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

[Alexander Potapenko]

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

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