linux-kernel - Re: [PATCH 1/4] platform/x86: think-lmi: Enable opcode support on BIOS settings

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Date:   Tue, 23 May 2023 08:36:49 -0400
From:   "Mark Pearson" <mpearson-lenovo@...ebb.ca>
To:     "Hans de Goede" <hdegoede@...hat.com>
Cc:     "markgross@...nel.org" <markgross@...nel.org>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] platform/x86: think-lmi: Enable opcode support on BIOS
 settings

Thanks Hans,

On Tue, May 23, 2023, at 6:46 AM, Hans de Goede wrote:
> Hi Mark,
>
> On 5/17/23 20:19, Mark Pearson wrote:
>> Whilst reviewing some documentation from the FW team on using WMI on
>> Lenovo system I noticed that we weren't using Opcode support when
>> changing BIOS settings in the thinkLMI driver.
>> 
>> We should be doing this to ensure we're future proof as the old
>> non-opcode mechanism has been deprecated.
>> 
>> Tested on X1 Carbon G10 and G11.
>> 
>> Signed-off-by: Mark Pearson <mpearson-lenovo@...ebb.ca>
>> ---
>>  drivers/platform/x86/think-lmi.c | 23 ++++++++++++++++++++++-
>>  1 file changed, 22 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/platform/x86/think-lmi.c b/drivers/platform/x86/think-lmi.c
>> index 1138f770149d..d9341305eba9 100644
>> --- a/drivers/platform/x86/think-lmi.c
>> +++ b/drivers/platform/x86/think-lmi.c
>> @@ -1001,7 +1001,28 @@ static ssize_t current_value_store(struct kobject *kobj,
>>  				tlmi_priv.pwd_admin->save_signature);
>>  		if (ret)
>>  			goto out;
>
>> -	} else { /* Non certiifcate based authentication */
>> +	} else if (tlmi_priv.opcode_support) {
>> +		/* If opcode support is present use that interface */
>> +		set_str = kasprintf(GFP_KERNEL, "%s,%s;", setting->display_name,
>> +					new_setting);
>> +		if (!set_str) {
>> +			ret = -ENOMEM;
>> +			goto out;
>> +		}
>> +
>> +		ret = tlmi_simple_call(LENOVO_SET_BIOS_SETTINGS_GUID, set_str);
>> +		if (ret)
>> +			goto out;
>> +
>> +		if (tlmi_priv.pwd_admin->valid && tlmi_priv.pwd_admin->password[0]) {
>> +			ret = tlmi_opcode_setting("WmiOpcodePasswordAdmin",
>> +					tlmi_priv.pwd_admin->password);
>> +			if (ret)
>> +				goto out;
>> +		}
>> +
>> +		ret = tlmi_save_bios_settings("");
>
> I'm a bit confused about how this works. You are calling the same
> LENOVO_SET_BIOS_SETTINGS_GUID as the old non opcode based authentication method
> without any auth string.
>
> And then afterwards you are calling LENOVO_OPCODE_IF_GUID with
> "WmiOpcodePasswordAdmin:<passwd>"
>
> Won't the initial LENOVO_SET_BIOS_SETTINGS_GUID get rejected since
> it does not include an auth-string and you have not authenticated
> yet using the opcode mechanism either. IOW shouldn't the opcode
> auth call go first ?
>
> And how does this work timing wise, vs races with userspace doing
> multiple sysfs writes at once.
>
> If the authentication done afterwards really acks the last
> LENOVO_SET_BIOS_SETTINGS_GUID call then a userspace based
> attacker could try to race and overwrite the last
> LENOVO_SET_BIOS_SETTINGS_GUID call before the ack happens... ?
>
> If this code really is correct I think we need to introduce
> a mutex to avoid this race.
>
> And this also needs some comments to explain what is going on.

Agreed - and looking at it now....I'm questioning it myself. This was tested so it works...but I wonder if that was more luck than judgement.
Let me do some checking - I think I may have messed up here.

Mark