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X-CSE-ConnectionGUID: H1c+sUGmSISL0dyH1AvFYA== X-CSE-MsgGUID: 06OuzOMuRLC5IwkTrfUKOQ== X-IronPort-AV: E=McAfee;i="6700,10204,11179"; a="48980319" X-IronPort-AV: E=Sophos;i="6.10,186,1719903600"; d="scan'208";a="48980319" Received: from orviesa003.jf.intel.com ([10.64.159.143]) by fmvoesa101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Aug 2024 15:28:29 -0700 X-CSE-ConnectionGUID: qYw4vOKuQzuKx7GshPMq8A== X-CSE-MsgGUID: qDzVw0jNTImtVwc3rondRg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.10,186,1719903600"; d="scan'208";a="68522895" Received: from tfalcon-desk.amr.corp.intel.com (HELO [10.124.220.59]) ([10.124.220.59]) by ORVIESA003-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Aug 2024 15:28:28 -0700 Message-ID: Date: Thu, 29 Aug 2024 15:28:13 -0700 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 v5] x86/entry_32: Use stack segment selector for VERW operand To: Pawan Gupta , Borislav Petkov , Dave Hansen Cc: linux-kernel@vger.kernel.org, x86@kernel.org, Robert Gill , Jari Ruusu , Brian Gerst , "Linux regression tracking (Thorsten Leemhuis)" , antonio.gomez.iglesias@linux.intel.com, daniel.sneddon@linux.intel.com, stable@vger.kernel.org References: <20240711-fix-dosemu-vm86-v5-1-e87dcd7368aa@linux.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: 7bit On 7/11/24 15:03, Pawan Gupta wrote: > +/* > + * Safer version of CLEAR_CPU_BUFFERS that uses %ss to reference VERW operand > + * mds_verw_sel. This ensures VERW will not #GP for an arbitrary user %ds. > + */ > +.macro CLEAR_CPU_BUFFERS_SAFE > + ALTERNATIVE "", __stringify(verw %ss:_ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF > +.endm One other thing... Instead of making a "_SAFE" variant, let's just make the 32-bit version always safe. Also, is there any downside to using %ss: on 64-bit? If not, let's just update the one and only CLEAR_CPU_BUFFERS use %ss:.