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

[Qian Cai <cai@lca.pw>]

On Fri, 2019-06-21 at 12:39 +0200, Alexander Potapenko wrote:
> On Fri, Jun 21, 2019 at 3:01 AM Kees Cook <keescook@chromium.org> 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.

> 
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> > 
> > -Kees
> > 
> > > 
> > > [1] https://patchwork.kernel.org/patch/10999465/
> > > 
> > > Signed-off-by: Qian Cai <cai@lca.pw>
> > > ---
> > > 
> > > 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
> 
> 
>