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@xxxxxxxxxx> 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
