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

[Hans de Goede <hdegoede@redhat.com>]

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@squebb.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.

Regards,

Hans





> +	} else { /* old non opcode based authentication method (deprecated)*/
>  		if (tlmi_priv.pwd_admin->valid && tlmi_priv.pwd_admin->password[0]) {
>  			auth_str = kasprintf(GFP_KERNEL, "%s,%s,%s;",
>  					tlmi_priv.pwd_admin->password,