linux-kernel - Re: [PATCH 05/26] vfio: KVM: Pass get/put helpers from KVM to VFIO, don't do circular lookup

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Message-ID: <20231203140756.GI1489931@ziepe.ca>
Date:   Sun, 3 Dec 2023 10:07:56 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Jason Herne <jjherne@...ux.ibm.com>,
        Harald Freudenberger <freude@...ux.ibm.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        Anish Ghulati <aghulati@...gle.com>,
        Venkatesh Srinivas <venkateshs@...omium.org>,
        Andrew Thornton <andrewth@...gle.com>
Subject: Re: [PATCH 05/26] vfio: KVM: Pass get/put helpers from KVM to VFIO,
 don't do circular lookup

On Fri, Dec 01, 2023 at 04:51:55PM -0800, Sean Christopherson wrote:

> There's one more wrinkle: this patch is buggy in that it doesn't ensure the liveliness
> of KVM-the-module, i.e. nothing prevents userspace from unloading kvm.ko while VFIO
> still holds a reference to a kvm structure, and so invoking ->put_kvm() could jump
> into freed code.  To fix that, KVM would also need to pass along a module pointer :-(

Maybe we should be refcounting the struct file not the struct kvm?

Then we don't need special helpers and it keeps the module alive correctly.

Jason