From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8F974C433F5 for ; Tue, 15 Feb 2022 02:47:17 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 20B4B6B0078; Mon, 14 Feb 2022 21:47:17 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 1BA9B6B007B; Mon, 14 Feb 2022 21:47:17 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 0AA4F6B007D; Mon, 14 Feb 2022 21:47:17 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0038.hostedemail.com [216.40.44.38]) by kanga.kvack.org (Postfix) with ESMTP id EDB186B0078 for ; Mon, 14 Feb 2022 21:47:16 -0500 (EST) Received: from smtpin22.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 9E247180AD837 for ; Tue, 15 Feb 2022 02:47:16 +0000 (UTC) X-FDA: 79143477672.22.5ABFE39 Received: from zeniv-ca.linux.org.uk (zeniv-ca.linux.org.uk [142.44.231.140]) by imf12.hostedemail.com (Postfix) with ESMTP id 1907040004 for ; Tue, 15 Feb 2022 02:47:15 +0000 (UTC) Received: from viro by zeniv-ca.linux.org.uk with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1nJnrt-001qwH-NG; Tue, 15 Feb 2022 02:47:05 +0000 Date: Tue, 15 Feb 2022 02:47:05 +0000 From: Al Viro To: Linus Torvalds Cc: Arnd Bergmann , Christoph Hellwig , Christoph Hellwig , linux-arch , Linux-MM , Linux API , Arnd Bergmann , Linux Kernel Mailing List , Mark Rutland , Rich Felker , linux-ia64@vger.kernel.org, Linux-sh list , Peter Zijlstra , Max Filippov , Guo Ren , sparclinux , "open list:QUALCOMM HEXAGON..." , linux-riscv , Will Deacon , Ard Biesheuvel , linux-s390 , Brian Cain , Helge Deller , the arch/x86 maintainers , Russell King - ARM Linux , linux-csky@vger.kernel.org, Ingo Molnar , Geert Uytterhoeven , "open list:SYNOPSYS ARC ARCHITECTURE" , "open list:TENSILICA XTENSA PORT (xtensa)" , Heiko Carstens , alpha , linux-um , linux-m68k , Openrisc , Greentime Hu , Stafford Horne , Linux ARM , Michal Simek , Thomas Bogendoerfer , Parisc List , Nick Hu , "open list:BROADCOM NVRAM DRIVER" , Dinh Nguyen , "Eric W . Biederman" , Richard Weinberger , Andrew Morton , linuxppc-dev , David Miller Subject: Re: [PATCH 04/14] x86: use more conventional access_ok() definition Message-ID: References: <20220214163452.1568807-1-arnd@kernel.org> <20220214163452.1568807-5-arnd@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 1907040004 X-Stat-Signature: tqemcpaupqe9erw7m7y1gn5up4bcmf9q Authentication-Results: imf12.hostedemail.com; dkim=none; dmarc=none; spf=none (imf12.hostedemail.com: domain of viro@ftp.linux.org.uk has no SPF policy when checking 142.44.231.140) smtp.mailfrom=viro@ftp.linux.org.uk X-HE-Tag: 1644893235-347508 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Feb 14, 2022 at 08:17:07PM +0000, Al Viro wrote: > On Mon, Feb 14, 2022 at 12:01:05PM -0800, Linus Torvalds wrote: > > On Mon, Feb 14, 2022 at 11:46 AM Arnd Bergmann wrote: > > > > > > As Al pointed out, they turned out to be necessary on sparc64, but the only > > > definitions are on sparc64 and x86, so it's possible that they serve a similar > > > purpose here, in which case changing the limit from TASK_SIZE to > > > TASK_SIZE_MAX is probably wrong as well. > > > > x86-64 has always(*) used TASK_SIZE_MAX for access_ok(), and the > > get_user() assembler implementation does the same. > > > > I think any __range_not_ok() users that use TASK_SIZE are entirely > > historical, and should be just fixed. > > IIRC, that was mostly userland stack trace collection in perf. > I'll try to dig in archives and see what shows up - it's been > a while ago... After some digging: access_ok() needs only to make sure that MMU won't go anywhere near the kernel page tables; address limit for 32bit threads is none of its concern, so TASK_SIZE_MAX is right for it. valid_user_frame() in arch/x86/events/core.c: used while walking the userland call chain. The reason it's not access_ok() is only that perf_callchain_user() might've been called from interrupt that came while we'd been under KERNEL_DS. That had been back in 2015 and it had been obsoleted since 2017, commit 88b0193d9418 (perf/callchain: Force USER_DS when invoking perf_callchain_user()). We had been guaranteed USER_DS ever since. IOW, it could've reverted to use of access_ok() at any point after that. TASK_SIZE vs TASK_SIZE_MAX is pretty much an accident there - might've been TASK_SIZE_MAX from the very beginning. copy_stack_frame() in arch/x86/kernel/stacktrace.c: similar story, except the commit that made sure callers will have USER_DS - cac9b9a4b083 (stacktrace: Force USER_DS for stack_trace_save_user()) in this case. Also could've been using access_ok() just fine. Amusingly, access_ok() used to be there, until it had been replaced with explicit check on Jul 22 2019 - 4 days after that had been made useless by fix in the caller... copy_from_user_nmi(). That one is a bit more interesting. We have a call chain from perf_output_sample_ustack() (covered by force_uaccess_begin() these days, not that it mattered for x86 now), there's something odd in dumpstack.c:copy_code() (with explicit check for TASK_SIZE_MAX in the caller) and there's a couple of callers in Intel PMU code. AFAICS, there's no reason whatsoever to use TASK_SIZE in that one - the point is to prevent copyin from the kernel memory, and in that respect TASK_SIZE_MAX isn't any worse. The check in copy_code() probably should go. So all of those guys should be simply switched to access_ok(). Might be worth making that a preliminary patch - it's independent from everything else and there's no point folding it into any of the patches in the series.