Re: [PATCH 0/3] kasan: Fix ordering between MTE tag colouring and page->flags

[Catalin Marinas <catalin.marinas@arm.com>]

On Sun, May 22, 2022 at 12:20:26AM +0200, Andrey Konovalov wrote:
> On Fri, May 20, 2022 at 3:01 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > This will reset the tags for all kinds of GFP_USER allocations, not
> > > only for the ones intended for MAP_ANONYMOUS and RAM-based file
> > > mappings, for which userspace can set tags, right? This will thus
> > > weaken in-kernel MTE for pages whose tags can't even be set by
> > > userspace. Is there a way to deal with this?
> >
> > That's correct, it will weaken some of the allocations where the user
> > doesn't care about MTE.
> 
> Well, while this is unfortunate, I don't mind the change.
> 
> I've left some comments on the patches.

Thanks. I'll update and post at -rc1.

> > > > Since clearing the flags in the arch code doesn't work, try to do this
> > > > at page allocation time by a new flag added to GFP_USER.
> 
> Does this have to be GFP_USER? Can we add new flags to
> GFP_HIGHUSER_MOVABLE instead?
> 
> For instance, Peter added __GFP_SKIP_KASAN_POISON to
> GFP_HIGHUSER_MOVABLE in c275c5c6d50a0.

The above commit was a performance improvement. Here we need to address
the correctness. However, looking through the GFP_USER cases, I don't
think any of them is at risk of ending up in user space with PROT_MTE.
There are places where GFP_USER is passed to kmalloc() for in-kernel
objects that would never be mapped to user, though the new gfp flag
won't be taken into account.

I'm ok to move the new flag to the GFP_HIGHUSER_MOVABLE but probably
still keep a page_kasan_tag_reset() on the set_pte_at() path together
with a WARN_ON_ONCE() if we miss anything.

> > > > Could we
> > > > instead add __GFP_SKIP_KASAN_UNPOISON rather than a new flag?
> 
> Adding __GFP_SKIP_KASAN_UNPOISON makes sense, but we still need to
> reset the tag in page->flags.

My thought was to reset the tag in page->flags based on 'unpoison'
alone without any extra flags. We use this flag for vmalloc() pages but
it seems we don't reset the page tags (as we do via
kasan_poison_slab()).

-- 
Catalin