linux-kernel - Re: [PATCH 5/5] platform/x86: think-lmi: mutex protection around multiple WMI calls

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Message-ID: <4b49873e-48ae-1164-739f-78d96ca3a7bb@redhat.com>
Date:   Fri, 26 May 2023 10:12:36 +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 5/5] platform/x86: think-lmi: mutex protection around
 multiple WMI calls

Hi,

On 5/25/23 21:50, Mark Pearson wrote:
> 
> 
> On Thu, May 25, 2023, at 3:41 PM, Hans de Goede wrote:
>> Hi Mark,
>>
>> On 5/25/23 21:31, Mark Pearson wrote:
>>> Add mutex protection around cases where an operation needs multiple
>>> WMI calls - e.g. setting password.
>>>
>>> Signed-off-by: Mark Pearson <mpearson-lenovo@...ebb.ca>
>>> ---
>>> Changes in V2: New commit added after review of other patches in series.
>>>
>>>  drivers/platform/x86/think-lmi.c | 46 ++++++++++++++++++++++++--------
>>>  1 file changed, 35 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/platform/x86/think-lmi.c b/drivers/platform/x86/think-lmi.c
>>> index 64cd453d6e7d..f3e1e4dacba2 100644
>>> --- a/drivers/platform/x86/think-lmi.c
>>> +++ b/drivers/platform/x86/think-lmi.c
>>> @@ -14,6 +14,7 @@
>>>  #include <linux/acpi.h>
>>>  #include <linux/errno.h>
>>>  #include <linux/fs.h>
>>> +#include <linux/mutex.h>
>>>  #include <linux/string.h>
>>>  #include <linux/types.h>
>>>  #include <linux/dmi.h>
>>> @@ -195,6 +196,7 @@ static const char * const level_options[] = {
>>>  };
>>>  static struct think_lmi tlmi_priv;
>>>  static struct class *fw_attr_class;
>>> +static DEFINE_MUTEX(tlmi_mutex);
>>>  
>>>  /* ------ Utility functions ------------*/
>>>  /* Strip out CR if one is present */
>>> @@ -463,23 +465,32 @@ static ssize_t new_password_store(struct kobject *kobj,
>>>  			sprintf(pwd_type, "%s", setting->pwd_type);
>>>  		}
>>>  
>>> +		mutex_lock(&tlmi_mutex);
>>>  		ret = tlmi_opcode_setting("WmiOpcodePasswordType", pwd_type);
>>> -		if (ret)
>>> +		if (ret) {
>>> +			mutex_unlock(&tlmi_mutex);
>>>  			goto out;
>>> -
>>> +		}
>>>  		if (tlmi_priv.pwd_admin->valid) {
>>>  			ret = tlmi_opcode_setting("WmiOpcodePasswordAdmin",
>>>  					tlmi_priv.pwd_admin->password);
>>> -			if (ret)
>>> +			if (ret) {
>>> +				mutex_unlock(&tlmi_mutex);
>>>  				goto out;
>>> +			}
>>>  		}
>>>  		ret = tlmi_opcode_setting("WmiOpcodePasswordCurrent01", setting->password);
>>> -		if (ret)
>>> +		if (ret) {
>>> +			mutex_unlock(&tlmi_mutex);
>>>  			goto out;
>>> +		}
>>>  		ret = tlmi_opcode_setting("WmiOpcodePasswordNew01", new_pwd);
>>> -		if (ret)
>>> +		if (ret) {
>>> +			mutex_unlock(&tlmi_mutex);
>>>  			goto out;
>>> +		}
>>>  		ret = tlmi_simple_call(LENOVO_OPCODE_IF_GUID, "WmiOpcodePasswordSetUpdate;");
>>> +		mutex_unlock(&tlmi_mutex);
>>>  	} else {
>>>  		/* Format: 'PasswordType,CurrentPw,NewPw,Encoding,KbdLang;' */
>>>  		auth_str = kasprintf(GFP_KERNEL, "%s,%s,%s,%s,%s;",
>>
>>
>> I haven't take a really close / good look yet. But at a first glance
>> I think it would be cleaner to just take the mutex at the top
>> and unlock it after the out label to which all the existing goto-s
>> already go ?
>>
> I did consider that - and it was in my first implementation; but then I got concerned
> about if the mutex_unlock could potentially get called without mutex_lock having been 
> called beforehand. I couldn't find any good reference as to whether that was safe or not.
> 
> I ended up deciding that a few extra brackets and unlock calls wasn't that ugly and was 'safer'...and 
> so went that route.
> 
> Happy to change it - but do you happen to know if it's safe to call unlock without a lock? If it is then
> that implementation is cleaner.

It is not allowed to unlock without a lock. But if you put the lock directly after the malloc for which the out: does the free then there should be no goto out paths which don't have the lock.

E.g. for new_password_store() put it here:

        new_pwd = kstrdup(buf, GFP_KERNEL);
        if (!new_pwd)
                return -ENOMEM;

	mutex_lock(&tlmi_mutex);

	/* Strip out CR if one is present, setting password won't work if it is present */
	...

This does mean also taking the lock in the case where the new password store is done with a single WMI call, but that is not an issue. It makes things a tiny bit slower but WMI calls already are not fast and it is not like we are going to change the password / settings 100-times per second.

And the same thing can be done in current_value_store():

        new_setting = kstrdup(buf, GFP_KERNEL);
        if (!new_setting)
                return -ENOMEM;

	mutex_lock(&tlmi_mutex);

        /* Strip out CR if one is present */
        ...

Regards,

Hans