Re: [PATCH 1/2] platform/x86: dell-ddv: Improve buffer handling

["David E. Box" <david.e.box@linux.intel.com>]

On Wed, 2022-11-02 at 22:23 +0100, Armin Wolf wrote:
> When the DDV interface returns a buffer, it actually
> returns a acpi buffer containing an integer (buffer size)
> and another acpi buffer (buffer content).
> The size of the buffer may be smaller than the size of
> the buffer content, which is perfectly valid and should not
> be treated as an error.

Is there documentation for this that you can refer to?

> Also use the buffer size instead of the buffer content size
> when accessing the buffer to prevent accessing bogus data.
> 
> Tested on a Dell Inspiron 3505.
> 
> Signed-off-by: Armin Wolf <W_Armin@gmx.de>
> ---
>  drivers/platform/x86/dell/dell-wmi-ddv.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/platform/x86/dell/dell-wmi-ddv.c
> b/drivers/platform/x86/dell/dell-wmi-ddv.c
> index 699feae3c435..1a001296e8c6 100644
> --- a/drivers/platform/x86/dell/dell-wmi-ddv.c
> +++ b/drivers/platform/x86/dell/dell-wmi-ddv.c
> @@ -129,9 +129,9 @@ static int dell_wmi_ddv_query_buffer(struct wmi_device
> *wdev, enum dell_ddv_meth
>  	if (obj->package.elements[1].type != ACPI_TYPE_BUFFER)
>  		goto err_free;
> 
> -	if (buffer_size != obj->package.elements[1].buffer.length) {
> +	if (buffer_size > obj->package.elements[1].buffer.length) {
>  		dev_warn(&wdev->dev,
> -			 FW_WARN "ACPI buffer size (%llu) does not match WMI
> buffer size (%d)\n",
> +			 FW_WARN "WMI buffer size (%llu) exceeds ACPI buffer
> size (%d)\n",
>  			 buffer_size, obj->package.elements[1].buffer.length);
> 
>  		goto err_free;
> @@ -271,15 +271,17 @@ static int dell_wmi_ddv_buffer_read(struct seq_file
> *seq, enum dell_ddv_method m
>  	struct device *dev = seq->private;
>  	struct dell_wmi_ddv_data *data = dev_get_drvdata(dev);
>  	union acpi_object *obj;
> -	union acpi_object buf;
> +	u64 size;
> +	u8 *buf;
>  	int ret;
> 
>  	ret = dell_wmi_ddv_query_buffer(data->wdev, method, 0, &obj);
>  	if (ret < 0)
>  		return ret;
> 
> -	buf = obj->package.elements[1];
> -	ret = seq_write(seq, buf.buffer.pointer, buf.buffer.length);
> +	size = obj->package.elements[0].integer.value;
> +	buf = obj->package.elements[1].buffer.pointer;
> +	ret = seq_write(seq, buf, size);

Okay, so the buffer may provide more space than what should actually be used
according to the size field. This looks like a bug that should have a fixes tag
on the original commit.

David

>  	kfree(obj);
> 
>  	return ret;
> --
> 2.30.2
>