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=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham 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 B27B6C04EB8 for ; Thu, 6 Dec 2018 11:10:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6CD3F20838 for ; Thu, 6 Dec 2018 11:10:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6CD3F20838 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729592AbeLFLKj (ORCPT ); Thu, 6 Dec 2018 06:10:39 -0500 Received: from foss.arm.com ([217.140.101.70]:47576 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727575AbeLFLKj (ORCPT ); Thu, 6 Dec 2018 06:10:39 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 028CF80D; Thu, 6 Dec 2018 03:10:38 -0800 (PST) Received: from edgewater-inn.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id C30103F5AF; Thu, 6 Dec 2018 03:10:37 -0800 (PST) Received: by edgewater-inn.cambridge.arm.com (Postfix, from userid 1000) id CC29C1AE0A0D; Thu, 6 Dec 2018 11:10:58 +0000 (GMT) Date: Thu, 6 Dec 2018 11:10:58 +0000 From: Will Deacon To: Ard Biesheuvel Cc: Andy Lutomirski , Rick Edgecombe , nadav.amit@gmail.com, Linux Kernel Mailing List , Daniel Borkmann , Jessica Yu , Steven Rostedt , Alexei Starovoitov , Linux-MM , Jann Horn , "Dock, Deneen T" , Peter Zijlstra , kristen@linux.intel.com, Andrew Morton , Ingo Molnar , anil.s.keshavamurthy@intel.com, Kernel Hardening , Masami Hiramatsu , naveen.n.rao@linux.vnet.ibm.com, "David S. Miller" , "" , Dave Hansen Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages Message-ID: <20181206111058.GD23697@arm.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 06, 2018 at 08:29:03AM +0100, Ard Biesheuvel wrote: > On Thu, 6 Dec 2018 at 00:16, Andy Lutomirski wrote: > > > > On Wed, Dec 5, 2018 at 3:41 AM Will Deacon wrote: > > > > > > On Tue, Dec 04, 2018 at 12:09:49PM -0800, Andy Lutomirski wrote: > > > > On Tue, Dec 4, 2018 at 12:02 PM Edgecombe, Rick P > > > > wrote: > > > > > > > > > > On Tue, 2018-12-04 at 16:03 +0000, Will Deacon wrote: > > > > > > On Mon, Dec 03, 2018 at 05:43:11PM -0800, Nadav Amit wrote: > > > > > > > > On Nov 27, 2018, at 4:07 PM, Rick Edgecombe > > > > > > > > wrote: > > > > > > > > > > > > > > > > Since vfree will lazily flush the TLB, but not lazily free the underlying > > > > > > > > pages, > > > > > > > > it often leaves stale TLB entries to freed pages that could get re-used. > > > > > > > > This is > > > > > > > > undesirable for cases where the memory being freed has special permissions > > > > > > > > such > > > > > > > > as executable. > > > > > > > > > > > > > > So I am trying to finish my patch-set for preventing transient W+X mappings > > > > > > > from taking space, by handling kprobes & ftrace that I missed (thanks again > > > > > > > for > > > > > > > pointing it out). > > > > > > > > > > > > > > But all of the sudden, I don’t understand why we have the problem that this > > > > > > > (your) patch-set deals with at all. We already change the mappings to make > > > > > > > the memory wrAcked-by: Ard Biesheuvel > itable before freeing the memory, so why can’t we make it > > > > > > > non-executable at the same time? Actually, why do we make the module memory, > > > > > > > including its data executable before freeing it??? > > > > > > > > > > > > Yeah, this is really confusing, but I have a suspicion it's a combination > > > > > > of the various different configurations and hysterical raisins. We can't > > > > > > rely on module_alloc() allocating from the vmalloc area (see nios2) nor > > > > > > can we rely on disable_ro_nx() being available at build time. > > > > > > > > > > > > If we *could* rely on module allocations always using vmalloc(), then > > > > > > we could pass in Rick's new flag and drop disable_ro_nx() altogether > > > > > > afaict -- who cares about the memory attributes of a mapping that's about > > > > > > to disappear anyway? > > > > > > > > > > > > Is it just nios2 that does something different? > > > > > > > > > > > Yea it is really intertwined. I think for x86, set_memory_nx everywhere would > > > > > solve it as well, in fact that was what I first thought the solution should be > > > > > until this was suggested. It's interesting that from the other thread Masami > > > > > Hiramatsu referenced, set_memory_nx was suggested last year and would have > > > > > inadvertently blocked this on x86. But, on the other architectures I have since > > > > > learned it is a bit different. > > > > > > > > > > It looks like actually most arch's don't re-define set_memory_*, and so all of > > > > > the frob_* functions are actually just noops. In which case allocating RWX is > > > > > needed to make it work at all, because that is what the allocation is going to > > > > > stay at. So in these archs, set_memory_nx won't solve it because it will do > > > > > nothing. > > > > > > > > > > On x86 I think you cannot get rid of disable_ro_nx fully because there is the > > > > > changing of the permissions on the directmap as well. You don't want some other > > > > > caller getting a page that was left RO when freed and then trying to write to > > > > > it, if I understand this. > > > > > > > > > > > > > Exactly. > > > > > > Of course, I forgot about the linear mapping. On arm64, we've just queued > > > support for reflecting changes to read-only permissions in the linear map > > > [1]. So, whilst the linear map is always non-executable, we will need to > > > make parts of it writable again when freeing the module. > > > > > > > After slightly more thought, I suggest renaming VM_IMMEDIATE_UNMAP to > > > > VM_MAY_ADJUST_PERMS or similar. It would have the semantics you want, > > > > but it would also call some arch hooks to put back the direct map > > > > permissions before the flush. Does that seem reasonable? It would > > > > need to be hooked up that implement set_memory_ro(), but that should > > > > be quite easy. If nothing else, it could fall back to set_memory_ro() > > > > in the absence of a better implementation. > > > > > > You mean set_memory_rw() here, right? Although, eliding the TLB invalidation > > > would open up a window where the vmap mapping is executable and the linear > > > mapping is writable, which is a bit rubbish. > > > > > > > Right, and Rick pointed out the same issue. Instead, we should set > > the direct map not-present or its ARM equivalent, then do the flush, > > then make it RW. I assume this also works on arm and arm64, although > > I don't know for sure. On x86, the CPU won't cache not-present PTEs. > > If we are going to unmap the linear alias, why not do it at vmalloc() > time rather than vfree() time? Right, that should be pretty straightforward. We're basically saying that RO in the vmalloc area implies PROT_NONE in the linear map, so we could just do this in our set_memory_ro() function. Will From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages Date: Thu, 6 Dec 2018 11:10:58 +0000 Message-ID: <20181206111058.GD23697@arm.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: Andy Lutomirski , Rick Edgecombe , nadav.amit@gmail.com, Linux Kernel Mailing List , Daniel Borkmann , Jessica Yu , Steven Rostedt , Alexei Starovoitov , Linux-MM , Jann Horn , "Dock, Deneen T" , Peter Zijlstra , kristen@linux.intel.com, Andrew Morton , Ingo Molnar , anil.s.keshavamurthy@intel.com, Kernel Hardening , Masami Hiramatsu , naveen.n.rao@linux.vnet.ibm.com, "David S. Miller" , " Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Thu, Dec 06, 2018 at 08:29:03AM +0100, Ard Biesheuvel wrote: > On Thu, 6 Dec 2018 at 00:16, Andy Lutomirski wrote: > > > > On Wed, Dec 5, 2018 at 3:41 AM Will Deacon wrote: > > > > > > On Tue, Dec 04, 2018 at 12:09:49PM -0800, Andy Lutomirski wrote: > > > > On Tue, Dec 4, 2018 at 12:02 PM Edgecombe, Rick P > > > > wrote: > > > > > > > > > > On Tue, 2018-12-04 at 16:03 +0000, Will Deacon wrote: > > > > > > On Mon, Dec 03, 2018 at 05:43:11PM -0800, Nadav Amit wrote: > > > > > > > > On Nov 27, 2018, at 4:07 PM, Rick Edgecombe > > > > > > > > wrote: > > > > > > > > > > > > > > > > Since vfree will lazily flush the TLB, but not lazily free the underlying > > > > > > > > pages, > > > > > > > > it often leaves stale TLB entries to freed pages that could get re-used. > > > > > > > > This is > > > > > > > > undesirable for cases where the memory being freed has special permissions > > > > > > > > such > > > > > > > > as executable. > > > > > > > > > > > > > > So I am trying to finish my patch-set for preventing transient W+X mappings > > > > > > > from taking space, by handling kprobes & ftrace that I missed (thanks again > > > > > > > for > > > > > > > pointing it out). > > > > > > > > > > > > > > But all of the sudden, I don’t understand why we have the problem that this > > > > > > > (your) patch-set deals with at all. We already change the mappings to make > > > > > > > the memory wrAcked-by: Ard Biesheuvel > itable before freeing the memory, so why can’t we make it > > > > > > > non-executable at the same time? Actually, why do we make the module memory, > > > > > > > including its data executable before freeing it??? > > > > > > > > > > > > Yeah, this is really confusing, but I have a suspicion it's a combination > > > > > > of the various different configurations and hysterical raisins. We can't > > > > > > rely on module_alloc() allocating from the vmalloc area (see nios2) nor > > > > > > can we rely on disable_ro_nx() being available at build time. > > > > > > > > > > > > If we *could* rely on module allocations always using vmalloc(), then > > > > > > we could pass in Rick's new flag and drop disable_ro_nx() altogether > > > > > > afaict -- who cares about the memory attributes of a mapping that's about > > > > > > to disappear anyway? > > > > > > > > > > > > Is it just nios2 that does something different? > > > > > > > > > > > Yea it is really intertwined. I think for x86, set_memory_nx everywhere would > > > > > solve it as well, in fact that was what I first thought the solution should be > > > > > until this was suggested. It's interesting that from the other thread Masami > > > > > Hiramatsu referenced, set_memory_nx was suggested last year and would have > > > > > inadvertently blocked this on x86. But, on the other architectures I have since > > > > > learned it is a bit different. > > > > > > > > > > It looks like actually most arch's don't re-define set_memory_*, and so all of > > > > > the frob_* functions are actually just noops. In which case allocating RWX is > > > > > needed to make it work at all, because that is what the allocation is going to > > > > > stay at. So in these archs, set_memory_nx won't solve it because it will do > > > > > nothing. > > > > > > > > > > On x86 I think you cannot get rid of disable_ro_nx fully because there is the > > > > > changing of the permissions on the directmap as well. You don't want some other > > > > > caller getting a page that was left RO when freed and then trying to write to > > > > > it, if I understand this. > > > > > > > > > > > > > Exactly. > > > > > > Of course, I forgot about the linear mapping. On arm64, we've just queued > > > support for reflecting changes to read-only permissions in the linear map > > > [1]. So, whilst the linear map is always non-executable, we will need to > > > make parts of it writable again when freeing the module. > > > > > > > After slightly more thought, I suggest renaming VM_IMMEDIATE_UNMAP to > > > > VM_MAY_ADJUST_PERMS or similar. It would have the semantics you want, > > > > but it would also call some arch hooks to put back the direct map > > > > permissions before the flush. Does that seem reasonable? It would > > > > need to be hooked up that implement set_memory_ro(), but that should > > > > be quite easy. If nothing else, it could fall back to set_memory_ro() > > > > in the absence of a better implementation. > > > > > > You mean set_memory_rw() here, right? Although, eliding the TLB invalidation > > > would open up a window where the vmap mapping is executable and the linear > > > mapping is writable, which is a bit rubbish. > > > > > > > Right, and Rick pointed out the same issue. Instead, we should set > > the direct map not-present or its ARM equivalent, then do the flush, > > then make it RW. I assume this also works on arm and arm64, although > > I don't know for sure. On x86, the CPU won't cache not-present PTEs. > > If we are going to unmap the linear alias, why not do it at vmalloc() > time rather than vfree() time? Right, that should be pretty straightforward. We're basically saying that RO in the vmalloc area implies PROT_NONE in the linear map, so we could just do this in our set_memory_ro() function. Will