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X-CSE-ConnectionGUID: v8cjRG6rR3S0uIdc/GP9QQ== X-CSE-MsgGUID: 1OeCHp7sS/iL4lbt9Az8ow== X-IronPort-AV: E=McAfee;i="6800,10657,11784"; a="79654953" X-IronPort-AV: E=Sophos;i="6.23,231,1770624000"; d="scan'208";a="79654953" Received: from fmviesa006.fm.intel.com ([10.60.135.146]) by fmvoesa109.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 May 2026 15:15:04 -0700 X-CSE-ConnectionGUID: kgtpRowkTYehQcXiPGeckg== X-CSE-MsgGUID: oyq6ueM4RNyt8R9mK+xJjQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.23,231,1770624000"; d="scan'208";a="233432807" Received: from ray2.jf.intel.com (HELO [10.24.81.183]) ([10.24.81.183]) by fmviesa006-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 May 2026 15:15:03 -0700 Message-ID: <7f7b8bfd-f39e-417c-991f-d224d58cb52a@intel.com> Date: Tue, 12 May 2026 15:14:54 -0700 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/tdx: Fix zero-extension for CPUID emulation To: "Edgecombe, Rick P" , "linux-coco@lists.linux.dev" , "clopez@suse.de" , "kas@kernel.org" , "x86@kernel.org" Cc: "ak@linux.intel.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "hpa@zytor.com" , "mingo@redhat.com" , "linux-kernel@vger.kernel.org" , "Luck, Tony" , "tglx@kernel.org" , "stable@vger.kernel.org" , "kvm@vger.kernel.org" References: <20260512213719.20974-1-clopez@suse.de> <81343db56b8df8f70a2e13a17e62c620bee36897.camel@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 5/12/26 14:48, Edgecombe, Rick P wrote: >> - regs->ax = args.r12; >> - regs->bx = args.r13; >> - regs->cx = args.r14; >> - regs->dx = args.r15; >> + regs->ax = lower_32_bits(args.r12); >> + regs->bx = lower_32_bits(args.r13); >> + regs->cx = lower_32_bits(args.r14); >> + regs->dx = lower_32_bits(args.r15); >>   > Can you explain the impact here? Why should the guest fixup what the VMM > emulates? Oh boy. args.r12-15 come from the VMM, right? So the VMM Can put whatever it wants in there. CPUID (the instruction) is defined to fill in eax/ebx/ecx/edx. Those are 32-bit registers so the normal register rules apply: "32-bit operands generate a 32-bit result, zero-extended to a 64-bit result in the destination general-purpose register." So a properly-behaving CPUID implementation will always end up with the top 32 bits empty on the four CPUID registers after a CPUID is executed. The VMM here obviously might be naughty and might put gunk in args.r12/r13/r14/r15 that gets copied to ptregs->ax/bx/cx/dx which are 'unsigned long' on 64-bit. The end result is that a TDX guest can use CPUID and end up having bits set in rax/rbx/rcx/rdx that are architecturally impossible. This patch is effectively fixing up the VMM naughtiness before the guest CPUID instance can see it. Does anybody disagree with any of that? Do we *want* to fix this up silently? If we catch a malicious VMM trying to stuff garbage into the guest, shouldn't we be a bit more upset than silently papering over it?