From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f72.google.com (mail-oi0-f72.google.com [209.85.218.72]) by kanga.kvack.org (Postfix) with ESMTP id F01736B0005 for ; Wed, 1 Aug 2018 13:43:05 -0400 (EDT) Received: by mail-oi0-f72.google.com with SMTP id b8-v6so17496226oib.4 for ; Wed, 01 Aug 2018 10:43:05 -0700 (PDT) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id a63-v6si11586179oii.56.2018.08.01.10.43.04 for ; Wed, 01 Aug 2018 10:43:04 -0700 (PDT) Date: Wed, 1 Aug 2018 18:42:56 +0100 From: Catalin Marinas Subject: Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel Message-ID: <20180801174256.5mbyf33eszml4nmu@armageddon.cambridge.arm.com> References: <20180626172900.ufclp2pfrhwkxjco@armageddon.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Andrey Konovalov Cc: Mark Rutland , Kate Stewart , linux-doc@vger.kernel.org, Will Deacon , Kostya Serebryany , linux-kselftest@vger.kernel.org, Chintan Pandya , Shuah Khan , Ingo Molnar , linux-arch@vger.kernel.org, Jacob Bramley , Dmitry Vyukov , Evgeniy Stepanov , Kees Cook , Ruben Ayrapetyan , Ramana Radhakrishnan , Al Viro , Linux ARM , Linux Memory Management List , Greg Kroah-Hartman , LKML , Lee Smith , Andrew Morton , Robin Murphy , "Kirill A . Shutemov" On Mon, Jul 16, 2018 at 01:25:59PM +0200, Andrey Konovalov wrote: > On Thu, Jun 28, 2018 at 9:30 PM, Andrey Konovalov wrote: > > On Wed, Jun 27, 2018 at 5:05 PM, Andrey Konovalov wrote: > >> On Tue, Jun 26, 2018 at 7:29 PM, Catalin Marinas > >> wrote: > >>> While I support this work, as a maintainer I'd like to understand > >>> whether we'd be in a continuous chase of ABI breaks with every kernel > >>> release or we have a better way to identify potential issues. Is there > >>> any way to statically analyse conversions from __user ptr to long for > >>> example? Or, could we get the compiler to do this for us? > >> > >> OK, got it, I'll try to figure out a way to find these conversions. > > > > I've prototyped a checker on top of clang static analyzer (initially > > looked at sparse, but couldn't find any documentation or examples). > > The results are here [1], search for "warning: user pointer cast". > > Sharing in case anybody wants to take a look, will look at them myself > > tomorrow. > > > > [1] https://gist.github.com/xairy/433edd5c86456a64026247cb2fef2115 > > So the checker reports ~100 different places where a __user pointer > being casted. I've looked through them and found 3 places where we > need to add untagging. Source code lines below come from 4.18-rc2+ > (6f0d349d). [...] > I'll add the 3 patches with fixes to v5 of this patchset. Thanks for investigating. You can fix those three places in your code but I was rather looking for a way to check such casting in the future for newly added code. While for the khwasan we can assume it's a debug option, the tagged user pointers are ABI and we need to keep it stable. We could we actually add some macros for explicit conversion between __user ptr and long and silence the warning there (I guess this would work better for sparse). We can then detect new ptr to long casts as they appear. I just hope that's not too intrusive. (I haven't tried the sparse patch yet, hopefully sometime this week) -- Catalin