From mboxrd@z Thu Jan  1 00:00:00 1970
From: Nicholas Piggin <npiggin@gmail.com>
Date: Thu, 24 Jun 2021 10:42:08 +0000
Subject: Re: [PATCH 2/6] KVM: mmu: also return page from gfn_to_pfn
Message-Id: <1624531085.fax3fcqpgc.astroid@bobo.none>
List-Id: <kvm-ppc.vger.kernel.org>
References: <20210624035749.4054934-1-stevensd@google.com>
        <20210624035749.4054934-3-stevensd@google.com>
        <1624524331.zsin3qejl9.astroid@bobo.none>
        <201b68a7-10ea-d656-0c1e-5511b1f22674@redhat.com>
        <1624528342.s2ezcyp90x.astroid@bobo.none>
        <1624529635.75a1ann91v.astroid@bobo.none>
        <fc2a88ed-6a98-857d-bb1f-73260b01ac30@redhat.com>
In-Reply-To: <fc2a88ed-6a98-857d-bb1f-73260b01ac30@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
To: Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>, Huacai Chen <chenhuacai@kernel.org>, Marc Zyngier <maz@kernel.org>, Paul Mackerras <paulus@ozlabs.org>, Paolo Bonzini <pbonzini@redhat.com>, David Stevens <stevensd@chromium.org>, Zhenyu Wang <zhenyuw@linux.intel.com>, Zhi Wang <zhi.a.wang@intel.com>
Cc: Alexandru Elisei <alexandru.elisei@arm.com>, dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, intel-gvt-dev@lists.freedesktop.org, James Morse <james.morse@arm.com>, Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>, kvmarm@lists.cs.columbia.edu, kvm-ppc@vger.kernel.org, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Sean Christopherson <seanjc@google.com>, Suzuki K Poulose <suzuki.poulose@arm.com>, Vitaly Kuznetsov <vkuznets@redhat.com>, Wanpeng Li <wanpengli@tencent.com>, Will Deacon <will@kernel.org>

Excerpts from Paolo Bonzini's message of June 24, 2021 8:21 pm:
> On 24/06/21 12:17, Nicholas Piggin wrote:
>>> If all callers were updated that is one thing, but from the changelog
>>> it sounds like that would not happen and there would be some gfn_to_pfn
>>> users left over.
>>>
>>> But yes in the end you would either need to make gfn_to_pfn never return
>>> a page found via follow_pte, or change all callers to the new way. If
>>> the plan is for the latter then I guess that's fine.
>>
>> Actually in that case anyway I don't see the need -- the existence of
>> gfn_to_pfn is enough to know it might be buggy. It can just as easily
>> be grepped for as kvm_pfn_page_unwrap.
> 
> Sure, but that would leave us with longer function names 
> (gfn_to_pfn_page* instead of gfn_to_pfn*).  So the "safe" use is the one 
> that looks worse and the unsafe use is the one that looks safe.

The churn isn't justified because of function name length. Choose g2pp() 
if you want a non-descriptive but short name.

The existing name isn't good anyway because it not only looks up a pfn 
but also a page, and more importantly it gets a ref on the page. The
name should be changed if you introduce a new API.

>> And are gfn_to_page cases also
>> vulernable to the same issue?
> 
> No, they're just broken for the VM_IO|VM_PFNMAP case.

No they aren't vulnerable, or they are vunlerable but also broken in 
other cases?

Thanks,
Nick