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([2600:1010:b05a:3e8f:cd65:4915:ea8d:fbab]) by smtp.gmail.com with ESMTPSA id g3sm45387367pfe.37.2018.12.04.17.57.27 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 04 Dec 2018 17:57:27 -0800 (PST) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (1.0) Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages From: Andy Lutomirski X-Mailer: iPhone Mail (16B92) In-Reply-To: <58a3b01c78b6c299f76c156f96211ff22ec28751.camel@intel.com> Date: Tue, 4 Dec 2018 17:57:26 -0800 Cc: "luto@kernel.org" , "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" , "nadav.amit@gmail.com" , "Dock, Deneen T" , "jannh@google.com" , "kristen@linux.intel.com" , "akpm@linux-foundation.org" , "peterz@infradead.org" , "will.deacon@arm.com" , "mingo@redhat.com" , "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: <1913CD9F-B912-490A-8DEC-8C24CFF0F6D6@amacapital.net> 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> <58a3b01c78b6c299f76c156f96211ff22ec28751.camel@intel.com> To: "Edgecombe, Rick P" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Dec 4, 2018, at 3:52 PM, Edgecombe, Rick P = wrote: >=20 >> On Tue, 2018-12-04 at 12:09 -0800, Andy Lutomirski wrote: >> On Tue, 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 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 >>>>> memory, >>>>> including its data executable before freeing it??? >>>>=20 >>>> Yeah, this is really confusing, but I have a suspicion it's a combinati= on >>>> 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 abo= ut >>>> 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 sho= uld >>> be >>> until this was suggested. It's interesting that from the other thread Ma= sami >>> Hiramatsu referenced, set_memory_nx was suggested last year and would ha= ve >>> inadvertently blocked this on x86. But, on the other architectures I hav= e >>> since >>> learned it is a bit different. >>>=20 >>> It looks like actually most arch's don't re-define set_memory_*, and so a= ll >>> of >>> the frob_* functions are actually just noops. In which case allocating R= WX >>> is >>> needed to make it work at all, because that is what the allocation is go= ing >>> 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 i= s >>> the >>> changing of the permissions on the directmap as well. You don't want som= e >>> other >>> caller getting a page that was left RO when freed and then trying to wri= te >>> to >>> it, if I understand this. >>>=20 >>=20 >> Exactly. >>=20 >> 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. >=20 > With arch hooks, I guess we could remove disable_ro_nx then. I think you w= ould > still have to flush twice on x86 to really have no W^X violating window fr= om the > direct map (I think x86 is the only one that sets permissions there?). But= this > could be down from sometimes 3. You could also directly vfree non exec RO m= emory > without set_memory_, like in BPF.=20 Just one flush if you=E2=80=99re careful. Set the memory not-present in the d= irect map and zap it from the vmap area, then flush, then set it RW in the=20= >=20 > The vfree deferred list would need to be moved since it then couldn't reus= e the > allocations since now the vfreed memory might be RO. It could kmalloc, or l= ookup > the vm_struct. So would probably be a little slower in the interrupt case.= Is > this ok? I=E2=80=99m fine with that. For eBPF, we should really have a lookaside list= for small allocations.