From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH] arm64: Preserve the user r/w register tpidr_el0 on context switch and fork in compat mode Date: Tue, 5 May 2015 18:15:58 +0100 Message-ID: <20150505171558.GO1550@arm.com> References: <55464BB2.7030401@dawncrow.de> <20150505105111.GB1550@arm.com> <5548F965.9070302@dawncrow.de> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <5548F965.9070302@dawncrow.de> Sender: linux-kernel-owner@vger.kernel.org To: =?iso-8859-1?Q?Andr=E9?= Hentschel Cc: "linux-arch@vger.kernel.org" , Russell King - ARM Linux , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "gregkh@linuxfoundation.org" , Jonathan Austin , Nathan Lynch , Catalin Marinas List-Id: linux-arch.vger.kernel.org On Tue, May 05, 2015 at 06:09:57PM +0100, Andr=C3=A9 Hentschel wrote: > Am 05.05.2015 um 12:51 schrieb Will Deacon: > > On Sun, May 03, 2015 at 05:24:18PM +0100, Andr=C3=A9 Hentschel wrot= e: > >> From: Andr=C3=A9 Hentschel > >> > >> Since commit a4780adeefd042482f624f5e0d577bf9cdcbb760 the user wri= teable TLS > >> register on ARM is preserved per thread. > >> > >> This patch does it analogous to the ARM patch, but for compat mode= on ARM64. > >> > >> Signed-off-by: Andr=C3=A9 Hentschel > >> Cc: Will Deacon > >> Cc: Jonathan Austin =20 > >> > >> --- > >> This patch is against Linux 4.1-rc1 (b787f68c36d49bb1d9236f4038136= 41efa74a031) > >=20 > > Curious, but why do you need this? iirc, we added this for arch/arm= / because > > of some windows rt (?) emulation in wine. Is that still the case he= re and is > > anybody actually using that? >=20 > Yes, Windows ARM binaries are the well known use case, but also the c= ompat > mode should do what the arm kernel is doing I=E2=80=99d think and the= code wasn't > adjusted yet. Sure, I was just curious. > What i'm curious about is why the main TLS register on arm64 is the u= ser > writeable, I'm not an security expert but this looks odd. I could eas= ily > provoke a crash by writing to it... You've probably got the wrong TLS. Allowing a program to clobber it's o= wn thread-local storage is no worse than allowing it to write to its gener= al purpose registers, pc, etc. I'm assuming the crash you saw was just a userspace crash, rather than the kernel? Will From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com ([217.140.101.70]:45468 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760724AbbEERQC (ORCPT ); Tue, 5 May 2015 13:16:02 -0400 Date: Tue, 5 May 2015 18:15:58 +0100 From: Will Deacon Subject: Re: [PATCH] arm64: Preserve the user r/w register tpidr_el0 on context switch and fork in compat mode Message-ID: <20150505171558.GO1550@arm.com> References: <55464BB2.7030401@dawncrow.de> <20150505105111.GB1550@arm.com> <5548F965.9070302@dawncrow.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <5548F965.9070302@dawncrow.de> Sender: linux-arch-owner@vger.kernel.org List-ID: To: =?iso-8859-1?Q?Andr=E9?= Hentschel Cc: "linux-arch@vger.kernel.org" , Russell King - ARM Linux , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "gregkh@linuxfoundation.org" , Jonathan Austin , Nathan Lynch , Catalin Marinas Message-ID: <20150505171558.SJAjVBLhDeYS8Aj7ujzxSLZwgV207FAo60aEXgX9qnw@z> On Tue, May 05, 2015 at 06:09:57PM +0100, André Hentschel wrote: > Am 05.05.2015 um 12:51 schrieb Will Deacon: > > On Sun, May 03, 2015 at 05:24:18PM +0100, André Hentschel wrote: > >> From: André Hentschel > >> > >> Since commit a4780adeefd042482f624f5e0d577bf9cdcbb760 the user writeable TLS > >> register on ARM is preserved per thread. > >> > >> This patch does it analogous to the ARM patch, but for compat mode on ARM64. > >> > >> Signed-off-by: André Hentschel > >> Cc: Will Deacon > >> Cc: Jonathan Austin > >> > >> --- > >> This patch is against Linux 4.1-rc1 (b787f68c36d49bb1d9236f403813641efa74a031) > > > > Curious, but why do you need this? iirc, we added this for arch/arm/ because > > of some windows rt (?) emulation in wine. Is that still the case here and is > > anybody actually using that? > > Yes, Windows ARM binaries are the well known use case, but also the compat > mode should do what the arm kernel is doing I’d think and the code wasn't > adjusted yet. Sure, I was just curious. > What i'm curious about is why the main TLS register on arm64 is the user > writeable, I'm not an security expert but this looks odd. I could easily > provoke a crash by writing to it... You've probably got the wrong TLS. Allowing a program to clobber it's own thread-local storage is no worse than allowing it to write to its general purpose registers, pc, etc. I'm assuming the crash you saw was just a userspace crash, rather than the kernel? Will