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

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

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.

> 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.

> 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?

-- 
Catalin