Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030669AbbDWSwR (ORCPT ); Thu, 23 Apr 2015 14:52:17 -0400 Received: from mail.skyhub.de ([78.46.96.112]:37830 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030437AbbDWSwP (ORCPT ); Thu, 23 Apr 2015 14:52:15 -0400 Date: Thu, 23 Apr 2015 20:52:02 +0200 From: Borislav Petkov To: Andy Lutomirski Cc: Denys Vlasenko , Denys Vlasenko , Brian Gerst , Steven Rostedt , Oleg Nesterov , Ingo Molnar , "H. Peter Anvin" , Linus Torvalds , Andy Lutomirski , Will Drewry , =?utf-8?B?RnLDqWTDqXJpYw==?= Weisbecker , Alexei Starovoitov , Linux Kernel Mailing List , Kees Cook , Thomas Gleixner , "linux-tip-commits@vger.kernel.org" Subject: Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32_DS to %ss Message-ID: <20150423185202.GQ28327@pd.tnic> References: <5538C1C5.7010408@redhat.com> <20150423101840.GC28327@pd.tnic> <5538C8E3.60009@redhat.com> <20150423104401.GF28327@pd.tnic> <20150423171440.GP28327@pd.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1944 Lines: 74 On Thu, Apr 23, 2015 at 11:24:14AM -0700, Andy Lutomirski wrote: > That nails it. We really do leak segment limits to other tasks on AMD > chips. I see at least two questions we should answer before fixing > this: Ok, WTF is going on?! Even this trivial test case causes a Bus Error: --- static unsigned short GDT3(int idx) { return (idx << 3) | 3; } static void *threadproc(void *ctx) { printf("Hello world\n"); return NULL; } int main() { pthread_t thread; if (pthread_create(&thread, 0, threadproc, 0) != 0) err(1, "pthread_create"); while (1) { usleep(1); } return 0; } --- $ make sysret_ss_attrs_32 gcc -m32 -o sysret_ss_attrs_32 -O2 -g -std=gnu99 -pthread -Wall sysret_ss_attrs.c -lrt -ldl sysret_ss_attrs.c:23:23: warning: ‘GDT3’ defined but not used [-Wunused-function] static unsigned short GDT3(int idx) ^ $ taskset -c 0 ./sysret_ss_attrs_32 Hello world Bus error in dmesg: [ 583.389368] traps: sysret_ss_attrs[2135] trap stack segment ip:f7784b87 sp:ffb640c0 error:0 This is insane. > 1. Do we consider this to be enough of a security issue that we want > to fix it for 64-bit userspace as well? > > 2. Do we fix it at sysret time (at the cost of an ss read even in the > best case on AMD chips) or at context switch time (with the risk of > more ss writes than necessary)? > > I slightly favor fixing it at sysret time for both the 32-bit and > 64-bit paths., but I'm not really convinced. Yeah, a "call amd_fixup_ss" which gets NOPped out on Intel with alternatives sounds nice and clean to me. Pending we have an explanation WTH is going on... -- Regards/Gruss, Boris. ECO tip #101: Trim your mails when you reply. -- -- 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/