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Tue, 04 Dec 2018 17:45:06 -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: <3c217322e990eba0269cc5ffea761cc1a5b17f4e.camel@intel.com> Date: Tue, 4 Dec 2018 17:45:04 -0800 Cc: "luto@kernel.org" , "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" , "will.deacon@arm.com" , "mingo@redhat.com" , "kernel-hardening@lists.openwall.com" , "Keshavamurthy, Anil S" , "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> <08141F66-F3E6-4CC5-AF91-1ED5F101A54C@gmail.com> <20CC2F71-308D-42E2-8C54-F64D7CC3863F@gmail.com> <3c217322e990eba0269cc5ffea761cc1a5b17f4e.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 5:09 PM, Edgecombe, Rick P = wrote: >=20 > On Tue, 2018-12-04 at 14:48 -0800, Nadav Amit wrote: >>> On Dec 4, 2018, at 11:48 AM, Andy Lutomirski = wrote: >>>=20 >>> On Tue, Dec 4, 2018 at 11:45 AM Nadav Amit = wrote: >>>>> On Dec 4, 2018, at 10:56 AM, Andy Lutomirski = wrote: >>>>>=20 >>>>> On Mon, Dec 3, 2018 at 5:43 PM 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 >>>>> 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. >>>>>=20 >>>>> 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? >>>>>=20 >>>>> 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. >>>>=20 >>>> Thanks for the explanation. I read it just after I realized that = indeed >>>> the >>>> whole purpose of this code is to get cpa_process_alias() >>>> update the corresponding direct mapping. >>>>=20 >>>> 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. >>>>=20 >>>> 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 >>>> module >>>> memory. Does it ever stop being W+X?? I=E2=80=99ll have another = look. >>>=20 >>> 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. >>=20 >> This piece of code causes me pain and misery. >>=20 >> So, it turns out that the direct map is *not* changed if you just = change >> the NX-bit. See change_page_attr_set_clr(): >>=20 >> /* No alias checking for _NX bit modifications */ >> checkalias =3D (pgprot_val(mask_set) | pgprot_val(mask_clr)) = !=3D >> _PAGE_NX; >>=20 >> How many levels of abstraction are broken in the way? What would = happen >> if somebody tries to change the NX-bit and some other bit in the PTE? >> Luckily, I don=E2=80=99t think someone does=E2=80=A6 at least for = now. >>=20 >> So, again, I think the change I proposed makes sense. nios2 does not = have >> set_memory_x() and it will not be affected. > Hold on...so on architectures that don't have set_memory_ but do have = something > like NX, wont the executable stale TLB continue to live to re-used = pages, and so > it doesn't fix the problem this patch is trying to address generally? = I see at > least a couple archs use vmalloc and have an exec bit, but don't = define > set_memory_*. Again, this does not come instead of your patch (the one in this = thread). And if you follow Andy=E2=80=99s suggestion, the patch I propose will = not be needed. However, in the meantime - I see no reason to mark data as executable, = even for a brief period of time.