arm64: Also reset KASAN tag if page is not PG_mte_tagged

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.

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 e74a684680 ("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: e5b8d92189 ("arm64: mte: reset the page tag in page->flags")
Cc: <stable@vger.kernel.org> # 5.15
Link: https://linux-review.googlesource.com/id/I7409cdd41acbcb215c2a7417c1e50d37b875beff
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20230420210945.2313627-1-pcc@google.com
Signed-off-by: Will Deacon <will@kernel.org>

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);
 		/* It's a new page, shouldn't have been tagged yet */
 		WARN_ON_ONCE(!try_page_mte_tagging(to));
 		mte_copy_page_tags(kto, kfrom);