From mboxrd@z Thu Jan 1 00:00:00 1970 From: Catalin Marinas Subject: Re: [PATCH RESEND 1/4] uaccess: Add user_read_access_begin/end and user_write_access_begin/end Date: Fri, 3 Apr 2020 12:26:10 +0100 Message-ID: <20200403112609.GB26633@mbp> References: <27106d62fdbd4ffb47796236050e418131cb837f.1585811416.git.christophe.leroy@c-s.fr> <20200402162942.GG23230@ZenIV.linux.org.uk> <67e21b65-0e2d-7ca5-7518-cec1b7abc46c@c-s.fr> <20200402175032.GH23230@ZenIV.linux.org.uk> <202004021132.813F8E88@keescook> <20200403005831.GI23230@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from foss.arm.com ([217.140.110.172]:51928 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728022AbgDCL0P (ORCPT ); Fri, 3 Apr 2020 07:26:15 -0400 Content-Disposition: inline In-Reply-To: <20200403005831.GI23230@ZenIV.linux.org.uk> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Al Viro Cc: Kees Cook , Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , airlied@linux.ie, daniel@ffwll.ch, torvalds@linux-foundation.org, akpm@linux-foundation.org, hpa@zytor.com, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Russell King , Christian Borntraeger On Fri, Apr 03, 2020 at 01:58:31AM +0100, Al Viro wrote: > On Thu, Apr 02, 2020 at 11:35:57AM -0700, Kees Cook wrote: > > Yup, I think it's a weakness of the ARM implementation and I'd like to > > not extend it further. AFAIK we should never nest, but I would not be > > surprised at all if we did. > > > > If we were looking at a design goal for all architectures, I'd like > > to be doing what the public PaX patchset did for their memory access > > switching, which is to alarm if calling into "enable" found the access > > already enabled, etc. Such a condition would show an unexpected nesting > > (like we've seen with similar constructs with set_fs() not getting reset > > during an exception handler, etc etc). > > FWIW, maybe I'm misreading the ARM uaccess logics, but... it smells like > KERNEL_DS is somewhat more dangerous there than on e.g. x86. > > Look: with CONFIG_CPU_DOMAINS, set_fs(KERNEL_DS) tells MMU to ignore > per-page permission bits in DOMAIN_KERNEL (i.e. for kernel address > ranges), allowing them even if they would normally be denied. We need > that for actual uaccess loads/stores, since those use insns that pretend > to be done in user mode and we want them to access the kernel pages. > But that affects the normal loads/stores as well; unless I'm misreading > that code, it will ignore (supervisor) r/o on a page. And that's not > just for the code inside the uaccess blocks; *everything* done under > KERNEL_DS is subject to that. That's correct. Luckily this only affects ARMv5 and earlier. From ARMv6 onwards, CONFIG_CPU_USE_DOMAINS is no longer selected and the uaccess instructions are just plain ldr/str. Russell should know the details on whether there was much choice. Since the kernel was living in the linear map with full rwx permissions, the KERNEL_DS overriding was probably not a concern and the ldrt/strt for uaccess deemed more secure. We also have weird permission setting pre-ARMv6 (or rather v6k) where RO user pages are writable from the kernel with standard str instructions (breaking CoW). I don't recall whether it was a choice made by the kernel or something the architecture enforced. The vectors page has to be kernel writable (and user RO) to store the TLS value in the absence of a TLS register but maybe we could do this via the linear alias together with the appropriate cache maintenance. >From ARMv6, the domain overriding had the side-effect of ignoring the XN bit and causing random instruction fetches from ioremap() areas. So we had to remove the domain switching. We also gained a dedicated TLS register. > Why do we do that (modify_domain(), that is) inside set_fs() and not > in uaccess_enable() et.al.? I think uaccess_enable() could indeed switch the kernel domain if KERNEL_DS is set and move this out of set_fs(). It would reduce the window the kernel domain permissions are overridden. Anyway, uaccess_enable() appeared much later on arm when Russell introduced PAN (SMAP) like support by switching the user domain. -- Catalin