linux-kernel - Re: [PATCH v2 2/2] KVM: s390: Use memdup

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Date:   Mon, 3 Oct 2016 15:47:46 +0200
From:   SF Markus Elfring <elfring@...rs.sourceforge.net>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        Christian Bornträger <borntraeger@...ibm.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        David Hildenbrand <dahi@...ux.vnet.ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org,
        Julia Lawall <julia.lawall@...6.fr>,
        Walter Harms <wharms@....de>
Subject: Re: [PATCH v2 2/2] KVM: s390: Use memdup_user() rather than
 duplicating code

>> Did you notice the check "IS_ERR(bp_data)" and the corresponding reaction
>> in this update suggestion?
> 
> Yes, but bp_data may still be a valid (as in "not an error") value.

Thanks for your constructive feedback.


> Your commit a1708a2eaded836b ("KVM: s390: Improve determination of sizes in
> kvm_s390_import_bp_data()") made the code more robust, as kmalloc_array() ha
>  a builtin overflow check, and will return NULL if overflow is detected.
> However, commit 0624a8eb82efd58e ("KVM: s390: Use memdup_user() rather than
> duplicating code") dropped that safety net again.

* How much are you concerned about the shown software evolution around
  multiplications for memory allocations?

* Does there really a probability remain that an inappropriate product
  would be calculated here (as the situation was before my two update steps
  for this software module)?

* Can it be that you are looking for a variant of a function like "memdup_user"
  where values can be passed as separate parameters "count" and "size" so that
  the needed multiplication and corresponding overflow check would be performed
  together as desired?

Regards,
Markus