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. > > > > > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > > > > > > -Kees > > > > > > > > > > > [1] https://patchwork.kernel.org/patch/10999465/ > > > > > > > > Signed-off-by: Qian Cai <cai@xxxxxx> > > > > --- > > > > > > > > v2: After further debugging, the issue after switching order is likely a > > > > separate issue as clear_page() should not cause issues with future > > > > accesses. > > > > > > > > mm/page_alloc.c | 3 ++- > > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > > > index 54dacf35d200..32bbd30c5f85 100644 > > > > --- a/mm/page_alloc.c > > > > +++ b/mm/page_alloc.c > > > > @@ -1172,9 +1172,10 @@ static __always_inline bool > > > > free_pages_prepare(struct page *page, > > > > PAGE_SIZE << order); > > > > } > > > > arch_free_page(page, order); > > > > - kernel_poison_pages(page, 1 << order, 0); > > > > if (want_init_on_free()) > > > > kernel_init_free_pages(page, 1 << order); > > > > + > > > > + kernel_poison_pages(page, 1 << order, 0); > > > > if (debug_pagealloc_enabled()) > > > > kernel_map_pages(page, 1 << order, 0); > > > > > > > > -- > > > > 1.8.3.1 > > > > > > > > > > -- > > > Kees Cook > > > > > > -- Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Straße, 33 80636 München Geschäftsführer: Paul Manicle, Halimah DeLaine Prado Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg
