Re: [PATCH] HID: logitech-hidpp: fix NULL pointer dereference in hidpp_get_report_length()

[Bastien Nocera <hadess@hadess.net>]

Hello,

Patch looks good, but you will need to resend it as plain text, as per
the submitting patches documentation:
https://www.kernel.org/doc/html/v6.17/process/submitting-patches.html#no-mime-no-links-no-compression-no-attachments-just-plain-text

Regards

On Thu, 2026-01-15 at 01:48 +0800, 齐柯宇 wrote:
> Add validation for report->maxfield and report->field[0] before
> dereferencing to prevent NULL pointer dereference.
> 
> The HID report descriptor is provided by the USB device firmware via
> USB control transfer (GET_DESCRIPTOR). A malicious device can craft
> a descriptor that defines an OUTPUT report without any usages
> (padding fields). When the HID subsystem parses such a descriptor:
> 
> 1. hid_add_field() calls hid_register_report() to create the report
>    object and stores it in report_id_hash[id]
> 2. Since parser->local.usage_index is 0, hid_add_field() returns
> early
>    without calling hid_register_field() to add any fields
> 3. Result: report exists with maxfield=0 and field[0]=NULL
> 
> When hidpp_probe() is called for a device matching this driver:
>   - hidpp_validate_device() calls hidpp_get_report_length()
>   - hidpp_get_report_length() retrieves the report from hash (not
> NULL)
>   - It then dereferences report->field[0]->report_count
>   - Since field[0] is NULL, this triggers a kernel NULL pointer
>     dereference
> 
> Data flow from attacker to crash:
>   Malicious USB Device
>        |
>        v (USB GET_DESCRIPTOR control transfer)
>   hid_get_class_descriptor() -- reads HID report descriptor from
> device
>        |
>        v
>   hid_parse_report() -- stores descriptor in hid->dev_rdesc
>        |
>        v
>   hid_open_report() -> hid_add_field()
>        |                    |
>        |                    v
>        |              hid_register_report() -- creates report,
> maxfield=0
>        |                    |
>        |                    v
>        |              returns early if usage_index==0, no field added
>        |
>        v
>   hidpp_validate_device() -> hidpp_get_report_length()
>        |
>        v
>   report->field[0]->report_count -- NULL pointer dereference!
> 
> This is triggerable by an attacker with physical access using a
> malicious USB device (e.g., BadUSB, programmable USB development
> boards).
> 
> Fixes: d71b18f7c7999 ("HID: logitech-hidpp: do not hardcode very long
> report length")
> Signed-off-by: Kery Qi <qikeyu2017@gmail.com>
> ---
>  drivers/hid/hid-logitech-hidpp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hid/hid-logitech-hidpp.c b/drivers/hid/hid-
> logitech-hidpp.c
> index d5011a5d0890..02ddbd658e89 100644
> --- a/drivers/hid/hid-logitech-hidpp.c
> +++ b/drivers/hid/hid-logitech-hidpp.c
> @@ -4314,7 +4314,7 @@ static int hidpp_get_report_length(struct
> hid_device *hdev, int id)
> 
>         re = &(hdev->report_enum[HID_OUTPUT_REPORT]);
>         report = re->report_id_hash[id];
> -       if (!report)
> +       if (!report || report->maxfield < 1 || !report->field[0])
>                 return 0;
> 
>         return report->field[0]->report_count + 1;
> --
> 2.34.1