From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753669AbcHPQTv (ORCPT ); Tue, 16 Aug 2016 12:19:51 -0400 Received: from foss.arm.com ([217.140.101.70]:48186 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753535AbcHPQS3 (ORCPT ); Tue, 16 Aug 2016 12:18:29 -0400 Date: Tue, 16 Aug 2016 17:18:24 +0100 From: Catalin Marinas To: Kees Cook Cc: Linux-MM , Will Deacon , LKML , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH] arm64: Introduce execute-only page access permissions Message-ID: <20160816161824.GB7609@e104818-lin.cambridge.arm.com> References: <1470937490-7375-1-git-send-email-catalin.marinas@arm.com> <20160815104751.GC22320@e104818-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 On Mon, Aug 15, 2016 at 10:45:09AM -0700, Kees Cook wrote: > On Mon, Aug 15, 2016 at 3:47 AM, Catalin Marinas > wrote: > > On Fri, Aug 12, 2016 at 11:23:03AM -0700, Kees Cook wrote: > >> On Thu, Aug 11, 2016 at 10:44 AM, Catalin Marinas > >> wrote: > >> > The ARMv8 architecture allows execute-only user permissions by clearing > >> > the PTE_UXN and PTE_USER bits. However, the kernel running on a CPU > >> > implementation without User Access Override (ARMv8.2 onwards) can still > >> > access such page, so execute-only page permission does not protect > >> > against read(2)/write(2) etc. accesses. Systems requiring such > >> > protection must enable features like SECCOMP. > >> > >> So, UAO CPUs will bypass this protection in userspace if using > >> read/write on a memory-mapped file? > > > > It's the other way around. CPUs prior to ARMv8.2 (when UAO was > > introduced) or with the CONFIG_ARM64_UAO disabled can still access > > user execute-only memory regions while running in kernel mode via the > > copy_*_user, (get|put)_user etc. routines. So a way user can bypass this > > protection is by using such address as argument to read/write file > > operations. > > Ah, okay. So exec-only for _userspace_ will always work, but exec-only > for _kernel_ will only work on ARMv8.2 with CONFIG_ARM64_UAO? Yes (mostly). With UAO, we changed the user access routines in the kernel to use the LDTR/STTR instructions which always behave unprivileged even when executed in kernel mode (unless the UAO bit is set to override this restriction, needed for set_fs(KERNEL_DS)). Even with UAO, we still have two cases where the kernel cannot perform unprivileged accesses (LDTR/STTR) since they don't have an exclusives equivalent (LDXR/STXR). These are in-user futex atomic ops and the SWP emulation for 32-bit binaries (armv8_deprecated.c). But these require write permission, so they would always fault even when running in the kernel. futex_atomic_cmpxchg_inatomic() is able to return the old value without a write (if it differs from "oldval") but it doesn't look like such value could leak to user space. -- Catalin