0x3a/0x170
	  vfs_read+0xa0/0x150
	  ksys_read+0x58/0xc0
	  __x64_sys_read+0x1a/0x20
	  do_syscall_64+0x55/0x110
	  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Grab driver_input_lock to make sure the input device exists throughout the
whole process of dumping the rdesc.

[[email protected]: update changelog a bit]
Signed-off-by: he, bo <[email protected]>
Signed-off-by: "Zhang, Jun" <[email protected]>
Signed-off-by: Jiri Kosina <[email protected]>

Author: bhe4

drivers/hid/hid-debug.c

@@ -1060,10 +1060,15 @@ static int hid_debug_rdesc_show(struct seq_file *f, void *p)
 	seq_printf(f, "\n\n");
 
 	/* dump parsed data and input mappings */
+	if (down_interruptible(&hdev->driver_input_lock))
+		return 0;
+
 	hid_dump_device(hdev, f);
 	seq_printf(f, "\n");
 	hid_dump_input_mapping(hdev, f);
 
+	up(&hdev->driver_input_lock);
+
 	return 0;
 }