init_on_alloc/init_on_free boot options

[Jirka Hladky]

Thanks a lot for the clarification! I was scratching my head if it
makes sense to enable both options simultaneously.


On Thu, Aug 20, 2020 at 1:36 AM Kees Cook <keescook at chromium.org> wrote:
>
> On Thu, Aug 20, 2020 at 12:18:33AM +0200, Jirka Hladky wrote:
> > Could you please help me to clarify the purpose of init_on_alloc=1
> > when init_on_free is enabled?
>
> It's to zero memory at allocation time. :) (They are independent
> options.)
>
> > If I get it right, init_on_free=1 alone guarantees that the memory
> > returned by the page allocator and SL[AU]B is initialized with zeroes.
>
> No, it's guarantees memory freed by the page/slab allocators are zeroed.
>
> > What is the purpose of init_on_alloc=1 in that case? We are zeroing
> > memory twice, or am I missing something?
>
> If you have both enabled, yes, you will zero twice. (In theory, if you
> have any kind of Use-After-Free/dangling pointers that get written
> through after free and before alloc, those contents wouldn't strictly be
> zero at alloc time without init_on_alloc. But that's pretty rare.
>
> I wouldn't expect many people to run with both options enabled;
> init_on_alloc is more performance-friendly (i.e. cache-friendly), and
> init_on_free minimizes the lifetime of stale data in memory.
>
> It appears that the shipping kernel defaults for several distros (Ubuntu,
> Arch, Debian, others?) and devices (Android, Chrome OS, others?) are using
> init_on_alloc=1. Will Fedora and/or RedHat be joining this trend?  :)
>
> --
> Kees Cook
>


-- 
-Jirka