On Wed, Oct 23, 2019 at 10:20:14AM +0200, David Hildenbrand wrote:
> On 22.10.19 19:12, David Hildenbrand wrote:
> > Right now, ZONE_DEVICE memory is always set PG_reserved. We want to
> > change that.
> >
> > Let's make sure that the logic in the function won't change. Once we no
> > longer set these pages to reserved, we can rework this function to
> > perform separate checks for ZONE_DEVICE (split from PG_reserved checks).
> >
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Kate Stewart <kstewart@xxxxxxxxxxxxxxxxxxx>
> > Cc: Allison Randal <allison@xxxxxxxxxxx>
> > Cc: "Isaac J. Manjarres" <isaacm@xxxxxxxxxxxxxx>
> > Cc: Qian Cai <cai@xxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> > mm/usercopy.c | 5 +++--
> > 1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/usercopy.c b/mm/usercopy.c
> > index 660717a1ea5c..a3ac4be35cde 100644
> > --- a/mm/usercopy.c
> > +++ b/mm/usercopy.c
> > @@ -203,14 +203,15 @@ static inline void check_page_span(const void *ptr, unsigned long n,
> > * device memory), or CMA. Otherwise, reject since the object spans
> > * several independently allocated pages.
> > */
> > - is_reserved = PageReserved(page);
> > + is_reserved = PageReserved(page) || is_zone_device_page(page);
> > is_cma = is_migrate_cma_page(page);
> > if (!is_reserved && !is_cma)
> > usercopy_abort("spans multiple pages", NULL, to_user, 0, n);
> > for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
> > page = virt_to_head_page(ptr);
> > - if (is_reserved && !PageReserved(page))
> > + if (is_reserved && !(PageReserved(page) ||
> > + is_zone_device_page(page)))
> > usercopy_abort("spans Reserved and non-Reserved pages",
> > NULL, to_user, 0, n);
> > if (is_cma && !is_migrate_cma_page(page))
> >
>
> @Kees, would it be okay to stop checking against ZONE_DEVICE pages here or
> is there a good rationale behind this?
>
> (I would turn this patch into a simple update of the comment if we agree
> that we don't care)
There has been work to actually remove the page span checks entirely,
but there wasn't consensus on what the right way forward was. I continue
to leaning toward just dropping it entirely, but Matthew Wilcox has some
alternative ideas that could use some further thought/testing.
--
Kees Cook
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