From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by kanga.kvack.org (Postfix) with ESMTP id 282126B716C for ; Tue, 4 Dec 2018 19:01:48 -0500 (EST) Received: by mail-wr1-f69.google.com with SMTP id q7so13841360wrw.8 for ; Tue, 04 Dec 2018 16:01:48 -0800 (PST) Received: from mail-sor-f41.google.com (mail-sor-f41.google.com. [209.85.220.41]) by mx.google.com with SMTPS id o4sor6398306wrj.2.2018.12.04.16.01.46 for (Google Transport Security); Tue, 04 Dec 2018 16:01:46 -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: Tue, 4 Dec 2018 16:01:38 -0800 Content-Transfer-Encoding: quoted-printable Message-Id: <843E4326-3426-4AEC-B0F7-2DC398A6E59A@gmail.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> Sender: owner-linux-mm@kvack.org List-ID: To: "Edgecombe, Rick P" Cc: "linux-kernel@vger.kernel.org" , "daniel@iogearbox.net" , "ard.biesheuvel@linaro.org" , "jeyu@kernel.org" , "rostedt@goodmis.org" , "ast@kernel.org" , "linux-mm@kvack.org" , "Dock, Deneen T" , "jannh@google.com" , "kristen@linux.intel.com" , "akpm@linux-foundation.org" , "peterz@infradead.org" , "will.deacon@arm.com" , "mingo@redhat.com" , "luto@kernel.org" , "Keshavamurthy, Anil S" , "kernel-hardening@lists.openwall.com" , "mhiramat@kernel.org" , "naveen.n.rao@linux.vnet.ibm.com" , "davem@davemloft.net" , "netdev@vger.kernel.org" , "Hansen, Dave" > On Dec 4, 2018, at 3:51 PM, Edgecombe, Rick P = wrote: >=20 > On Tue, 2018-12-04 at 12:36 -0800, Nadav Amit wrote: >>> On Dec 4, 2018, at 12:02 PM, Edgecombe, Rick P = >>> wrote: >>>=20 >>> 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 < >>>>>> rick.p.edgecombe@intel.com> >>>>>> wrote: >>>>>>=20 >>>>>> 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. >>>>>=20 >>>>> 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). >>>>>=20 >>>>> But all of the sudden, I don=E2=80=99t understand why we have the = problem 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 >>>>> memory, >>>>> including its data executable before freeing it??? >>>>=20 >>>> 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. >>>>=20 >>>> 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? >>>>=20 >>>> Is it just nios2 that does something different? >>>>=20 >>>> Will >>>=20 >>> 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. >>>=20 >>> 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. >>>=20 >>> 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. >>>=20 >>> The other reasoning was that calling set_memory_nx isn't doing what = we are >>> actually trying to do which is prevent the pages from getting = released too >>> early. >>>=20 >>> A more clear solution for all of this might involve refactoring some = of the >>> set_memory_ de-allocation logic out into __weak functions in either = modules >>> or >>> vmalloc. As Jessica points out in the other thread though, modules = does a >>> lot >>> more stuff there than the other module_alloc callers. I think it may = take >>> some >>> thought to centralize AND make it optimal for every >>> module_alloc/vmalloc_exec >>> user and arch. >>>=20 >>> But for now with the change in vmalloc, we can block the executable = mapping >>> freed page re-use issue in a cross platform way. >>=20 >> Please understand me correctly - I didn=E2=80=99t mean that your = patches are not >> needed. > Ok, I think I understand. I have been pondering these same things = after Masami > Hiramatsu's comments on this thread the other day. >=20 >> All I did is asking - how come the PTEs are executable when they are = cleared >> they are executable, when in fact we manipulate them when the module = is >> removed. > I think the directmap used to be RWX so maybe historically its trying = to return > it to its default state? Not sure. >=20 >> I think I try to deal with a similar problem to the one you encounter = - >> broken W^X. The only thing that bothered me in regard to your patches = (and >> only after I played with the code) is that there is still a = time-window in >> which W^X is broken due to disable_ro_nx(). > Totally agree there is overlap in the fixes and we should sync. >=20 > What do you think about Andy's suggestion for doing the vfree cleanup = in vmalloc > with arch hooks? So the allocation goes into vfree fully setup and = vmalloc frees > it and on x86 resets the direct map. As long as you do it, I have no problem ;-) You would need to consider all the callers of module_memfree(), and = probably to untangle at least part of the mess in pageattr.c . If you are up to = it, just say so, and I=E2=80=99ll drop this patch. All I can say is =E2=80=9Cg= ood luck with all that=E2=80=9D.