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Tue, 25 Feb 2025 06:35:41 -0800 (PST) Date: Tue, 25 Feb 2025 06:35:40 -0800 In-Reply-To: X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Mime-Version: 1.0 References: <20250224235542.2562848-1-seanjc@google.com> <20250224235542.2562848-4-seanjc@google.com> Message-ID: Subject: Re: [PATCH 3/7] KVM: Assert that a destroyed/freed vCPU is no longer visible From: Sean Christopherson To: Yan Zhao Cc: Marc Zyngier , Oliver Upton , Tianrui Zhao , Bibo Mao , Huacai Chen , Madhavan Srinivasan , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Paolo Bonzini , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, Aaron Lewis , Jim Mattson , Rick P Edgecombe , Kai Huang , Isaku Yamahata Content-Type: text/plain; charset="us-ascii" On Tue, Feb 25, 2025, Yan Zhao wrote: > On Mon, Feb 24, 2025 at 03:55:38PM -0800, Sean Christopherson wrote: > > After freeing a vCPU, assert that it is no longer reachable, and that > > kvm_get_vcpu() doesn't return garbage or a pointer to some other vCPU. > > While KVM obviously shouldn't be attempting to access a freed vCPU, it's > > all too easy for KVM to make a VM-wide request, e.g. via KVM_BUG_ON() or > > kvm_flush_remote_tlbs(). > > > > Alternatively, KVM could short-circuit problematic paths if the VM's > > refcount has gone to zero, e.g. in kvm_make_all_cpus_request(), or KVM > > could try disallow making global requests during teardown. But given that > > deleting the vCPU from the array Just Works, adding logic to the requests > > path is unnecessary, and trying to make requests illegal during teardown > > would be a fool's errand. > > > > Signed-off-by: Sean Christopherson > > --- > > virt/kvm/kvm_main.c | 8 ++++++++ > > 1 file changed, 8 insertions(+) > > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > index 201c14ff476f..991e8111e88b 100644 > > --- a/virt/kvm/kvm_main.c > > +++ b/virt/kvm/kvm_main.c > > @@ -489,6 +489,14 @@ void kvm_destroy_vcpus(struct kvm *kvm) > > kvm_for_each_vcpu(i, vcpu, kvm) { > > kvm_vcpu_destroy(vcpu); > > xa_erase(&kvm->vcpu_array, i); > > + > > + /* > > + * Assert that the vCPU isn't visible in any way, to ensure KVM > > + * doesn't trigger a use-after-free if destroying vCPUs results > > + * in VM-wide request, e.g. to flush remote TLBs when tearing > > + * down MMUs, or to mark the VM dead if a KVM_BUG_ON() fires. > > + */ > > + WARN_ON_ONCE(xa_load(&kvm->vcpu_array, i) || kvm_get_vcpu(kvm, i)); > As xa_erase() says "After this function returns, loading from @index will return > %NULL", is this checking of xa_load() necessary? None of this is "necessary". My goal with the assert is to (a) document that KVM relies the vCPU to be NULL/unreachable and (b) to help ensure that doesn't change in the future. Checking xa_load() is mostly about (a). That said, I agree checking xa_load() is more than a bit gratuitous. I have no objection to checking only kvm_get_vcpu().