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

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Message-ID: <a27660bd-fa7c-20c5-b7f2-f2ff20fe6cb8@redhat.com>
Date:   Thu, 25 May 2023 11:52:28 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Mark Pearson <mpearson-lenovo@...ebb.ca>
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

Hi Mark,

On 5/24/23 20:20, Mark Pearson wrote:
> Hi Hans,
> 
> On Tue, May 23, 2023, at 8:36 AM, Mark Pearson wrote:
>> 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.
>>
> 
> Looked at this and the code is correct - even if it is a bit weird :)
> https://docs.lenovocdrt.com/#/bios/wmi/wmi_guide?id=set-and-save-a-bios-setting-on-newer-models
> 
> The save_bios_settings would fail if a password was not set (if it's required).

Ok, can you add some comments to the next revision explaining this ?
(no need to write a novel, just some short comments)

> With regards to race conditions - that does seem somewhat unlikely in real life but I can add a mutex around this to catch that condition. I think I should probably do the same in a couple of other places (e.g. certificate_store and new_password_store) where multiple WMI calls are needed to complete an operation. 

Ack for also adding the mutex in other places where there is more
then 1 WMI call involved.

> Is it OK if I do that as a separate commit on the end of the series or would you rather it was included in this commit? As the scope is, I think, more than just this function I'm leaning towards a separate commit but let me know what best practice is.

Adding this in a separate commit is fine with me.

Regards,

Hans