From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f197.google.com (mail-pl1-f197.google.com [209.85.214.197]) by kanga.kvack.org (Postfix) with ESMTP id 140506B7BCD for ; Thu, 6 Dec 2018 14:39:38 -0500 (EST) Received: by mail-pl1-f197.google.com with SMTP id g7so932561plp.10 for ; Thu, 06 Dec 2018 11:39:38 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id v13sor2152216pgn.66.2018.12.06.11.39.36 for (Google Transport Security); Thu, 06 Dec 2018 11:39:36 -0800 (PST) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 12.1 \(3445.101.1\)) Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages From: Nadav Amit In-Reply-To: Date: Thu, 6 Dec 2018 11:39:32 -0800 Content-Transfer-Encoding: quoted-printable Message-Id: References: <20181128000754.18056-1-rick.p.edgecombe@intel.com> <20181128000754.18056-2-rick.p.edgecombe@intel.com> <4883FED1-D0EC-41B0-A90F-1A697756D41D@gmail.com> <20181204160304.GB7195@arm.com> <51281e69a3722014f718a6840f43b2e6773eed90.camel@intel.com> <20181205114148.GA15160@arm.com> <20181206190115.GC10086@cisco> Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: Tycho Andersen , Ard Biesheuvel , Will Deacon , Rick Edgecombe , LKML , Daniel Borkmann , Jessica Yu , Steven Rostedt , Alexei Starovoitov , Linux-MM , Jann Horn , "Dock, Deneen T" , Peter Zijlstra , Kristen Carlson Accardi , Andrew Morton , Ingo Molnar , Anil S Keshavamurthy , Kernel Hardening , Masami Hiramatsu , "Naveen N . Rao" , "David S. Miller" , Network Development , Dave Hansen > On Dec 6, 2018, at 11:19 AM, Andy Lutomirski wrote: >=20 > On Thu, Dec 6, 2018 at 11:01 AM Tycho Andersen wrote: >> On Thu, Dec 06, 2018 at 10:53:50AM -0800, Andy Lutomirski wrote: >>>> If we are going to unmap the linear alias, why not do it at = vmalloc() >>>> time rather than vfree() time? >>>=20 >>> That=E2=80=99s not totally nuts. Do we ever have code that expects = __va() to >>> work on module data? Perhaps crypto code trying to encrypt static >>> data because our APIs don=E2=80=99t understand virtual addresses. I = guess if >>> highmem is ever used for modules, then we should be fine. >>>=20 >>> RO instead of not present might be safer. But I do like the idea of >>> renaming Rick's flag to something like VM_XPFO or VM_NO_DIRECT_MAP = and >>> making it do all of this. >>=20 >> Yeah, doing it for everything automatically seemed like it was/is >> going to be a lot of work to debug all the corner cases where things >> expect memory to be mapped but don't explicitly say it. And in >> particular, the XPFO series only does it for user memory, whereas an >> additional flag like this would work for extra paranoid allocations >> of kernel memory too. >=20 > I just read the code, and I looks like vmalloc() is already using > highmem (__GFP_HIGH) if available, so, on big x86_32 systems, for > example, we already don't have modules in the direct map. >=20 > So I say we go for it. This should be quite simple to implement -- > the pageattr code already has almost all the needed logic on x86. The > only arch support we should need is a pair of functions to remove a > vmalloc address range from the address map (if it was present in the > first place) and a function to put it back. On x86, this should only > be a few lines of code. >=20 > What do you all think? This should solve most of the problems we = have. >=20 > If we really wanted to optimize this, we'd make it so that > module_alloc() allocates memory the normal way, then, later on, we > call some function that, all at once, removes the memory from the > direct map and applies the right permissions to the vmalloc alias (or > just makes the vmalloc alias not-present so we can add permissions > later without flushing), and flushes the TLB. And we arrange for > vunmap to zap the vmalloc range, then put the memory back into the > direct map, then free the pages back to the page allocator, with the > flush in the appropriate place. >=20 > I don't see why the page allocator needs to know about any of this. > It's already okay with the permissions being changed out from under it > on x86, and it seems fine. Rick, do you want to give some variant of > this a try? Setting it as read-only may work (and already happens for the read-only module data). I am not sure about setting it as non-present. At some point, a discussion about a threat-model, as Rick indicated, = would be required. I presume ROP attacks can easily call = set_all_modules_text_rw() and override all the protections.