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 X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CD77DC004C9 for ; Wed, 8 May 2019 02:00:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 94F4A20656 for ; Wed, 8 May 2019 02:00:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="bWmIb+W1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726701AbfEHCA1 (ORCPT ); Tue, 7 May 2019 22:00:27 -0400 Received: from mail-pl1-f194.google.com ([209.85.214.194]:33036 "EHLO mail-pl1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726378AbfEHCA1 (ORCPT ); Tue, 7 May 2019 22:00:27 -0400 Received: by mail-pl1-f194.google.com with SMTP id y3so9101322plp.0 for ; Tue, 07 May 2019 19:00:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=7lkKlWarMcmCQ856FYfrh6ypdVg/VPirY77fSPvi4dc=; b=bWmIb+W13o7hQ7Arb4CrEiaXAnQXSkX6/b5Z/ZXsZRuwnYaV9W5ZyVrP6vAj5UADmr oJBIJtY494JAKpjGSlw9hLqo2E9zSMLkLnNkFodbwUgsdhs/UVyDYv5AI5RxGL7P7j2X w0F41/ZsHo+idE6xBP6NWAzvrTtQJuSJSd890= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=7lkKlWarMcmCQ856FYfrh6ypdVg/VPirY77fSPvi4dc=; b=W2c3pSo+D5upZmfR8IvkLNbBGX1dwVPvDKOs6nh3WZv4lEbI4rHxntRYbHCFZ19aA9 CfrFIc1Fc4ZcCHHxmtX+Ptga5u1n3QG375pr1xCKaUO93A5QcSzmTCMxmGBNzxPGxlBL rdnmHfB4Xy1Af7YviAJj0sya2oiwZP1OCT8+RNxxXJ9+haOOf9rOmCqMeCj2OlH0Ljsy OfombGNwD1Cq63RJTX9szfWDE4cgzF9f5m5JG4HX+GimcpkI8LqMDxLVznr3muyUTtny cj6VKhHn29qM4jewbWypVX2Xi0hzc2HI6cyIEVjeur0GQSFS8OW4ouFQWapZWYb3/M07 4iAw== X-Gm-Message-State: APjAAAXB/2RuAYTCFwipjXXySEkDpEA2spWXBo//57Hf9+furGhFSyuD VWjb6orFibe4LgIf7DQIWa/gxA== X-Google-Smtp-Source: APXvYqw57+9AV/J9ht8xnAsWf4KN8plduZMRXaxqJxdD7UnLJjK/X1KkU9TLvi4LUBZ/1oPLu+PALw== X-Received: by 2002:a17:902:e305:: with SMTP id cg5mr43978820plb.112.1557280826492; Tue, 07 May 2019 19:00:26 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id s198sm30036178pfs.34.2019.05.07.19.00.24 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 07 May 2019 19:00:25 -0700 (PDT) Date: Tue, 7 May 2019 22:00:23 -0400 From: Joel Fernandes To: Will Deacon Cc: Qais Yousef , linux-kernel@vger.kernel.org, Michal Gregorczyk , Adrian Ratiu , Mohammad Husain , Srinivas Ramana , duyuchao , Manjo Raja Rao , Karim Yaghmour , Tamir Carmeli , Yonghong Song , Alexei Starovoitov , Brendan Gregg , Masami Hiramatsu , Peter Ziljstra , Steven Rostedt , Kees Cook , kernel-team@android.com, Daniel Borkmann , Ingo Molnar , netdev@vger.kernel.org, Mark Rutland Subject: Re: [PATCH RFC] bpf: Add support for reading user pointers Message-ID: <20190508020023.GA184577@google.com> References: <20190503134935.GA253329@google.com> <20190505110423.u7g3f2viovvgzbtn@e107158-lin.cambridge.arm.com> <20190505132949.GB3076@localhost> <20190505144608.u3vsxyz5huveuskx@e107158-lin.cambridge.arm.com> <20190505155223.GA4976@localhost> <20190505180313.GA80924@google.com> <20190506183506.GD2875@brain-police> <20190506205807.GA223956@google.com> <20190506215737.cuugrrxbhkp2uknn@e107158-lin.cambridge.arm.com> <20190507095242.GA17052@fuggles.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190507095242.GA17052@fuggles.cambridge.arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Tue, May 07, 2019 at 10:52:42AM +0100, Will Deacon wrote: > On Mon, May 06, 2019 at 10:57:37PM +0100, Qais Yousef wrote: > > On 05/06/19 16:58, Joel Fernandes wrote: > > > > If you're trying to dereference a pointer to userspace using > > > > probe_kernel_read(), that clearly isn't going to work. > > > > > > Ok. Thanks for confirming as well. The existing code has this bug and these > > > patches fix it. > > > > 5.1-rc7 and 4.9.173 stable both managed to read the path in do_sys_open() on my > > Juno-r2 board using the defconfig in the tree. > > That's not surprising: Juno-r2 only features v8.0 CPUs, so doesn't have PAN > or UAO capabilities. The SoC Joel is talking about is 8.2, so has both of > those. > > Here's some background which might help... > > PAN (Privileged Access Never) prevents the kernel from inadvertently > accessing userspace and will cause a page (permission) fault if such an > access is made outside of the standard uaccess routines. This means that > in those routines (e.g. get_user()) we have to toggle the PAN state in the > same way that x86 toggles SMAP. This can be expensive and was part of the > motivation for the adoption of things like unsafe_get_user() on x86. > > On arm64, we have a set of so-called "translated" memory access instructions > which can be used to perform unprivileged accesses to userspace from within > the kernel even when PAN is enabled. Using these special instructions (e.g. > LDTR) in our uaccess routines can therefore remove the need to toggle PAN. > Sounds great, right? Well, that all falls apart when the uaccess routines > are used on kernel addresses as in probe_kernel_read() because they will > fault when trying to dereference a kernel pointer. > > The answer is UAO (User Access Override). When UAO is set, the translated > memory access instructions behave the same as non-translated accesses. > Therefore we can toggle UAO in set_fs() so that it is set when we're using > KERNEL_DS and cleared when we're using USER_DS. > > The side-effect of this is that when KERNEL_DS is set on a system that > implements both PAN and UAO, passing a user pointer to our uaccess routines > will return -EFAULT. In other words, set_fs() can be thought of as a > selector between the kernel and user address spaces, which are distinct. > > Joel -- does disabling UAO in your .config "fix" the issue? If so, feel > free to use some of the above text in a commit message if it helps to > justify your change. Disabling CONFIG_ARM64_UAO does "fix" it. I will use the description above in the commit message as you suggested, thanks a lot for the explantation of this! - Joel