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Message-ID: <8dee4d22-8b32-e2b0-4f4f-8c1921fce5b3@intel.com>
Date:   Mon, 18 Apr 2022 20:49:05 +0800
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Subject: Re: [PATCH v8 9/9] KVM: VMX: enable IPI virtualization
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To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Luck, Tony" <tony.luck@intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Kim Phillips <kim.phillips@amd.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Jethro Beekman <jethro@fortanix.com>,
        "Huang, Kai" <kai.huang@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Hu, Robert" <robert.hu@intel.com>,
        "Gao, Chao" <chao.gao@intel.com>
References: <20220411090447.5928-1-guang.zeng@intel.com>
 <20220411090447.5928-10-guang.zeng@intel.com> <YlmOUtXgIdQcUTO1@google.com>
From:   Zeng Guang <guang.zeng@intel.com>
In-Reply-To: <YlmOUtXgIdQcUTO1@google.com>
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Precedence: bulk


On 4/15/2022 11:25 PM, 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.


This is the tricky exploit that make max_vcpu_ids update more times. I 
think we
can avoid this issue by checking pid table availability during the 
pre-creation
of the first vCPU. If it's already allocated, free it and re-allocate to 
accommodate
table size to new max_vcpu_ids if updated.

> 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