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Tue, 04 Dec 2018 16:53:06 -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: <3dc0492f209c630e40e93e9c657722041da0ed29.camel@intel.com> Date: Tue, 4 Dec 2018 16:53:03 -0800 Cc: "linux-kernel@vger.kernel.org" , "daniel@iogearbox.net" , "ard.biesheuvel@linaro.org" , "jeyu@kernel.org" , "rostedt@goodmis.org" , "linux-mm@kvack.org" , "jannh@google.com" , "ast@kernel.org" , "Dock, Deneen T" , "peterz@infradead.org" , "kristen@linux.intel.com" , "akpm@linux-foundation.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" 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> <843E4326-3426-4AEC-B0F7-2DC398A6E59A@gmail.com> <3dc0492f209c630e40e93e9c657722041da0ed29.camel@intel.com> To: "Edgecombe, Rick P" X-Mailer: Apple Mail (2.3445.101.1) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Dec 4, 2018, at 4:29 PM, Edgecombe, Rick P = wrote: >=20 > On Tue, 2018-12-04 at 16:01 -0800, Nadav Amit wrote: >>> 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 < >>>>> rick.p.edgecombe@intel.com> >>>>> 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=99= 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??? >>>>>>=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. >>>=20 >>> 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. >>>=20 >>> 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(). >>>=20 >>> 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. >>=20 >> As long as you do it, I have no problem ;-) >>=20 >> 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=9Cgood luck with all >> that=E2=80=9D. > I thought you were trying to prevent having any memory that at any = time was W+X, > how does vfree help with the module load time issues, where it starts = WRX on > x86? I didn=E2=80=99t say it does. The patch I submitted before [1] should = deal with the issue of module loading, and I still think it is required. I also = addressed the kprobe and ftrace issues that you raised. Perhaps it makes more sense that I will include the patch I proposed for module cleanup to make the patch-set =E2=80=9Ccomplete=E2=80=9D. If you = finish the changes you propose before the patch is applied, it could be dropped. I just = want to get rid of this series, as it keeps collecting more and more patches. I suspect it will not be the last version anyhow. [1] https://lkml.org/lkml/2018/11/21/305=