Re: [PATCH -next v2] mm/page_alloc: fix a false memory corruption

[Alexander Potapenko]

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?

> 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

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