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a="1760118" X-IronPort-AV: E=Sophos;i="6.27,181,1787036400"; d="scan'208";a="1760118" Received: from orviesa002.jf.intel.com ([10.64.159.142]) by fmvoesa105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 Oct 2026 15:01:36 -0700 X-CSE-ConnectionGUID: TJ2b8k3wS+WqtCeua/5yDA== X-CSE-MsgGUID: O7S8McqDQtKS8l2yZ1bmdg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.27,181,1787036400"; d="scan'208";a="4669267" Received: from jmaxwel1-mobl.amr.corp.intel.com (HELO [10.125.108.191]) ([10.125.108.191]) by smtpauth.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 Oct 2026 15:01:35 -0700 Message-ID: Date: Tue, 27 Oct 2026 15:01:35 -0700 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/entry/32: Harden return-to-user To: Andrey Zakharov , x86@kernel.org Cc: Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , hpa@zytor.com, Peter Zijlstra , Kees Cook , Brian Gerst , Pawan Gupta , linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org References: <20261027215251.15275-1-dedemorow@gmail.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 10/27/26 14:52, Andrey Zakharov wrote: > --- a/arch/x86/entry/entry_32.S > +++ b/arch/x86/entry/entry_32.S > @@ -952,6 +952,14 @@ restore_all_switch_stack: > > /* Restore user state */ > RESTORE_REGS pop=4 # skip orig_eax/error_code > + > + /* Assert that the IRET frame returns to user mode or vm86. */ > + testb $USER_SEGMENT_RPL_MASK, 4(%esp) > + jnz .Lreturn_to_user_ok > + testl $X86_EFLAGS_VM, 8(%esp) > + jnz .Lreturn_to_user_ok > + ud2 > +.Lreturn_to_user_ok: I'm curious who cares. Is anyone who is actually still running a 32-bit kernel concerned about this kind of hardening? If no one cares, then I'd rather pass on the complexity, even four instructions of it.