Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031010AbbDWViB (ORCPT ); Thu, 23 Apr 2015 17:38:01 -0400 Received: from terminus.zytor.com ([198.137.202.10]:57022 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030729AbbDWViA (ORCPT ); Thu, 23 Apr 2015 17:38:00 -0400 Message-ID: <55396617.4020303@zytor.com> Date: Thu, 23 Apr 2015 14:37:27 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0 MIME-Version: 1.0 To: Borislav Petkov , Denys Vlasenko CC: Andy Lutomirski , Linus Torvalds , Brian Gerst , Denys Vlasenko , Ingo Molnar , Steven Rostedt , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , the arch/x86 maintainers , Linux Kernel Mailing List Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary References: <1429792491-5978-1-git-send-email-dvlasenk@redhat.com> <20150423211015.GS28327@pd.tnic> In-Reply-To: <20150423211015.GS28327@pd.tnic> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1075 Lines: 26 On 04/23/2015 02:10 PM, Borislav Petkov wrote: > On Thu, Apr 23, 2015 at 10:01:16PM +0200, Denys Vlasenko wrote: >> Naturally, CS can't be NULL, and up until today >> I thought SS also can't. But the bit is probably implemented >> for all eight cached descriptors. > > There's this section about NULL selector in APM v2. It says that NULL > selectors are used to invalidate segment registers and software can load > a NULL selector in SS in CPL0. > > So, if an interrupt happens and as you quoted earlier that SS gets set > to NULL as a result of an interrupt, there's that SS leak causing the SS > exception. > Yes, the NULL SS is a special thing in 64-bit mode. I agree that context-switching it is probably the way to go; it should be cheap enough. We might even be able to conditionalize it on an X86_BUG_ flag. -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/