linux-kernel - Re: [PATCH v3 24/25] KVM: TDX: Guard VM state transitions with "all" the locks

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Message-ID: <aPtOtzGLigbY0Vqw@yzhao56-desk.sh.intel.com>
Date: Fri, 24 Oct 2025 18:02:31 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>,
	Huacai Chen <chenhuacai@...nel.org>, Madhavan Srinivasan
	<maddy@...ux.ibm.com>, Anup Patel <anup@...infault.org>, Paul Walmsley
	<pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
	<aou@...s.berkeley.edu>, Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda
	<imbrenda@...ux.ibm.com>, Paolo Bonzini <pbonzini@...hat.com>, "Kirill A.
 Shutemov" <kas@...nel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<kvmarm@...ts.linux.dev>, <kvm@...r.kernel.org>, <loongarch@...ts.linux.dev>,
	<linux-mips@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
	<kvm-riscv@...ts.infradead.org>, <linux-riscv@...ts.infradead.org>,
	<x86@...nel.org>, <linux-coco@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, Ira Weiny <ira.weiny@...el.com>, Kai Huang
	<kai.huang@...el.com>, Michael Roth <michael.roth@....com>, Vishal Annapurve
	<vannapurve@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Ackerley Tng <ackerleytng@...gle.com>, Binbin Wu <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v3 24/25] KVM: TDX: Guard VM state transitions with "all"
 the locks

On Thu, Oct 16, 2025 at 05:32:42PM -0700, Sean Christopherson wrote:
> Acquire kvm->lock, kvm->slots_lock, and all vcpu->mutex locks when
> servicing ioctls that (a) transition the TD to a new state, i.e. when
> doing INIT or FINALIZE or (b) are only valid if the TD is in a specific
> state, i.e. when initializing a vCPU or memory region.  Acquiring "all"
> the locks fixes several KVM_BUG_ON() situations where a SEAMCALL can fail
> due to racing actions, e.g. if tdh_vp_create() contends with either
> tdh_mr_extend() or tdh_mr_finalize().
> 
> For all intents and purposes, the paths in question are fully serialized,
> i.e. there's no reason to try and allow anything remotely interesting to
> happen.  Smack 'em with a big hammer instead of trying to be "nice".
> 
> Acquire kvm->lock to prevent VM-wide things from happening, slots_lock to
> prevent kvm_mmu_zap_all_fast(), and _all_ vCPU mutexes to prevent vCPUs
slots_lock to prevent kvm_mmu_zap_memslot()?
kvm_mmu_zap_all_fast() does not operate on the mirror root.

We may have missed a zap in the guest_memfd punch hole path:

The SEAMCALLs tdh_mem_range_block(), tdh_mem_track() tdh_mem_page_remove()
in the guest_memfd punch hole path are only protected by the filemap invaliate
lock and mmu_lock, so they could contend with v1 version of tdh_vp_init().
(I'm writing a selftest to verify this, haven't been able to reproduce
tdh_vp_init(v1) returning BUSY yet. However, this race condition should be
theoretically possible.)

Resources              SHARED  users              EXCLUSIVE users
------------------------------------------------------------------------
(1) TDR                tdh_mng_rdwr               tdh_mng_create
                       tdh_vp_create              tdh_mng_add_cx
                       tdh_vp_addcx               tdh_mng_init
                       tdh_vp_init(v0)            tdh_mng_vpflushdone
                       tdh_vp_enter               tdh_mng_key_config
                       tdh_vp_flush               tdh_mng_key_freeid
                       tdh_vp_rd_wr               tdh_mr_extend
                       tdh_mem_sept_add           tdh_mr_finalize
                       tdh_mem_sept_remove        tdh_vp_init(v1)
                       tdh_mem_page_aug           tdh_mem_page_add
                       tdh_mem_page_remove
                       tdh_mem_range_block
                       tdh_mem_track
                       tdh_mem_range_unblock
                       tdh_phymem_page_reclaim

Do you think we can acquire the mmu_lock for cmd KVM_TDX_INIT_VCPU?

> @@ -3155,12 +3198,13 @@ int tdx_vcpu_unlocked_ioctl(struct kvm_vcpu *vcpu, void __user *argp)
>  	if (r)
>  		return r;
>  
> +	CLASS(tdx_vm_state_guard, guard)(kvm);
Should we move the guard to inside each cmd? Then there's no need to acquire the
locks in the default cases.