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=-4.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 098A1C433F8 for ; Tue, 28 Jul 2020 15:14:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DA69D2070B for ; Tue, 28 Jul 2020 15:14:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730859AbgG1POK convert rfc822-to-8bit (ORCPT ); Tue, 28 Jul 2020 11:14:10 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:34985 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730721AbgG1PNT (ORCPT ); Tue, 28 Jul 2020 11:13:19 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-235-JJgRaiMhNeeB2BdH3JMmbA-1; Tue, 28 Jul 2020 16:13:13 +0100 X-MC-Unique: JJgRaiMhNeeB2BdH3JMmbA-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Tue, 28 Jul 2020 16:13:12 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Tue, 28 Jul 2020 16:13:12 +0100 From: David Laight To: "'madvenka@linux.microsoft.com'" , "kernel-hardening@lists.openwall.com" , "linux-api@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "linux-fsdevel@vger.kernel.org" , "linux-integrity@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-security-module@vger.kernel.org" , "oleg@redhat.com" , "x86@kernel.org" Subject: RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor Thread-Topic: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor Thread-Index: AQHWZOCQT+e4gDrzGEmP/30MMvDTCqkdFOrw Date: Tue, 28 Jul 2020 15:13:12 +0000 Message-ID: References: <20200728131050.24443-1-madvenka@linux.microsoft.com> In-Reply-To: <20200728131050.24443-1-madvenka@linux.microsoft.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: From: madvenka@linux.microsoft.com > Sent: 28 July 2020 14:11 ... > The kernel creates the trampoline mapping without any permissions. When > the trampoline is executed by user code, a page fault happens and the > kernel gets control. The kernel recognizes that this is a trampoline > invocation. It sets up the user registers based on the specified > register context, and/or pushes values on the user stack based on the > specified stack context, and sets the user PC to the requested target > PC. When the kernel returns, execution continues at the target PC. > So, the kernel does the work of the trampoline on behalf of the > application. Isn't the performance of this going to be horrid? If you don't care that much about performance the fixup can all be done in userspace within the fault signal handler. Since whatever you do needs the application changed why not change the implementation of nested functions to not need on-stack executable trampolines. I can think of other alternatives that don't need much more than an array of 'push constant; jump trampoline' instructions be created (all jump to the same place). You might want something to create an executable page of such instructions. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)