Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758048AbbDWPWo (ORCPT ); Thu, 23 Apr 2015 11:22:44 -0400 Received: from mail-ig0-f175.google.com ([209.85.213.175]:33762 "EHLO mail-ig0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752618AbbDWPWm (ORCPT ); Thu, 23 Apr 2015 11:22:42 -0400 MIME-Version: 1.0 In-Reply-To: <1429792491-5978-1-git-send-email-dvlasenk@redhat.com> References: <1429792491-5978-1-git-send-email-dvlasenk@redhat.com> Date: Thu, 23 Apr 2015 08:22:41 -0700 X-Google-Sender-Auth: I43r4OEmH36wHw4i3rhBlJiUQqk Message-ID: Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary From: Linus Torvalds To: Denys Vlasenko Cc: Ingo Molnar , Brian Gerst , Steven Rostedt , Borislav Petkov , "H. Peter Anvin" , Andy Lutomirski , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , "the arch/x86 maintainers" , Linux Kernel Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1741 Lines: 36 On Thu, Apr 23, 2015 at 5:34 AM, Denys Vlasenko wrote: > > It was observed to cause Wine crashes. Conjectured sequence of events > causing it is as follows: > > 1. Wine process enters kernel via syscall insn. > 2. Context switch to any other task. > 3. Interrupt or exception happens, CPU loads %ss with 0. > (This happens according to both Intel and AMD docs.) > %ss cached descriptor is set to "invalid" state. > 4. Context switch back to Wine. > 5. sysret to 32-bit userspace. %ss selector has correct value but its > cached descriptor is still invalid. I really don't like the patch, as it just feels very hacky to me. It is a bit scary to me that apparently we leak %ss values between processes, so that while we run in the kernel we can randomly have the ss descriptor either be 0 or __KERNEL_DS. That sounds like an information leak to me, even in 64-bit mode. The value of %ss may not *matter* in 64-bit mode, but leaking that difference between processes sounds nasty. I can't offhand thing of any way to actually read the present bit in the cached descriptor (I was thinking something like the "LSL" instruction, but that takes a new segment selector, not the segment itself), but it just smells odd to me. Also, why does this only happen with Wine? In regular 32-bit mode the segment valid bit in the cached descriptor should also matter. So how come this doesn't trigger for any 32-bit user land on a 64-bit kernel? Linus -- 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/