Revert "arm64: kasan: Revert "arm64: mte: reset the page tag in page->flags""

This reverts commit add4bc9281.

On Mon, Sep 12, 2022 at 10:52:45AM +0100, Catalin Marinas wrote:
>I missed this (holidays) and it looks like it's in stable already. On
>its own it will likely break kasan_hw if used together with user-space
>MTE as this change relies on two previous commits:
>
>70c248aca9e7 ("mm: kasan: Skip unpoisoning of user pages")
>6d05141a3930 ("mm: kasan: Skip page unpoisoning only if __GFP_SKIP_KASAN_UNPOISON")
>
>The reason I did not cc stable is that there are other dependencies in
>this area. The potential issues without the above commits were rather
>theoretical, so take these patches rather as clean-ups/refactoring than
>fixes.

Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/arm64/kernel/hibernate.c

@@ -326,6 +326,11 @@ static void swsusp_mte_restore_tags(void)
 		unsigned long pfn = xa_state.xa_index;
 		struct page *page = pfn_to_online_page(pfn);
 
+		/*
+		 * It is not required to invoke page_kasan_tag_reset(page)
+		 * at this point since the tags stored in page->flags are
+		 * already restored.
+		 */
 		mte_restore_page_tags(page_address(page), tags);
 
 		mte_free_tag_storage(tags);

arch/arm64/kernel/mte.c

@@ -44,6 +44,15 @@ static void mte_sync_page_tags(struct page *page, pte_t old_pte,
 	if (!pte_is_tagged)
 		return;
 
+	page_kasan_tag_reset(page);
+	/*
+	 * We need smp_wmb() in between setting the flags and clearing the
+	 * tags because if another thread reads page->flags and builds a
+	 * tagged address out of it, there is an actual dependency to the
+	 * memory access, but on the current thread we do not guarantee that
+	 * the new page->flags are visible before the tags were updated.
+	 */
+	smp_wmb();
 	mte_clear_page_tags(page_address(page));
 }
 

arch/arm64/mm/copypage.c

@@ -23,6 +23,15 @@ void copy_highpage(struct page *to, struct page *from)
 
 	if (system_supports_mte() && test_bit(PG_mte_tagged, &from->flags)) {
 		set_bit(PG_mte_tagged, &to->flags);
+		page_kasan_tag_reset(to);
+		/*
+		 * We need smp_wmb() in between setting the flags and clearing the
+		 * tags because if another thread reads page->flags and builds a
+		 * tagged address out of it, there is an actual dependency to the
+		 * memory access, but on the current thread we do not guarantee that
+		 * the new page->flags are visible before the tags were updated.
+		 */
+		smp_wmb();
 		mte_copy_page_tags(kto, kfrom);
 	}
 }

arch/arm64/mm/mteswap.c

@@ -53,6 +53,15 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
 	if (!tags)
 		return false;
 
+	page_kasan_tag_reset(page);
+	/*
+	 * We need smp_wmb() in between setting the flags and clearing the
+	 * tags because if another thread reads page->flags and builds a
+	 * tagged address out of it, there is an actual dependency to the
+	 * memory access, but on the current thread we do not guarantee that
+	 * the new page->flags are visible before the tags were updated.
+	 */
+	smp_wmb();
 	mte_restore_page_tags(page_address(page), tags);
 
 	return true;