FAILED: patch "[PATCH] arm64: mte: Avoid setting PG_mte_tagged if no tags cleared or" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Possible dependencies:

a8e5e5146ad0 ("arm64: mte: Avoid setting PG_mte_tagged if no tags cleared or restored") 20794545c146 ("arm64: kasan: Revert "arm64: mte: reset the page tag in page->flags"")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From a8e5e5146ad08d794c58252bab00b261045ef16d Mon Sep 17 00:00:00 2001
From: Catalin Marinas <[email protected]>
Date: Thu, 6 Oct 2022 17:33:54 +0100
Subject: [PATCH] arm64: mte: Avoid setting PG_mte_tagged if no tags cleared or
restored

Prior to commit 69e3b846d8a7 ("arm64: mte: Sync tags for pages where PTE is untagged"), mte_sync_tags() was only called for pte_tagged() entries (those mapped with PROT_MTE). Therefore mte_sync_tags() could safely use test_and_set_bit(PG_mte_tagged, &page->flags) without inadvertently setting PG_mte_tagged on an untagged page.

The above commit was required as guests may enable MTE without any control at the stage 2 mapping, nor a PROT_MTE mapping in the VMM. However, the side-effect was that any page with a PTE that looked like swap (or migration) was getting PG_mte_tagged set automatically. A subsequent page copy (e.g. migration) copied the tags to the destination page even if the tags were owned by KASAN.

This issue was masked by the page_kasan_tag_reset() call introduced in commit e5b8d9218951 ("arm64: mte: reset the page tag in page->flags"). When this commit was reverted (20794545c146), KASAN started reporting access faults because the overriding tags in a page did not match the original page->flags (with CONFIG_KASAN_HW_TAGS=y):

  BUG: KASAN: invalid-access in copy_page+0x10/0xd0 arch/arm64/lib/copy_page.S:26
  Read at addr f5ff000017f2e000 by task syz-executor.1/2218
  Pointer tag: [f5], memory tag: [f2]

Move the PG_mte_tagged bit setting from mte_sync_tags() to the actual place where tags are cleared (mte_sync_page_tags()) or restored (mte_restore_tags()).

index aca88470fb69..7467217c1eaf 100644
@@ -48,7 +48,12 @@ static void mte_sync_page_tags(struct page *page, pte_t old_pte,
 	if (!pte_is_tagged)
 		return;
 
-	mte_clear_page_tags(page_address(page));
+	/*
+	 * Test PG_mte_tagged again in case it was racing with another
+	 * set_pte_at().
+	 */
+	if (!test_and_set_bit(PG_mte_tagged, &page->flags))
+		mte_clear_page_tags(page_address(page));
 }
 
 void mte_sync_tags(pte_t old_pte, pte_t pte)

@@ -64,7 +69,7 @@ void mte_sync_tags(pte_t old_pte, pte_t pte)
 
 	/* if PG_mte_tagged is set, tags have already been initialised */
 	for (i = 0; i < nr_pages; i++, page++) {
-		if (!test_and_set_bit(PG_mte_tagged, &page->flags))
+		if (!test_bit(PG_mte_tagged, &page->flags))
 			mte_sync_page_tags(page, old_pte, check_swap,
 					   pte_is_tagged);
 	}

index 4334dec93bd4..bed803d8e158 100644
@@ -53,7 +53,12 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
 	if (!tags)
 		return false;
 
-	mte_restore_page_tags(page_address(page), tags);
+	/*
+	 * Test PG_mte_tagged again in case it was racing with another
+	 * set_pte_at().
+	 */
+	if (!test_and_set_bit(PG_mte_tagged, &page->flags))
+		mte_restore_page_tags(page_address(page), tags);
 
 	return true;
 }