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@xxxxxxxxxxxx> 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
