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Message-ID: <8dee4d22-8b32-e2b0-4f4f-8c1921fce5b3@intel.com>
Date: Mon, 18 Apr 2022 20:49:05 +0800
From: Zeng Guang <guang.zeng@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: 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" <kvm@...r.kernel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"Luck, Tony" <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>,
"Huang, Kai" <kai.huang@...el.com>,
"x86@...nel.org" <x86@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"Hu, Robert" <robert.hu@...el.com>,
"Gao, Chao" <chao.gao@...el.com>
Subject: Re: [PATCH v8 9/9] KVM: VMX: enable IPI virtualization
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
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