Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752020AbdCSLZN (ORCPT ); Sun, 19 Mar 2017 07:25:13 -0400 Received: from terminus.zytor.com ([65.50.211.136]:43358 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751286AbdCSLZK (ORCPT ); Sun, 19 Mar 2017 07:25:10 -0400 Date: Sun, 19 Mar 2017 04:24:49 -0700 From: tip-bot for Andy Lutomirski Message-ID: Cc: mingo@kernel.org, bp@alien8.de, thgarnie@google.com, torvalds@linux-foundation.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, hpa@zytor.com, luto@kernel.org Reply-To: bp@alien8.de, thgarnie@google.com, mingo@kernel.org, hpa@zytor.com, luto@kernel.org, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, tglx@linutronix.de In-Reply-To: <62b7748542df0164af7e0a5231283b9b13858c45.1489900519.git.luto@kernel.org> References: <62b7748542df0164af7e0a5231283b9b13858c45.1489900519.git.luto@kernel.org> To: linux-tip-commits@vger.kernel.org Subject: [tip:x86/mm] x86/tls: Forcibly set the accessed bit in TLS segments Git-Commit-ID: 5b781c7e317fcf9f74475dc82bfce2e359dfca13 X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2230 Lines: 60 Commit-ID: 5b781c7e317fcf9f74475dc82bfce2e359dfca13 Gitweb: http://git.kernel.org/tip/5b781c7e317fcf9f74475dc82bfce2e359dfca13 Author: Andy Lutomirski AuthorDate: Sat, 18 Mar 2017 22:17:24 -0700 Committer: Thomas Gleixner CommitDate: Sun, 19 Mar 2017 12:14:35 +0100 x86/tls: Forcibly set the accessed bit in TLS segments For mysterious historical reasons, struct user_desc doesn't indicate whether segments are accessed. set_thread_area() has always programmed segments as non-accessed, so the first write will set the accessed bit. This will fault if the GDT is read-only. Fix it by making TLS segments start out accessed. If this ends up breaking something, we could, in principle, leave TLS segments non-accessed and fix them up when we get the page fault. I'd be surprised, though -- AFAIK all the nasty legacy segmented programs (DOSEMU, Wine, things that run on DOSEMU and Wine, etc.) do their nasty segmented things using the LDT and not the GDT. I assume this is mainly because old OSes (Linux and otherwise) didn't historically provide APIs to do nasty things in the GDT. Fixes: 45fc8757d1d2 ("x86: Make the GDT remapping read-only on 64-bit") Signed-off-by: Andy Lutomirski Cc: Linus Torvalds Cc: Borislav Petkov Cc: Thomas Garnier Link: http://lkml.kernel.org/r/62b7748542df0164af7e0a5231283b9b13858c45.1489900519.git.luto@kernel.org Signed-off-by: Thomas Gleixner --- arch/x86/kernel/tls.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c index 6c89344..dcd699b 100644 --- a/arch/x86/kernel/tls.c +++ b/arch/x86/kernel/tls.c @@ -92,10 +92,17 @@ static void set_tls_desc(struct task_struct *p, int idx, cpu = get_cpu(); while (n-- > 0) { - if (LDT_empty(info) || LDT_zero(info)) + if (LDT_empty(info) || LDT_zero(info)) { desc->a = desc->b = 0; - else + } else { fill_ldt(desc, info); + + /* + * Always set the accessed bit so that the CPU + * doesn't try to write to the (read-only) GDT. + */ + desc->type |= 1; + } ++info; ++desc; }