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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 DDD45C43381 for ; Fri, 8 Mar 2019 19:01:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ADF8320661 for ; Fri, 8 Mar 2019 19:01:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726751AbfCHTB1 (ORCPT ); Fri, 8 Mar 2019 14:01:27 -0500 Received: from mga04.intel.com ([192.55.52.120]:29937 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726254AbfCHTB0 (ORCPT ); Fri, 8 Mar 2019 14:01:26 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga104.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 08 Mar 2019 11:01:26 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.58,456,1544515200"; d="scan'208";a="132727032" Received: from ray.jf.intel.com (HELO [10.24.9.79]) ([10.24.9.79]) by fmsmga007.fm.intel.com with ESMTP; 08 Mar 2019 11:01:25 -0800 Subject: Re: [PATCH 14/22] x86/fpu: Eager switch PKRU state To: Sebastian Andrzej Siewior Cc: linux-kernel@vger.kernel.org, x86@kernel.org, Andy Lutomirski , Paolo Bonzini , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , kvm@vger.kernel.org, "Jason A. Donenfeld" , Rik van Riel , Dave Hansen References: <20190221115020.12385-1-bigeasy@linutronix.de> <20190221115020.12385-15-bigeasy@linutronix.de> <20190308180849.hy22jejhffp4n6uh@linutronix.de> From: Dave Hansen Openpgp: preference=signencrypt Autocrypt: addr=dave.hansen@intel.com; keydata= mQINBE6HMP0BEADIMA3XYkQfF3dwHlj58Yjsc4E5y5G67cfbt8dvaUq2fx1lR0K9h1bOI6fC oAiUXvGAOxPDsB/P6UEOISPpLl5IuYsSwAeZGkdQ5g6m1xq7AlDJQZddhr/1DC/nMVa/2BoY 2UnKuZuSBu7lgOE193+7Uks3416N2hTkyKUSNkduyoZ9F5twiBhxPJwPtn/wnch6n5RsoXsb ygOEDxLEsSk/7eyFycjE+btUtAWZtx+HseyaGfqkZK0Z9bT1lsaHecmB203xShwCPT49Blxz VOab8668QpaEOdLGhtvrVYVK7x4skyT3nGWcgDCl5/Vp3TWA4K+IofwvXzX2ON/Mj7aQwf5W iC+3nWC7q0uxKwwsddJ0Nu+dpA/UORQWa1NiAftEoSpk5+nUUi0WE+5DRm0H+TXKBWMGNCFn c6+EKg5zQaa8KqymHcOrSXNPmzJuXvDQ8uj2J8XuzCZfK4uy1+YdIr0yyEMI7mdh4KX50LO1 pmowEqDh7dLShTOif/7UtQYrzYq9cPnjU2ZW4qd5Qz2joSGTG9eCXLz5PRe5SqHxv6ljk8mb ApNuY7bOXO/A7T2j5RwXIlcmssqIjBcxsRRoIbpCwWWGjkYjzYCjgsNFL6rt4OL11OUF37wL QcTl7fbCGv53KfKPdYD5hcbguLKi/aCccJK18ZwNjFhqr4MliQARAQABtEVEYXZpZCBDaHJp c3RvcGhlciBIYW5zZW4gKEludGVsIFdvcmsgQWRkcmVzcykgPGRhdmUuaGFuc2VuQGludGVs LmNvbT6JAjgEEwECACIFAlQ+9J0CGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEGg1 lTBwyZKwLZUP/0dnbhDc229u2u6WtK1s1cSd9WsflGXGagkR6liJ4um3XCfYWDHvIdkHYC1t MNcVHFBwmQkawxsYvgO8kXT3SaFZe4ISfB4K4CL2qp4JO+nJdlFUbZI7cz/Td9z8nHjMcWYF IQuTsWOLs/LBMTs+ANumibtw6UkiGVD3dfHJAOPNApjVr+M0P/lVmTeP8w0uVcd2syiaU5jB aht9CYATn+ytFGWZnBEEQFnqcibIaOrmoBLu2b3fKJEd8Jp7NHDSIdrvrMjYynmc6sZKUqH2 I1qOevaa8jUg7wlLJAWGfIqnu85kkqrVOkbNbk4TPub7VOqA6qG5GCNEIv6ZY7HLYd/vAkVY E8Plzq/NwLAuOWxvGrOl7OPuwVeR4hBDfcrNb990MFPpjGgACzAZyjdmYoMu8j3/MAEW4P0z F5+EYJAOZ+z212y1pchNNauehORXgjrNKsZwxwKpPY9qb84E3O9KYpwfATsqOoQ6tTgr+1BR CCwP712H+E9U5HJ0iibN/CDZFVPL1bRerHziuwuQuvE0qWg0+0SChFe9oq0KAwEkVs6ZDMB2 P16MieEEQ6StQRlvy2YBv80L1TMl3T90Bo1UUn6ARXEpcbFE0/aORH/jEXcRteb+vuik5UGY 5TsyLYdPur3TXm7XDBdmmyQVJjnJKYK9AQxj95KlXLVO38lcuQINBFRjzmoBEACyAxbvUEhd GDGNg0JhDdezyTdN8C9BFsdxyTLnSH31NRiyp1QtuxvcqGZjb2trDVuCbIzRrgMZLVgo3upr MIOx1CXEgmn23Zhh0EpdVHM8IKx9Z7V0r+rrpRWFE8/wQZngKYVi49PGoZj50ZEifEJ5qn/H Nsp2+Y+bTUjDdgWMATg9DiFMyv8fvoqgNsNyrrZTnSgoLzdxr89FGHZCoSoAK8gfgFHuO54B lI8QOfPDG9WDPJ66HCodjTlBEr/Cwq6GruxS5i2Y33YVqxvFvDa1tUtl+iJ2SWKS9kCai2DR 3BwVONJEYSDQaven/EHMlY1q8Vln3lGPsS11vSUK3QcNJjmrgYxH5KsVsf6PNRj9mp8Z1kIG qjRx08+nnyStWC0gZH6NrYyS9rpqH3j+hA2WcI7De51L4Rv9pFwzp161mvtc6eC/GxaiUGuH BNAVP0PY0fqvIC68p3rLIAW3f97uv4ce2RSQ7LbsPsimOeCo/5vgS6YQsj83E+AipPr09Caj 0hloj+hFoqiticNpmsxdWKoOsV0PftcQvBCCYuhKbZV9s5hjt9qn8CE86A5g5KqDf83Fxqm/ vXKgHNFHE5zgXGZnrmaf6resQzbvJHO0Fb0CcIohzrpPaL3YepcLDoCCgElGMGQjdCcSQ+Ci FCRl0Bvyj1YZUql+ZkptgGjikQARAQABiQIfBBgBAgAJBQJUY85qAhsMAAoJEGg1lTBwyZKw l4IQAIKHs/9po4spZDFyfDjunimEhVHqlUt7ggR1Hsl/tkvTSze8pI1P6dGp2XW6AnH1iayn yRcoyT0ZJ+Zmm4xAH1zqKjWplzqdb/dO28qk0bPso8+1oPO8oDhLm1+tY+cOvufXkBTm+whm +AyNTjaCRt6aSMnA/QHVGSJ8grrTJCoACVNhnXg/R0g90g8iV8Q+IBZyDkG0tBThaDdw1B2l asInUTeb9EiVfL/Zjdg5VWiF9LL7iS+9hTeVdR09vThQ/DhVbCNxVk+DtyBHsjOKifrVsYep WpRGBIAu3bK8eXtyvrw1igWTNs2wazJ71+0z2jMzbclKAyRHKU9JdN6Hkkgr2nPb561yjcB8 sIq1pFXKyO+nKy6SZYxOvHxCcjk2fkw6UmPU6/j/nQlj2lfOAgNVKuDLothIxzi8pndB8Jju KktE5HJqUUMXePkAYIxEQ0mMc8Po7tuXdejgPMwgP7x65xtfEqI0RuzbUioFltsp1jUaRwQZ MTsCeQDdjpgHsj+P2ZDeEKCbma4m6Ez/YWs4+zDm1X8uZDkZcfQlD9NldbKDJEXLIjYWo1PH hYepSffIWPyvBMBTW2W5FRjJ4vLRrJSUoEfJuPQ3vW9Y73foyo/qFoURHO48AinGPZ7PC7TF vUaNOTjKedrqHkaOcqB185ahG2had0xnFsDPlx5y Message-ID: Date: Fri, 8 Mar 2019 11:01:25 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <20190308180849.hy22jejhffp4n6uh@linutronix.de> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/8/19 10:08 AM, Sebastian Andrzej Siewior wrote: > On 2019-02-25 10:16:24 [-0800], Dave Hansen wrote: >>> + if (!cpu_feature_enabled(X86_FEATURE_OSPKE)) >>> + return; >>> + >>> + if (current->mm) { >>> + pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU); >>> + WARN_ON_ONCE(!pk); >> >> This can trip on us of the 'init optimization' is in play because >> get_xsave_addr() checks xsave->header.xfeatures. That's unlikely today >> because we usually set PKRU to a restrictive value. But, it's also not >> *guaranteed*. >> >> Userspace could easily do an XRSTOR that puts PKRU back in its init >> state if it wanted to, then this would end up with pk==NULL. >> >> We might actually want a selftest that *does* that. I don't think we >> have one. > > So you are saying that the above warning might trigger and be "okay"? Nothing will break, but the warning will trigger, which isn't nice. > My understanding is that the in-kernel XSAVE will always save everything > so we should never "lose" the XFEATURE_PKRU no matter what user space > does. > > So as test case you want > xsave (-1 & ~XFEATURE_PKRU) > xrestore (-1 & ~XFEATURE_PKRU) > > in userland and then a context switch to see if the warning above > triggers? I think you need an XRSTOR with RFBM=-1 (or at least with the PKRU bit set) and the PKRU bit in the XFEATURES field in the XSAVE buffer set to 0. >>> + if (pk) >>> + pkru_val = pk->pkru; >>> + }> + __write_pkru(pkru_val); >>> } >> >> A comment above __write_pkru() would be nice to say that it only >> actually does the slow instruction on changes to the value. > > Could we please not do this? It is a comment above one of the callers > function and we have two or three. And we have that comment already > within __write_pkru(). I looked at this code and thought "writing PKRU is slow", and "this writes PKRU unconditionally", and "the __ version of the function shoudn't have much logic in it". I got 2/3 wrong. To me that means this site needs a 1-line comment. Feel free to move one of the other comments to here if you think it's over-commented, but this site needs one. >> BTW, this has the implicit behavior of always trying to do a >> __write_pkru(0) on switches to kernel threads. That seems a bit weird >> and it is likely to impose WRPKRU overhead on switches between user and >> kernel threads. >> >> The 0 value is also the most permissive, which is not great considering >> that user mm's can be active the in page tables when running kernel >> threads if we're being lazy. >> >> Seems like we should either leave PKRU alone or have 'init_pkru_value' >> be the default. That gives good security properties and is likely to >> match the application value, removing the WRPKRU overhead. > > Last time we talked about this we agreed (or this was my impression) that > 0 should be written so that the kernel thread should always be able to > write to user space in case it borrowed its mm (otherwise it has none > and it would fail anyway). We can't write to userspace when borrowing an mm. If the kernel borrows an mm, we might as well be on the init_mm which has no userspace mappings. > We didn't want to leave PKRU alone because the outcome (whether or not > the write by the kernel thread succeeds) should not depend on the last > running task (and be random) but deterministic. Right, so let's make it deterministically restrictive: either init_pkru_value, or -1 since kernel threads shouldn't be touching userspace in the first place.