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([2601:647:4580:b719:9448:6a51:fd0f:790f]) by smtp.gmail.com with ESMTPSA id g11sm22596415pfo.139.2018.12.04.11.44.58 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 04 Dec 2018 11:45:00 -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 11:44:58 -0800 Cc: Rick Edgecombe , Andrew Morton , Will Deacon , Linux-MM , LKML , Kernel Hardening , "Naveen N . Rao" , Anil S Keshavamurthy , "David S. Miller" , Masami Hiramatsu , Steven Rostedt , Ingo Molnar , Alexei Starovoitov , Daniel Borkmann , jeyu@kernel.org, Network Development , Ard Biesheuvel , Jann Horn , Kristen Carlson Accardi , Dave Hansen , "Dock, Deneen T" , Peter Zijlstra Content-Transfer-Encoding: quoted-printable Message-Id: <08141F66-F3E6-4CC5-AF91-1ED5F101A54C@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> To: Andy Lutomirski 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 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 = 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. 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()=20 update the corresponding direct mapping. 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. 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.