Re: [PATCH 1/7] KVM: x86: Free vCPUs before freeing VM state

[Paolo Bonzini <pbonzini@redhat.com>]

On 2/25/25 00:55, Sean Christopherson wrote:
> Free vCPUs before freeing any VM state, as both SVM and VMX may access
> VM state when "freeing" a vCPU that is currently "in" L2, i.e. that needs
> to be kicked out of nested guest mode.
> 
> Commit 6fcee03df6a1 ("KVM: x86: avoid loading a vCPU after .vm_destroy was
> called") partially fixed the issue, but for unknown reasons only moved the
> MMU unloading before VM destruction.  Complete the change, and free all
> vCPU state prior to destroying VM state, as nVMX accesses even more state
> than nSVM.

I applied this to kvm-coco-queue, I will place it in kvm/master too 
unless you shout.

Paolo

> In addition to the AVIC, KVM can hit a use-after-free on MSR filters:
> 
>    kvm_msr_allowed+0x4c/0xd0
>    __kvm_set_msr+0x12d/0x1e0
>    kvm_set_msr+0x19/0x40
>    load_vmcs12_host_state+0x2d8/0x6e0 [kvm_intel]
>    nested_vmx_vmexit+0x715/0xbd0 [kvm_intel]
>    nested_vmx_free_vcpu+0x33/0x50 [kvm_intel]
>    vmx_free_vcpu+0x54/0xc0 [kvm_intel]
>    kvm_arch_vcpu_destroy+0x28/0xf0
>    kvm_vcpu_destroy+0x12/0x50
>    kvm_arch_destroy_vm+0x12c/0x1c0
>    kvm_put_kvm+0x263/0x3c0
>    kvm_vm_release+0x21/0x30
> 
> and an upcoming fix to process injectable interrupts on nested VM-Exit
> will access the PIC:
> 
>    BUG: kernel NULL pointer dereference, address: 0000000000000090
>    #PF: supervisor read access in kernel mode
>    #PF: error_code(0x0000) - not-present page
>    CPU: 23 UID: 1000 PID: 2658 Comm: kvm-nx-lpage-re
>    RIP: 0010:kvm_cpu_has_extint+0x2f/0x60 [kvm]
>    Call Trace:
>     <TASK>
>     kvm_cpu_has_injectable_intr+0xe/0x60 [kvm]
>     nested_vmx_vmexit+0x2d7/0xdf0 [kvm_intel]
>     nested_vmx_free_vcpu+0x40/0x50 [kvm_intel]
>     vmx_vcpu_free+0x2d/0x80 [kvm_intel]
>     kvm_arch_vcpu_destroy+0x2d/0x130 [kvm]
>     kvm_destroy_vcpus+0x8a/0x100 [kvm]
>     kvm_arch_destroy_vm+0xa7/0x1d0 [kvm]
>     kvm_destroy_vm+0x172/0x300 [kvm]
>     kvm_vcpu_release+0x31/0x50 [kvm]
> 
> Inarguably, both nSVM and nVMX need to be fixed, but punt on those
> cleanups for the moment.  Conceptually, vCPUs should be freed before VM
> state.  Assets like the I/O APIC and PIC _must_ be allocated before vCPUs
> are created, so it stands to reason that they must be freed _after_ vCPUs
> are destroyed.
> 
> Reported-by: Aaron Lewis <aaronlewis@google.com>
> Closes: https://lore.kernel.org/all/20240703175618.2304869-2-aaronlewis@google.com
> Cc: Jim Mattson <jmattson@google.com>
> Cc: Yan Zhao <yan.y.zhao@intel.com>
> Cc: Rick P Edgecombe <rick.p.edgecombe@intel.com>
> Cc: Kai Huang <kai.huang@intel.com>
> Cc: Isaku Yamahata <isaku.yamahata@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/x86.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 58b82d6fd77c..045c61cc7e54 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12890,11 +12890,11 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
>   		mutex_unlock(&kvm->slots_lock);
>   	}
>   	kvm_unload_vcpu_mmus(kvm);
> +	kvm_destroy_vcpus(kvm);
>   	kvm_x86_call(vm_destroy)(kvm);
>   	kvm_free_msr_filter(srcu_dereference_check(kvm->arch.msr_filter, &kvm->srcu, 1));
>   	kvm_pic_destroy(kvm);
>   	kvm_ioapic_destroy(kvm);
> -	kvm_destroy_vcpus(kvm);
>   	kvfree(rcu_dereference_check(kvm->arch.apic_map, 1));
>   	kfree(srcu_dereference_check(kvm->arch.pmu_event_filter, &kvm->srcu, 1));
>   	kvm_mmu_uninit_vm(kvm);


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