Date: Fri, 17 May 2019 19:11:15 +0200 From: Michal Hocko <mhocko@...nel.org> To: Kees Cook <keescook@...omium.org> Cc: Alexander Potapenko <glider@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Masahiro Yamada <yamada.masahiro@...ionext.com>, James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, Nick Desaulniers <ndesaulniers@...gle.com>, Kostya Serebryany <kcc@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Sandeep Patil <sspatil@...roid.com>, Laura Abbott <labbott@...hat.com>, Randy Dunlap <rdunlap@...radead.org>, Jann Horn <jannh@...gle.com>, Mark Rutland <mark.rutland@....com>, Linux Memory Management List <linux-mm@...ck.org>, linux-security-module <linux-security-module@...r.kernel.org> Subject: Re: [PATCH v2 1/4] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options On Fri 17-05-19 09:36:36, Kees Cook wrote: > On Fri, May 17, 2019 at 04:20:48PM +0200, Michal Hocko wrote: > > On Fri 17-05-19 16:11:32, Alexander Potapenko wrote: > > > On Fri, May 17, 2019 at 4:04 PM Michal Hocko <mhocko@...nel.org> wrote: > > > > > > > > On Tue 14-05-19 16:35:34, Alexander Potapenko wrote: > > > > > The new options are needed to prevent possible information leaks and > > > > > make control-flow bugs that depend on uninitialized values more > > > > > deterministic. > > > > > > > > > > init_on_alloc=1 makes the kernel initialize newly allocated pages and heap > > > > > objects with zeroes. Initialization is done at allocation time at the > > > > > places where checks for __GFP_ZERO are performed. > > > > > > > > > > init_on_free=1 makes the kernel initialize freed pages and heap objects > > > > > with zeroes upon their deletion. This helps to ensure sensitive data > > > > > doesn't leak via use-after-free accesses. > > > > > > > > Why do we need both? The later is more robust because even free memory > > > > cannot be sniffed and the overhead might be shifted from the allocation > > > > context (e.g. to RCU) but why cannot we stick to a single model? > > > init_on_free appears to be slower because of cache effects. It's > > > several % in the best case vs. <1% for init_on_alloc. > > > > This doesn't really explain why we need both. > > There are a couple reasons. The first is that once we have hardware with > memory tagging (e.g. arm64's MTE) we'll need both on_alloc and on_free > hooks to do change the tags. With MTE, zeroing comes for "free" with > tagging (though tagging is as slow as zeroing, so it's really the tagging > that is free...), so we'll need to re-use the init_on_free infrastructure. I am not sure I follow, but ... > > The second reason is for very paranoid use-cases where in-memory > data lifetime is desired to be minimized. There are various arguments > for/against the realism of the associated threat models, but given that > we'll need the infrastructre for MTE anyway, and there are people who > want wipe-on-free behavior no matter what the performance cost, it seems > reasonable to include it in this series. > > All that said, init_on_alloc looks desirable enough that distros will > likely build with it enabled by default (I hope), and the very paranoid > users will switch to (or additionally enable) init_on_free for their > systems. ... this should all be part of the changelog. -- Michal Hocko SUSE Labs
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