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X-CSE-ConnectionGUID: EDSJwTN6SjiGOT+OwsQO5Q== X-CSE-MsgGUID: Yi/8wDDpQO6jIcZX03BgOQ== X-IronPort-AV: E=McAfee;i="6800,10657,11936"; a="900067" X-IronPort-AV: E=Sophos;i="6.27,160,1787036400"; d="scan'208";a="900067" Received: from fmviesa002.fm.intel.com ([10.60.135.142]) by fmvoesa111.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Oct 2026 19:12:31 -0700 X-CSE-ConnectionGUID: H49bFA3cTFKQGL0uSmGcmA== X-CSE-MsgGUID: jxlX/bbSSTuv0lpVX7vmsQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.27,161,1787036400"; d="scan'208";a="1927201" Received: from binbinwu-mobl.ccr.corp.intel.com (HELO [10.124.245.162]) ([10.124.245.162]) by smtpauth.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Oct 2026 19:12:25 -0700 Message-ID: <86b530c0-65ea-4b9c-a020-124fa5477275@linux.intel.com> Date: Fri, 16 Oct 2026 10:12:22 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] KVM: TDX: Set SIGNIFCANT_INDEX flag for supported CPUIDs To: Sean Christopherson Cc: Changyuan Lyu , rick.p.edgecombe@intel.com, binbin.wu@intel.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, isaku.yamahata@intel.com, kas@kernel.org, kvm@vger.kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org, mingo@redhat.com, pbonzini@redhat.com, tglx@kernel.org, x86@kernel.org, xiaoyao.li@intel.com References: <213d614fe73e183a230c8f4e0c8fa1cc3d45df39.camel@intel.com> <20260224212939.2081828-1-changyuanl@google.com> Content-Language: en-US From: Binbin Wu In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/16/2026 9:47 AM, Sean Christopherson wrote: > On Tue, Oct 13, 2026, Binbin Wu wrote: >> On 2/25/2026 5:29 AM, Changyuan Lyu wrote: >> >> [...] >> >>> ----------------------- >>> >>> >From 18b967b718911c09872c3717d8ab083fa59c4a70 Mon Sep 17 00:00:00 2001 >>> From: Changyuan Lyu >>> Date: Fri, 20 Feb 2026 09:55:28 -0800 >>> Subject: [PATCH] KVM: TDX: Set SIGNIFCANT_INDEX flag for supported CPUIDs >>> >>> Set the KVM_CPUID_FLAG_SIGNIFCANT_INDEX flag in the kvm_cpuid_entry2 >>> structures returned by KVM_TDX_CAPABILITIES if the CPUID is indexed. >>> This ensures consistency with the CPUID entries returned by >>> KVM_GET_SUPPORTED_CPUID. >>> >>> Suggested-by: Sean Christopherson >>> Signed-off-by: Changyuan Lyu >> >> >> Hi Sean, >> >> When I was preparing the v5 of the TDX CPUID patch series, the internal sashiko >> reported a issue: >> > @@ -2359,6 +2359,15 @@ static int setup_tdparams_cpuids(struct kvm_cpuid2 *cpuid, >> > if (!entry) >> > continue; >> > >> > + /* >> > + * Reject entries whose index doesn't match the expected one. >> > + * This catches userspace passing a CPUID entry with the >> > + * KVM_CPUID_FLAG_SIGNIFCANT_INDEX flag cleared when the index >> > + * is significant. >> > + */ >> > + if (entry->index != tmp.index) >> > + return -EINVAL; >> >> [Severity: Medium] >> Does this unconditional check inadvertently break userspace VMMs that provide >> valid CPUID entries but leave the index field uninitialized for >> non-significant leaves? >> >> Since kvm_find_cpuid_entry2() ignores the index field when >> KVM_CPUID_FLAG_SIGNIFCANT_INDEX is cleared, VMMs are not architecturally >> required to zero the index for non-significant leaves. This check forces >> the index to be exactly 0 (since tmp.index is normalized to 0) even for >> leaves that do not have architecturally significant subleaves. >> >> >> I think the issue is valid, > > Yeah, requiring the index to be a specific value without SIGNIFCANT_INDEX set > could break usrspace. > >> so it needs to know whether the entry is a subleaf or not. I think it's more > > By "it", I assume you mean userspace? I meant the code to check the index mismatch in my previous implementation when preparing the v5 of TDX CPUID validation patch series. But I changed the implementation, which doesn't rely on the flag. I.e. KVM validates the unsupported features against the same subleaf that the values are copied into. > >> reliable to do it via the information reported by the TDX module. i.e. my >> patch set will rely on this patch to set KVM_CPUID_FLAG_SIGNIFCANT_INDEX for >> entries that are indexed. >> >> Will you apply this patch? >> Or I can include this patch in my patch set. Also, 0x23 is still missing in >> cpuid_function_is_indexed(), I can add a patch to add 0x23 as well. >> >>> --- >>> arch/x86/kvm/vmx/tdx.c | 4 ++++ >>> 1 file changed, 4 insertions(+) >>> >>> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c >>> index 2d7a4d52ccfb4..1c039eab2f3d8 100644 >>> --- a/arch/x86/kvm/vmx/tdx.c >>> +++ b/arch/x86/kvm/vmx/tdx.c >>> @@ -167,6 +167,7 @@ static void td_init_cpuid_entry2(struct kvm_cpuid_entry2 *entry, unsigned char i >>> >>> entry->function = (u32)td_conf->cpuid_config_leaves[idx]; >>> entry->index = td_conf->cpuid_config_leaves[idx] >> 32; >>> + entry->flags = 0; >>> entry->eax = (u32)td_conf->cpuid_config_values[idx][0]; >>> entry->ebx = td_conf->cpuid_config_values[idx][0] >> 32; >>> entry->ecx = (u32)td_conf->cpuid_config_values[idx][1]; >>> @@ -174,6 +175,9 @@ static void td_init_cpuid_entry2(struct kvm_cpuid_entry2 *entry, unsigned char i >>> >>> if (entry->index == KVM_TDX_CPUID_NO_SUBLEAF) >>> entry->index = 0; >>> + else >>> + entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX; > > Hrm, I'm getting deja vu. Oh, this is an old thread. *sigh* I was expecting > a new patch/thread, but it got "posted" inline. > > I'll send a small v2. Thanks.