From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5C90CC47082 for ; Tue, 8 Jun 2021 14:47:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 382AC61183 for ; Tue, 8 Jun 2021 14:47:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233489AbhFHOtq (ORCPT ); Tue, 8 Jun 2021 10:49:46 -0400 Received: from mga05.intel.com ([192.55.52.43]:34602 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233288AbhFHOtq (ORCPT ); Tue, 8 Jun 2021 10:49:46 -0400 IronPort-SDR: zW40cu/NiVtF04V0Og58Ua+2OfLanUHV+cUeEaTa8x0uFU+ckpgFJVkGqZhm0xpGlt/kHAKObD pxbZPh3Nfk2g== X-IronPort-AV: E=McAfee;i="6200,9189,10009"; a="290487698" X-IronPort-AV: E=Sophos;i="5.83,258,1616482800"; d="scan'208";a="290487698" Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by fmsmga105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 Jun 2021 07:47:53 -0700 IronPort-SDR: c5iW+XRYk6tdKb/CLbJVQhiL0xz4jYzmy9nkSVuqkZNkJ9WVC5ZQ2q7TDGBAM6Zb/RZydDCbFg CVFCuOFHPG+A== X-IronPort-AV: E=Sophos;i="5.83,258,1616482800"; d="scan'208";a="637655818" Received: from ciball-mobl1.amr.corp.intel.com (HELO [10.252.140.48]) ([10.252.140.48]) by fmsmga005-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 Jun 2021 07:47:52 -0700 Subject: Re: [patch V2 00/14] x86/fpu: Mop up XSAVES and related damage To: Thomas Gleixner , LKML Cc: x86@kernel.org, Andy Lutomirski , Dave Hansen , Fenghua Yu , Tony Luck , Yu-cheng Yu , Sebastian Andrzej Siewior References: <20210605234742.712464974@linutronix.de> <87h7i9zv3r.ffs@nanos.tec.linutronix.de> <87eeddzs0l.ffs@nanos.tec.linutronix.de> <37df631f-9d3d-3035-6eeb-85ef33e580d5@intel.com> <87bl8hz3sl.ffs@nanos.tec.linutronix.de> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <87bl8hz3sl.ffs@nanos.tec.linutronix.de> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/7/21 3:51 PM, Thomas Gleixner wrote: ... > But it creates a few new problems: > > 1) Where to put the PKRU value in the sigframe? > > For 64bit sigframes that's easy as there is padding space, for > 32bit sigframes that's a problem because there is no space. > > 2) Backward compatibility > > As much as we wish to have a time machine there is a rule not to > break existing user space. > > Now fortunately there is a way out: > > 1) User space cannot rely on PKRU being XSTATE managed unless PKRU is > enabled in XCR0. XCR0 enablement is part of the UABI so any > complaint about missing XCR0 support is futile So... One more gem from the manpages: > It is recommended that > applications wanting to use protection keys should simply call > pkey_alloc(2) and test whether the call succeeds, instead of > attempting to detect support for the feature in any other way. I kinda wrote that thinking that folks could avoid doing the CPUID/XGETBV dance and just use the syscall instead. *If* they do what is suggested, they'll never notice the lack of PKRU in XCR0. The pkey selftest, for instance, blindly assumes that pkeys is enabled in XCR0. It would probably end up scribbling somewhere on the stack. Now the same person who wrote that also wrote the manpages, so those are not exactly two separate data points. ... > So the proposed solution is to: > > A) Exclude PKRU from XSTATE managed state, i.e. do not set the PKRU > bit in XCR0 > > B) Exclude 32bit applications on 64bit kernels from using PKEYS by > returning an error code from pkey_alloc(). That's fine because the > man page requires them to handle the fail which they need to do > anyway because 32bit kernel do not support PKEYS and never will. > > C) Replace the current context switch mechanism which is partially > XSAVE based by a software managed one. > > D) Store the PKRU value in one of the reserved slots of the 64bit > signal frame which is possible because of #B so that a signal > handler has the chance to override the interrupted task's PKRU > setting. > > Thoughts? The thing that makes me most nervous is changing the signal stack ABI for PKRU. Careful apps (not the selftest) will probably have more careful enumeration and might bug out due to the missing XCR0 bit. Or, they might at least check xfeatures (aka. XSTATE_BV) in the signal stack XSAVE buffer. On the bright side, rudely masking PKRU out of XCR0: xcr0 &= ~XFEATURE_MASK_PKRU; still results in a kernel that boots.