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

Markus Elfring
2019-11-07T13:46:16Z

>> 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