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Tue, 04 Dec 2018 12:36:46 -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: <51281e69a3722014f718a6840f43b2e6773eed90.camel@intel.com> Date: Tue, 4 Dec 2018 12:36:44 -0800 Cc: "will.deacon@arm.com" , "linux-kernel@vger.kernel.org" , "daniel@iogearbox.net" , "jeyu@kernel.org" , "rostedt@goodmis.org" , "ast@kernel.org" , "ard.biesheuvel@linaro.org" , "linux-mm@kvack.org" , "jannh@google.com" , "Dock, Deneen T" , "peterz@infradead.org" , "kristen@linux.intel.com" , "akpm@linux-foundation.org" , "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> 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 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 = >>>> 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. Please understand me correctly - I didn=E2=80=99t mean that your patches = are not needed. 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 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().