linux-kernel - Re: [PATCH V4 0/1] KVM: TDX: Decrease TDX VM shutdown time

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: <3ef581f1-1ff1-4b99-b216-b316f6415318@intel.com>
Date: Mon, 14 Jul 2025 11:20:05 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "seanjc@...gle.com" <seanjc@...gle.com>
Cc: "Gao, Chao" <chao.gao@...el.com>, "Huang, Kai" <kai.huang@...el.com>,
 "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "Hunter, Adrian" <adrian.hunter@...el.com>,
 "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>,
 "Chatre, Reinette" <reinette.chatre@...el.com>,
 "Yamahata, Isaku" <isaku.yamahata@...el.com>,
 "Zhao, Yan Y" <yan.y.zhao@...el.com>,
 "tony.lindgren@...ux.intel.com" <tony.lindgren@...ux.intel.com>
Subject: Re: [PATCH V4 0/1] KVM: TDX: Decrease TDX VM shutdown time

On 7/12/2025 7:17 AM, Edgecombe, Rick P wrote:
> On Fri, 2025-07-11 at 16:05 -0700, Sean Christopherson wrote:
>>> Zero the reserved area in struct kvm_tdx_capabilities so that fields added
>>> in
>>> the reserved area won't disturb any userspace that previously had garbage
>>> there.
>>
>> It's not only about disturbing userspace, it's also about actually being able
>> to repurpose the reserved fields in the future without needing *another* flag
>> to tell userspace that it's ok to read the previously-reserved fields.  I care
>> about this much more than I care about userspace using reserved fields as
>> scratch space.
> 
> If, before calling KVM_TDX_CAPABILITIES, userspace zeros the new field that it
> knows about, but isn't sure if the kernel does, it's the same no?
> 
> Did you see that the way KVM_TDX_CAPABILITIES is implemented today is a little
> weird? It actually copies the whole struct kvm_tdx_capabilities from userspace
> and then sets some fields (not reserved) and then copies it back. So userspace
> can zero any fields it wants to know about before calling KVM_TDX_CAPABILITIES.
> Then it could know the same things as if the kernel zeroed it.
> 
> I was actually wondering if we want to change the kernel to zero reserved, if it
> might make more sense to just copy caps->cpuid.nent field from userspace, and
> then populate the whole thing starting from a zero'd buffer in the kernel.

+1 to zero the whole buffer of *caps in the kernel.

current code seems to have issue on the 
caps->kernel_tdvmcallinfo_1_r11/kernel_tdvmcallinfo_1_r12/user_tdvmcallinfo_1_r12, 
as KVM cannot guarantee zero'ed value are returned to userspace.