[PATCH v2] kasan, mm: fix crash with HW_TAGS and DEBUG_PAGEALLOC

[Andrey Konovalov]

On Sat, Mar 6, 2021 at 12:50 AM Andrew Morton <akpm at linux-foundation.org> wrote:
>
> On Sat,  6 Mar 2021 00:36:33 +0100 Andrey Konovalov <andreyknvl at google.com> wrote:
>
> > Currently, kasan_free_nondeferred_pages()->kasan_free_pages() is called
> > after debug_pagealloc_unmap_pages(). This causes a crash when
> > debug_pagealloc is enabled, as HW_TAGS KASAN can't set tags on an
> > unmapped page.
> >
> > This patch puts kasan_free_nondeferred_pages() before
> > debug_pagealloc_unmap_pages() and arch_free_page(), which can also make
> > the page unavailable.
> >
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1304,6 +1304,12 @@ static __always_inline bool free_pages_prepare(struct page *page,
> >
> >       kernel_poison_pages(page, 1 << order);
> >
> > +     /*
> > +      * With hardware tag-based KASAN, memory tags must be set before the
> > +      * page becomes unavailable via debug_pagealloc or arch_free_page.
> > +      */
> > +     kasan_free_nondeferred_pages(page, order, fpi_flags);
> > +
> >       /*
> >        * arch_free_page() can make the page's contents inaccessible.  s390
> >        * does this.  So nothing which can access the page's contents should
> > @@ -1313,8 +1319,6 @@ static __always_inline bool free_pages_prepare(struct page *page,
> >
> >       debug_pagealloc_unmap_pages(page, 1 << order);
> >
> > -     kasan_free_nondeferred_pages(page, order, fpi_flags);
> > -
> >       return true;
> >  }
>
> kasan_free_nondeferred_pages() has only two args in current mainline.

Ah, yes, forgot to mention: this goes on top of:

kasan: initialize shadow to TAG_INVALID for SW_TAGS
mm, kasan: don't poison boot memory with tag-based modes

>
> I fixed that in the obvious manner...

Thanks!

If you changed this patch, you'll also need to change the other one though.