Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756814AbaDLXta (ORCPT ); Sat, 12 Apr 2014 19:49:30 -0400 Received: from out5-smtp.messagingengine.com ([66.111.4.29]:51467 "EHLO out5-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756223AbaDLXt3 (ORCPT ); Sat, 12 Apr 2014 19:49:29 -0400 Message-Id: <1397346568.13490.105824481.72FA2B8D@webmail.messagingengine.com> X-Sasl-Enc: s4H/BqldMyf8QYiW3sWD4cuuXePCoqYGoSutYTjfMRHI 1397346568 From: Alexander van Heukelum To: "H. Peter Anvin" , Andy Lutomirski , Brian Gerst , Ingo Molnar , Linux Kernel Mailing List , Linus Torvalds , Thomas Gleixner , stable@jasper.es MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain X-Mailer: MessagingEngine.com Webmail Interface - ajax-5be85a12 In-Reply-To: <5349CCE3.3000003@zytor.com> References: <53483487.6030103@zytor.com> <53485BB8.1000106@mit.edu> <53485D95.9030301@zytor.com> <1397345179.1772.105823721.224E1140@webmail.messagingengine.com> <5349CCE3.3000003@zytor.com> Subject: Re: [tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels Date: Sun, 13 Apr 2014 01:49:28 +0200 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Apr 13, 2014, at 1:31, H. Peter Anvin wrote: > >>> d. Trampoline in user space > >>> > >>> A return to the vdso with values set up in registers r8-r15 would enable > >>> a trampoline in user space. Unfortunately there is no way > >>> to do a far JMP entirely with register state so this would require > >>> touching user space memory, possibly in an unsafe manner. > > > > d.2. trampoline in user space via long mode > > > > Return from the kernel to a user space trampoline via long mode. > > The kernel changes the stack frame just before executing the iret > > instruction. (the CS and RIP slots are set to run the trampoline code, > > where CS is a long mode segment.) The trampoline code in userspace > > is set up to this single instruction: a far jump to the final CS:EIP > > (compatibility mode). > > This still requires user space memory that the kernel can write to. > Long mode is actually exactly identical to what I was suggesting above, > except that I would avoid using self-modifying code in favor of just > parameterization using the high registers. No self modifying code... The far jump must be in the indirect form anyhow. The CS:EIP must be accessible from user mode, but not necessarily from compatibility mode. So the trampoline (the jump) and data (CS:EIP) can live pretty much anywhere in virtual memory. But indeed, I see what you meant now. Greetings, Alexander > > -hpa > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/