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

This reverts commit e5b8d92.

Pages mapped in user-space with PROT_MTE have the allocation tags either
zeroed or copied/restored to some user values. In order for the kernel
to access such pages via page_address(), resetting the tag in
page->flags was necessary. This tag resetting was deferred to
set_pte_at() -> mte_sync_page_tags() but it can race with another CPU
reading the flags (via page_to_virt()):

P0 (mte_sync_page_tags):	P1 (memcpy from virt_to_page):
				  Rflags!=0xff
  Wflags=0xff
  DMB (doesn't help)
  Wtags=0
				  Rtags=0   // fault

Since now the post_alloc_hook() function resets the page->flags tag when
unpoisoning is skipped for user pages (including the __GFP_ZEROTAGS
case), revert the arm64 commit calling page_kasan_tag_reset().

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Peter Collingbourne <pcc@google.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://lore.kernel.org/r/20220610152141.2148929-5-catalin.marinas@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/hibernate.c

@@ -300,11 +300,6 @@ 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

@@ -48,15 +48,6 @@ 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

@@ -23,15 +23,6 @@ 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

@@ -53,15 +53,6 @@ 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;