arm64: Reset KASAN tag in copy_highpage with HW tags only

During page migration, the copy_highpage function is used to copy the
page data to the target page. If the source page is a userspace page
with MTE tags, the KASAN tag of the target page must have the match-all
tag in order to avoid tag check faults during subsequent accesses to the
page by the kernel. However, the target page may have been allocated in
a number of ways, some of which will use the KASAN allocator and will
therefore end up setting the KASAN tag to a non-match-all tag. Therefore,
update the target page's KASAN tag to match the source page.

We ended up unintentionally fixing this issue as a result of a bad
merge conflict resolution between commit e059853d14 ("arm64: mte:
Fix/clarify the PG_mte_tagged semantics") and commit 20794545c1 ("arm64:
kasan: Revert "arm64: mte: reset the page tag in page->flags""), which
preserved a tag reset for PG_mte_tagged pages which was considered to be
unnecessary at the time. Because SW tags KASAN uses separate tag storage,
update the code to only reset the tags when HW tags KASAN is enabled.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/If303d8a709438d3ff5af5fd85706505830f52e0c
Reported-by: "Kuan-Ying Lee (李冠穎)" <Kuan-Ying.Lee@mediatek.com>
Cc: <stable@vger.kernel.org> # 6.1
Fixes: 20794545c1 ("arm64: kasan: Revert "arm64: mte: reset the page tag in page->flags"")
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://lore.kernel.org/r/20230215050911.1433132-1-pcc@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/copypage.c

@@ -22,7 +22,8 @@ void copy_highpage(struct page *to, struct page *from)
 	copy_page(kto, kfrom);
 
 	if (system_supports_mte() && page_mte_tagged(from)) {
-		page_kasan_tag_reset(to);
+		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);