linux-kernel - Re: [PATCH v8 9/9] KVM: VMX: enable IPI virtualization

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Date:   Mon, 18 Apr 2022 17:25:05 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Zeng Guang <guang.zeng@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kim Phillips <kim.phillips@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jethro Beekman <jethro@...tanix.com>,
        Kai Huang <kai.huang@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Robert Hu <robert.hu@...el.com>
Subject: Re: [PATCH v8 9/9] KVM: VMX: enable IPI virtualization

On Fri, Apr 15, 2022 at 03:25:06PM +0000, Sean Christopherson wrote:
>On Mon, Apr 11, 2022, Zeng Guang wrote:
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index d1a39285deab..23fbf52f7bea 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -11180,11 +11180,15 @@ static int sync_regs(struct kvm_vcpu *vcpu)
>>  
>>  int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
>>  {
>> +	int ret = 0;
>> +
>>  	if (kvm_check_tsc_unstable() && atomic_read(&kvm->online_vcpus) != 0)
>>  		pr_warn_once("kvm: SMP vm created on host with unstable TSC; "
>>  			     "guest TSC will not be reliable\n");
>>  
>> -	return 0;
>> +	if (kvm_x86_ops.alloc_ipiv_pid_table)
>> +		ret = static_call(kvm_x86_alloc_ipiv_pid_table)(kvm);
>
>Add a generic kvm_x86_ops.vcpu_precreate, no reason to make this so specific.
>And use KVM_X86_OP_RET0 instead of KVM_X86_OP_OPTIONAL, then this can simply be
>
>	return static_call(kvm_x86_vcpu_precreate);
>
>That said, there's a flaw in my genius plan.
>
>  1. KVM_CREATE_VM
>  2. KVM_CAP_MAX_VCPU_ID, set max_vcpu_ids=1
>  3. KVM_CREATE_VCPU, create IPIv table but ultimately fails
>  4. KVM decrements created_vcpus back to '0'
>  5. KVM_CAP_MAX_VCPU_ID, set max_vcpu_ids=4096
>  6. KVM_CREATE_VCPU w/ ID out of range
>
>In other words, malicious userspace could trigger buffer overflow.

can we simply return an error (e.g., -EEXIST) on step 5 (i.e.,
max_vcpu_ids cannot be changed after being set once)?

or

can we detect the change of max_vcpu_ids in step 6 and re-allocate PID
table?

>
>That could be solved by adding an arch hook to undo precreate, but that's gross
>and a good indication that we're trying to solve this the wrong way.
>
>I think it's high time we add KVM_FINALIZE_VM, though that's probably a bad name
>since e.g. TDX wants to use that name for VM really, really, being finalized[*],
>i.e. after all vCPUs have been created.
>
>KVM_POST_CREATE_VM?  That's not very good either.
>
>Paolo or anyone else, thoughts?
>
>[*] https://lore.kernel.org/all/83768bf0f786d24f49d9b698a45ba65441ef5ef0.1646422845.git.isaku.yamahata@intel.com