Re: [PATCH] arm64: Also reset KASAN tag if page is not PG_mte_tagged

[Peter Collingbourne <pcc@google.com>]

On Fri, Apr 21, 2023 at 5:24 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Thu, Apr 20, 2023 at 02:09:45PM -0700, Peter Collingbourne wrote:
> > Consider the following sequence of events:
> >
> > 1) A page in a PROT_READ|PROT_WRITE VMA is faulted.
> > 2) Page migration allocates a page with the KASAN allocator,
> >    causing it to receive a non-match-all tag, and uses it
> >    to replace the page faulted in 1.
> > 3) The program uses mprotect() to enable PROT_MTE on the page faulted in 1.
>
> Ah, so there is no race here, it's simply because the page allocation
> for migration has a non-match-all kasan tag in page->flags.
>
> How do we handle the non-migration case with mprotect()? IIRC
> post_alloc_hook() always resets the page->flags since
> GFP_HIGHUSER_MOVABLE has the __GFP_SKIP_KASAN_UNPOISON flag.

Yes, that's how it normally works.

> > As a result of step 3, we are left with a non-match-all tag for a page
> > with tags accessible to userspace, which can lead to the same kind of
> > tag check faults that commit e74a68468062 ("arm64: Reset KASAN tag in
> > copy_highpage with HW tags only") intended to fix.
> >
> > The general invariant that we have for pages in a VMA with VM_MTE_ALLOWED
> > is that they cannot have a non-match-all tag. As a result of step 2, the
> > invariant is broken. This means that the fix in the referenced commit
> > was incomplete and we also need to reset the tag for pages without
> > PG_mte_tagged.
> >
> > Fixes: e5b8d9218951 ("arm64: mte: reset the page tag in page->flags")
>
> This commit was reverted in 20794545c146 (arm64: kasan: Revert "arm64:
> mte: reset the page tag in page->flags"). It looks a bit strange to fix
> it up.

It does seem strange but I think it is correct because that is when
the bug (resetting tag only if PG_mte_tagged) was introduced. The
revert preserved the bug because it did not account for the migration
case, which means that it didn't account for migration+mprotect
either.

> > diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
> > index 4aadcfb01754..a7bb20055ce0 100644
> > --- a/arch/arm64/mm/copypage.c
> > +++ b/arch/arm64/mm/copypage.c
> > @@ -21,9 +21,10 @@ void copy_highpage(struct page *to, struct page *from)
> >
> >       copy_page(kto, kfrom);
> >
> > +     if (kasan_hw_tags_enabled())
> > +             page_kasan_tag_reset(to);
> > +
> >       if (system_supports_mte() && page_mte_tagged(from)) {
> > -             if (kasan_hw_tags_enabled())
> > -                     page_kasan_tag_reset(to);
>
> This should work but can we not do this at allocation time like we do
> for the source page and remove any page_kasan_tag_reset() here
> altogether?

That would be difficult because of the number of different ways that
the page can be allocated. That's why we also decided to reset it here
in commit e74a68468062.

Peter