linux-kernel - Re: [PATCH 5/5] KVM: TDX: Add sub-ioctl KVM_TDX_TERMINATE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <b27f807e-b04f-487d-be13-74a8b0a61b42@intel.com>
Date: Fri, 1 Aug 2025 16:56:36 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>, Paolo Bonzini <pbonzini@...hat.com>
CC: <linux-arm-kernel@...ts.infradead.org>, <kvmarm@...ts.linux.dev>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Vishal Annapurve
	<vannapurve@...gle.com>, Xiaoyao Li <xiaoyao.li@...el.com>, Rick Edgecombe
	<rick.p.edgecombe@...el.com>, Nikolay Borisov <nik.borisov@...e.com>, "Zhao,
 Yan Y" <yan.y.zhao@...el.com>, "Huang, Kai" <kai.huang@...el.com>,
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH 5/5] KVM: TDX: Add sub-ioctl KVM_TDX_TERMINATE_VM

On 29/07/2025 22:33, Sean Christopherson wrote:
> +static int tdx_terminate_vm(struct kvm *kvm)
> +{
> +	if (kvm_trylock_all_vcpus(kvm))
> +		return -EBUSY;
> +
> +	kvm_vm_dead(kvm);
> +	to_kvm_tdx(kvm)->vm_terminated = true;
> +
> +	kvm_unlock_all_vcpus(kvm);
> +
> +	tdx_mmu_release_hkid(kvm);
> +
> +	return 0;
> +}

As I think I mentioned when removing vm_dead first came up,
I think we need more checks.  I spent some time going through
the code and came up with what is below:

First, we need to avoid TDX VCPU sub-IOCTLs from racing with
tdx_mmu_release_hkid().  But having any TDX sub-IOCTL run after
KVM_TDX_TERMINATE_VM raises questions of what might happen, so
it is much simpler to understand, if that is not possible.
There are 3 options:

1. Require that KVM_TDX_TERMINATE_VM is valid only if
kvm_tdx->state == TD_STATE_RUNNABLE.  Since currently all
the TDX sub-IOCTLs are for initialization, that would block
the opportunity for any to run after KVM_TDX_TERMINATE_VM.

2. Check vm_terminated in tdx_vm_ioctl() and tdx_vcpu_ioctl()

3. Test KVM_REQ_VM_DEAD in tdx_vm_ioctl() and tdx_vcpu_ioctl()

[ Note cannot check is_hkid_assigned() because that is racy ]

Secondly, I suggest we avoid SEAMCALLs that will fail and
result in KVM_BUG_ON() if HKID has been released.

There are 2 groups of those: MMU-related and TDVPS_ACCESSORS.

For the MMU-related, the following 2 functions should return
an error immediately if vm_terminated:

	tdx_sept_link_private_spt()
	tdx_sept_set_private_spte()

For that not be racy, extra synchronization is needed so that
vm_terminated can be reliably checked when holding mmu lock
i.e.

static int tdx_terminate_vm(struct kvm *kvm)
{
	if (kvm_trylock_all_vcpus(kvm))
		return -EBUSY;

	kvm_vm_dead(kvm);
+
+       write_lock(&kvm->mmu_lock);
	to_kvm_tdx(kvm)->vm_terminated = true;
+       write_unlock(&kvm->mmu_lock);

	kvm_unlock_all_vcpus(kvm);

	tdx_mmu_release_hkid(kvm);

	return 0;
}

Finally, there are 2 TDVPS_ACCESSORS that need avoiding:

	tdx_load_mmu_pgd()
		skip td_vmcs_write64() if vm_terminated

	tdx_protected_apic_has_interrupt()
		skip td_state_non_arch_read64() if vm_terminated