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 D88D96B7063 for ; Tue, 4 Dec 2018 14:48:57 -0500 (EST) Received: by mail-pl1-f197.google.com with SMTP id y2so13220644plr.8 for ; Tue, 04 Dec 2018 11:48:57 -0800 (PST) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id z14si15813374pgj.73.2018.12.04.11.48.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 04 Dec 2018 11:48:56 -0800 (PST) Received: from mail-wm1-f45.google.com (mail-wm1-f45.google.com [209.85.128.45]) (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 E5F9D20878 for ; Tue, 4 Dec 2018 19:48:55 +0000 (UTC) Received: by mail-wm1-f45.google.com with SMTP id c126so10562561wmh.0 for ; Tue, 04 Dec 2018 11:48:55 -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> <08141F66-F3E6-4CC5-AF91-1ED5F101A54C@gmail.com> In-Reply-To: <08141F66-F3E6-4CC5-AF91-1ED5F101A54C@gmail.com> From: Andy Lutomirski Date: Tue, 4 Dec 2018 11:48:42 -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: Nadav Amit Cc: Andrew Lutomirski , Rick Edgecombe , Andrew Morton , Will Deacon , Linux-MM , LKML , Kernel Hardening , "Naveen N . Rao" , Anil S Keshavamurthy , "David S. Miller" , Masami Hiramatsu , Steven Rostedt , Ingo Molnar , Alexei Starovoitov , Daniel Borkmann , jeyu@kernel.org, Network Development , Ard Biesheuvel , Jann Horn , Kristen Carlson Accardi , Dave Hansen , "Dock, Deneen T" , Peter Zijlstra On Tue, Dec 4, 2018 at 11:45 AM Nadav Amit wrote: > > > On Dec 4, 2018, at 10:56 AM, Andy Lutomirski wrote: > > > > On Mon, Dec 3, 2018 at 5:43 PM 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 underl= ying pages, > >>> it often leaves stale TLB entries to freed pages that could get re-us= ed. This is > >>> undesirable for cases where the memory being freed has special permis= sions such > >>> as executable. > >> > >> So I am trying to finish my patch-set for preventing transient W+X map= pings > >> from taking space, by handling kprobes & ftrace that I missed (thanks = again for > >> pointing it out). > >> > >> But all of the sudden, I don=E2=80=99t understand why we have the prob= lem that this > >> (your) patch-set deals with at all. We already change the mappings to = make > >> the memory writable before freeing the memory, so why can=E2=80=99t we= make it > >> non-executable at the same time? Actually, why do we make the module m= emory, > >> including its data executable before freeing it??? > > > > All the code you're looking at is IMO a very awkward and possibly > > incorrect of doing what's actually necessary: putting the direct map > > the way it wants to be. > > > > Can't we shove this entirely mess into vunmap? Have a flag (as part > > of vmalloc like in Rick's patch or as a flag passed to a vfree variant > > directly) that makes the vunmap code that frees the underlying pages > > also reset their permissions? > > > > Right now, we muck with set_memory_rw() and set_memory_nx(), which > > both have very awkward (and inconsistent with each other!) semantics > > when called on vmalloc memory. And they have their own flushes, which > > is inefficient. Maybe the right solution is for vunmap to remove the > > vmap area PTEs, call into a function like set_memory_rw() that resets > > the direct maps to their default permissions *without* flushing, and > > then to do a single flush for everything. Or, even better, to cause > > the change_page_attr code to do the flush and also to flush the vmap > > area all at once so that very small free operations can flush single > > pages instead of flushing globally. > > Thanks for the explanation. I read it just after I realized that indeed t= he > whole purpose of this code is to get cpa_process_alias() > update the corresponding direct mapping. > > This thing (pageattr.c) indeed seems over-engineered and very unintuitive= . > Right now I have a list of patch-sets that I owe, so I don=E2=80=99t have= the time > to deal with it. > > But, I still think that disable_ro_nx() should not call set_memory_x(). > IIUC, this breaks W+X of the direct-mapping which correspond with the mod= ule > memory. Does it ever stop being W+X?? I=E2=80=99ll have another look. > Dunno. I did once chase down a bug where some memory got freed while it was still read-only, and the results were hilarious and hard to debug, since the explosion happened long after the buggy code finished. --Andy