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X-CSE-ConnectionGUID: WvVIW+waQ0+iT5mYMg2Taw== X-CSE-MsgGUID: OfoGsmPORoOEdRiEHCNgsg== X-IronPort-AV: E=McAfee;i="6700,10204,11324"; a="38089833" X-IronPort-AV: E=Sophos;i="6.13,229,1732608000"; d="scan'208";a="38089833" Received: from orviesa004.jf.intel.com ([10.64.159.144]) by orvoesa113.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Jan 2025 15:06:26 -0800 X-CSE-ConnectionGUID: 4pKjc1dHRyG6K33oMP+cOA== X-CSE-MsgGUID: uyj/U51pQPui0JjEvhTdRg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.13,229,1732608000"; d="scan'208";a="112628081" Received: from lbogdanm-mobl3.ger.corp.intel.com (HELO [10.124.223.89]) ([10.124.223.89]) by orviesa004-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Jan 2025 15:06:25 -0800 Message-ID: <6c1785c8-e74e-4912-95bb-88b6e94f544f@intel.com> Date: Thu, 23 Jan 2025 15:06:26 -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: [RFC][PATCH 0/8] x86/mm: Simplify PAE page table handling To: Peter Zijlstra , Dave Hansen Cc: linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de, bp@alien8.de, joro@8bytes.org, luto@kernel.org, kirill.shutemov@linux.intel.com, rick.p.edgecombe@intel.com, jgross@suse.com References: <20250123172428.D6D8C8D9@davehans-spike.ostc.intel.com> <20250123214911.GB969@noisy.programming.kicks-ass.net> Content-Language: en-US From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 1/23/25 13:49, Peter Zijlstra wrote: > On Thu, Jan 23, 2025 at 09:24:28AM -0800, Dave Hansen wrote: >> tl;dr: 32-bit PAE page table handing is a bit different when PTI >> is on and off. Making the handling uniform removes a good amount >> of code at the cost of not sharing kernel PMDs. The downside of >> this simplification is bloating non-PTI PAE kernels by ~2 pages >> per process. >> >> Anyone who cares about security on 32-bit is running with PTI and >> PAE because PAE has the No-eXecute page table bit. They are already >> paying the 2-page penalty. Anyone who cares more about memory >> footprint than security is probably already running a !PAE kernel >> and will not be affected by this. > > The reality is that many of the mitigations we have are 64bit only. > 32bit is known insecure. There is absolutely no point in using PTI on > 32bit at all. > > Can't we just rip it out? 32-bit+PTI or 32-bit in general? ;) I'm curious what Joerg and the other folks that worked on 32-bit PTI think about it in retrospect. The 32 vs. 64-bit security gap was probably modest in 2018 and it can only have grown since then. I definitely haven't seen a lot of 32-bit PTI bug reports.