On Fri, 2019-06-21 at 16:37 +0200, Alexander Potapenko wrote: > On Fri, Jun 21, 2019 at 2:26 PM Qian Cai <cai@xxxxxx> wrote: > > > > On Fri, 2019-06-21 at 12:39 +0200, Alexander Potapenko wrote: > > > On Fri, Jun 21, 2019 at 3:01 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > > > > > On Thu, Jun 20, 2019 at 04:46:06PM -0400, Qian Cai wrote: > > > > > The linux-next commit "mm: security: introduce init_on_alloc=1 and > > > > > init_on_free=1 boot options" [1] introduced a false positive when > > > > > init_on_free=1 and page_poison=on, due to the page_poison expects the > > > > > pattern 0xaa when allocating pages which were overwritten by > > > > > init_on_free=1 with 0. > > > > > > > > > > Fix it by switching the order between kernel_init_free_pages() and > > > > > kernel_poison_pages() in free_pages_prepare(). > > > > > > > > Cool; this seems like the right approach. Alexander, what do you think? > > > > > > Can using init_on_free together with page_poison bring any value at all? > > > Isn't it better to decide at boot time which of the two features we're > > > going to enable? > > > > I think the typical use case is people are using init_on_free=1, and then > > decide > > to debug something by enabling page_poison=on. Definitely, don't want > > init_on_free=1 to disable page_poison as the later has additional checking > > in > > the allocation time to make sure that poison pattern set in the free time is > > still there. > > In addition to information lifetime reduction the idea of init_on_free > is to ensure the newly allocated objects have predictable contents. > Therefore it's handy (although not strictly necessary) to keep them > zero-initialized regardless of other boot-time flags. > Right now free_pages_prezeroed() relies on that, though this can be changed. > > On the other hand, since page_poison already initializes freed memory, > we can probably make want_init_on_free() return false in that case to > avoid extra initialization. > > Side note: if we make it possible to switch betwen 0x00 and 0xAA in > init_on_free mode, we can merge it with page_poison, performing the > initialization depending on a boot-time flag and doing heavyweight > checks under a separate config. Yes, that would be great which will reduce code duplication.
