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

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Date:   Mon, 7 Nov 2022 20:49:49 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     david.e.box@...ux.intel.com, Armin Wolf <W_Armin@....de>,
        markgross@...nel.org
Cc:     platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] platform/x86: dell-ddv: Improve buffer handling

Hi,

On 11/7/22 19:54, David E. Box wrote:
> 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@....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.

I have already merged this and both the original commit as well as
this fix will land in 6.2, so I don't think a Fixes commit is
really necessary in this case.

Also the old code checked that the 2 sizes matched, so it was more
strict and as such running only the original patch should not lead
to buffer overruns or anything like that.

Regards,

Hans