linux-kernel - Re: [PATCH 0/7] KVM: TDX: TD vcpu enter/exit

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: <d27b4e076c3ad2f5d7d71135f112e6a45e067ae7.camel@intel.com>
Date: Tue, 26 Nov 2024 03:52:04 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Li, Xiaoyao"
	<xiaoyao.li@...el.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "Hunter, Adrian"
	<adrian.hunter@...el.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>,
	"Yang, Weijiang" <weijiang.yang@...el.com>, "Chatre, Reinette"
	<reinette.chatre@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "tony.lindgren@...ux.intel.com"
	<tony.lindgren@...ux.intel.com>, "nik.borisov@...e.com"
	<nik.borisov@...e.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Gao, Chao" <chao.gao@...el.com>, "Edgecombe,
 Rick P" <rick.p.edgecombe@...el.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 0/7] KVM: TDX: TD vcpu enter/exit

On Tue, 2024-11-26 at 09:44 +0800, Binbin Wu wrote:
> 
> 
> On 11/26/2024 6:51 AM, Sean Christopherson wrote:
> 
> [...]
> > When an NMI happens in non-root, the NMI is acknowledged by the CPU prior to
> > performing VM-Exit.  In regular VMX, NMIs are blocked after such VM-Exits.  With
> > TDX, that blocking happens for SEAM root, but the SEAMRET back to VMX root will
> > load interruptibility from the SEAMCALL VMCS, and I don't see any code in the
> > TDX-Module that propagates that blocking to SEAMCALL VMCS.
> I see, thanks for the explanation!
> 
> > 
> > Hmm, actually, this means that TDX has a causality inversion, which may become
> > visible with FRED's NMI source reporting.  E.g. NMI X arrives in SEAM non-root
> > and triggers a VM-Exit.  NMI X+1 becomes pending while SEAM root is active.
> > TDX-Module SEAMRETs to VMX root, NMIs are unblocked, and so NMI X+1 is delivered
> > and handled before NMI X.
> 
> This example can also cause an issue without FRED.
> 1. NMI X arrives in SEAM non-root and triggers a VM-Exit.
> 2. NMI X+1 becomes pending while SEAM root is active.
> 3. TDX-Module SEAMRETs to VMX root, NMIs are unblocked.
> 4. NMI X+1 is delivered and handled before NMI X.
>     (NMI handler could handle all NMI source events, including the source
>      triggered NMI X)
> 5. KVM calls exc_nmi() to handle the VM Exit caused by NMI X
> In step 5, because the source event caused NMI X has been handled, and NMI X
> will not be detected as a second half of back-to-back NMIs, according to
> Linux NMI handler, it will be considered as an unknown NMI.

I don't think KVM should call exc_nmi() anymore if NMI is unblocked upon
SEAMRET.

> 
> Actually, the issue could happen if NMI X+1 occurs after exiting to SEAM root
> mode and before KVM handling the VM-Exit caused by NMI X.
> 

If we can make sure NMI is still blocked upon SEAMRET then everything follows
the current VMX flow IIUC.  We should make that happen IMHO.