From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755309AbaEDPvw (ORCPT ); Sun, 4 May 2014 11:51:52 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:36820 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755006AbaEDPmx (ORCPT ); Sun, 4 May 2014 11:42:53 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Linus Torvalds , "H. Peter Anvin" Subject: [PATCH 3.4 14/43] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels Date: Sun, 4 May 2014 11:42:08 -0400 Message-Id: <20140504154226.182340048@linuxfoundation.org> X-Mailer: git-send-email 1.9.0 In-Reply-To: <20140504154224.211508175@linuxfoundation.org> References: <20140504154224.211508175@linuxfoundation.org> User-Agent: quilt/0.60-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: "H. Peter Anvin" commit b3b42ac2cbae1f3cecbb6229964a4d48af31d382 upstream. The IRET instruction, when returning to a 16-bit segment, only restores the bottom 16 bits of the user space stack pointer. We have a software workaround for that ("espfix") for the 32-bit kernel, but it relies on a nonzero stack segment base which is not available in 32-bit mode. Since 16-bit support is somewhat crippled anyway on a 64-bit kernel (no V86 mode), and most (if not quite all) 64-bit processors support virtualization for the users who really need it, simply reject attempts at creating a 16-bit segment when running on top of a 64-bit kernel. Cc: Linus Torvalds Signed-off-by: H. Peter Anvin Link: http://lkml.kernel.org/n/tip-kicdm89kzw9lldryb1br9od0@git.kernel.org Signed-off-by: Greg Kroah-Hartman --- arch/x86/kernel/ldt.c | 11 +++++++++++ 1 file changed, 11 insertions(+) --- a/arch/x86/kernel/ldt.c +++ b/arch/x86/kernel/ldt.c @@ -229,6 +229,17 @@ static int write_ldt(void __user *ptr, u } } + /* + * On x86-64 we do not support 16-bit segments due to + * IRET leaking the high bits of the kernel stack address. + */ +#ifdef CONFIG_X86_64 + if (!ldt_info.seg_32bit) { + error = -EINVAL; + goto out_unlock; + } +#endif + fill_ldt(&ldt, &ldt_info); if (oldmode) ldt.avl = 0;