From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f200.google.com (mail-pf1-f200.google.com [209.85.210.200]) by kanga.kvack.org (Postfix) with ESMTP id 5F7846B7C04 for ; Thu, 6 Dec 2018 15:26:34 -0500 (EST) Received: by mail-pf1-f200.google.com with SMTP id y88so1255242pfi.9 for ; Thu, 06 Dec 2018 12:26:34 -0800 (PST) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id k2si1018798pgh.63.2018.12.06.12.26.32 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 06 Dec 2018 12:26:32 -0800 (PST) Received: from mail-wm1-f46.google.com (mail-wm1-f46.google.com [209.85.128.46]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 4F27F2154B for ; Thu, 6 Dec 2018 20:26:32 +0000 (UTC) Received: by mail-wm1-f46.google.com with SMTP id g67so2375861wmd.2 for ; Thu, 06 Dec 2018 12:26:32 -0800 (PST) MIME-Version: 1.0 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> In-Reply-To: From: Andy Lutomirski Date: Thu, 6 Dec 2018 12:26:18 -0800 Message-ID: Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Rick Edgecombe Cc: Andrew Lutomirski , Tycho Andersen , LKML , Daniel Borkmann , Ard Biesheuvel , Alexei Starovoitov , Steven Rostedt , Jessica Yu , Linux-MM , Jann Horn , Nadav Amit , "Dock, Deneen T" , Peter Zijlstra , Kristen Carlson Accardi , Andrew Morton , Will Deacon , Ingo Molnar , Anil S Keshavamurthy , Kernel Hardening , Masami Hiramatsu , "Naveen N . Rao" , "David S. Miller" , Network Development , Dave Hansen On Thu, Dec 6, 2018 at 12:20 PM Edgecombe, Rick P wrote: > > On Thu, 2018-12-06 at 11:19 -0800, Andy Lutomirski wrote: > > 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 vmall= oc() > > > > > time rather than vfree() time? > > > > > > > > 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. > > > > > > > > RO instead of not present might be safer. But I do like the idea o= f > > > > renaming Rick's flag to something like VM_XPFO or VM_NO_DIRECT_MAP = and > > > > making it do all of this. > > > > > > 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. > > > > > > > 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. > > > > 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. > > > > What do you all think? This should solve most of the problems we have. > > > > 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. > > > > 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? > Hi, > > Sorry, I've been having email troubles today. > > I found some cases where vmap with PAGE_KERNEL_RO happens, which would no= t set > NP/RO in the directmap, so it would be sort of inconsistent whether the > directmap of vmalloc range allocations were readable or not. I couldn't s= ee any > places where it would cause problems today though. > > I was ready to assume that all TLBs don't cache NP, because I don't know = how > usages where a page fault is used to load something could work without lo= ts of > flushes. Or the architecture just fixes up the spurious faults, I suppose. I'm only well-educated on the x86 mmu. > If that's the case, then all archs with directmap permissions could > share a single vmalloc special permission flush implementation that works= like > Andy described originally. It could be controlled with an > ARCH_HAS_DIRECT_MAP_PERMS. We would just need something like set_pages_np= and > set_pages_rw on any archs with directmap permissions. So seems simpler to= me > (and what I have been doing) unless I'm missing the problem. Hmm. The only reason I've proposed anything fancier was because I was thinking of minimizing flushes, but I think I'm being silly. This sequence ought to work optimally: - vmalloc(..., VM_HAS_DIRECT_MAP_PERMS); /* no flushes */ - Write some data, via vmalloc's return address. - Use some set_memory_whatever() functions to update permissions, which will flush, hopefully just once. - Run the module code! - vunmap -- this will do a single flush that will fix everything. This does require that set_pages_np() or set_memory_np() or whatever exists and that it's safe to do that, then flush, and then set_pages_rw(). So maybe you want set_pages_np_noflush() and set_pages_rw_noflush() to make it totally clear what's supposed to happen. --Andy