linux-kernel - Re: [PATCH v2] s390/pkey: Use memdup_user() rather than duplicating its implementation

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Date:   Thu, 7 Nov 2019 14:45:54 +0100
From:   Markus Elfring <Markus.Elfring@....de>
To:     Christian Bornträger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org, Joe Perches <joe@...ches.com>,
        Harald Freudenberger <freude@...ux.ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Ingo Franzki <ifranzki@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org, Kangjie Lu <kjlu@....edu>,
        Navid Emamdoost <emamd001@....edu>,
        Stephen McCamant <smccaman@....edu>
Subject: Re: [PATCH v2] s390/pkey: Use memdup_user() rather than duplicating
 its implementation

>> Reuse existing functionality from memdup_user() instead of keeping
>> duplicate source code.
>>
>> Generated by: scripts/coccinelle/api/memdup_user.cocci
>>
>> Delete local variables which became unnecessary with this refactoring
>> in two function implementations.
>>
>> Fixes: f2bbc96e7cfad3891b7bf9bd3e566b9b7ab4553d ("s390/pkey: add CCA AES cipher key support")
>
> With that patch description, the Fixes tag is wrong...but (see below)

I wonder about such a conclusion together with your subsequent feedback.


>>  static void *_copy_apqns_from_user(void __user *uapqns, size_t nr_apqns)
>>  {
>
> This part below is not an equivalent replacement.

The shown refactoring provides also different run time characteristics,
doesn't it?


> In fact you are fixing a bug here...

Thanks for your acknowledgement.


>> -	void *kapqns = NULL;
>> -	size_t nbytes;
>> -
>> -	if (uapqns && nr_apqns > 0) {
>> -		nbytes = nr_apqns * sizeof(struct pkey_apqn);
>> -		kapqns = kmalloc(nbytes, GFP_KERNEL);
>> -		if (!kapqns)
>> -			return ERR_PTR(-ENOMEM);
>> -		if (copy_from_user(kapqns, uapqns, nbytes))
>
> 	.... here we would need to kfree kapqns, but we do not. So this is
> a memory leak. Isnt it?

This is another undesirable software weakness because of incomplete
exception handling in the previous copy approach.


> So indeed this is fixing something. But please rework your the patch
> description accordingly.

Can the final committer pick the opportunity up to extend the change
description another bit?


>> +	if (!uapqns || nr_apqns <= 0)
>> +		return NULL;
>>
>> -	return kapqns;
>> +	return memdup_user(uapqns, nr_apqns * sizeof(struct pkey_apqn));
>>  }


Would you like to add any tags for the presented software improvement?

Regards,
Markus