linux-kernel - Re: [PATCH 7/8] KVM: gmem: Avoid race with kvm_gmem

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Date:   Fri, 18 Aug 2023 11:15:28 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     isaku.yamahata@...el.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        isaku.yamahata@...il.com, Michael Roth <michael.roth@....com>,
        Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Kai Huang <kai.huang@...el.com>,
        Zhi Wang <zhi.wang.linux@...il.com>, chen.bo@...el.com,
        linux-coco@...ts.linux.dev,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yuan Yao <yuan.yao@...ux.intel.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Xu Yilun <yilun.xu@...el.com>,
        Quentin Perret <qperret@...gle.com>, wei.w.wang@...el.com,
        Fuad Tabba <tabba@...gle.com>
Subject: Re: [PATCH 7/8] KVM: gmem: Avoid race with kvm_gmem_release and mmu notifier

On Tue, Aug 15, 2023, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> Add slots_lock around kvm_flush_shadow_all().  kvm_gmem_release() via
> fput() and kvm_mmu_notifier_release() via mmput() can be called
> simultaneously on process exit because vhost, /dev/vhost_{net, vsock}, can
> delay the call to release mmu_notifier, kvm_mmu_notifier_release() by its
> kernel thread.  Vhost uses get_task_mm() and mmput() for the kernel thread
> to access process memory.  mmput() can defer after closing the file.
> 
> kvm_flush_shadow_all() and kvm_gmem_release() can be called simultaneously.

KVM shouldn't reclaim memory on file release, it should instead do that on the
inode being "evicted": https://lore.kernel.org/all/ZLGiEfJZTyl7M8mS@google.com

> With TDX KVM, HKID releasing by kvm_flush_shadow_all() and private memory
> releasing by kvm_gmem_release() can race.  Add slots_lock to
> kvm_mmu_notifier_release().

No, the right answer is to not release the HKID until the VM is destroyed.  gmem
has a reference to its associated kvm instance, and so that will naturally ensure
memory all memory encrypted with the HKID is freed before the HKID is released.
kvm_flush_shadow_all() should only tear down page tables, it shouldn't be freeing
guest_memfd memory.

Then patches 6-8 go away.