arm64: mte: Do not set PG_mte_tagged if tags were not initialized

The mte_sync_page_tags() function sets PG_mte_tagged if it initializes
page tags. Then we return to mte_sync_tags(), which sets PG_mte_tagged
again. At best, this is redundant. However, it is possible for
mte_sync_page_tags() to return without having initialized tags for the
page, i.e. in the case where check_swap is true (non-compound page),
is_swap_pte(old_pte) is false and pte_is_tagged is false. So at worst,
we set PG_mte_tagged on a page with uninitialized tags. This can happen
if, for example, page migration causes a PTE for an untagged page to
be replaced. If the userspace program subsequently uses mprotect() to
enable PROT_MTE for that page, the uninitialized tags will be exposed
to userspace.

Fix it by removing the redundant call to set_page_mte_tagged().

Fixes: e059853d ("arm64: mte: Fix/clarify the PG_mte_tagged semantics")
Signed-off-by: Peter Collingbourne <pcc@google.com>
Cc: <stable@vger.kernel.org> # 6.1
Link: https://linux-review.googlesource.com/id/Ib02d004d435b2ed87603b858ef7480f7b1463052Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20230420214327.2357985-1-pcc@google.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/mte.c

@@ -66,13 +66,10 @@ void mte_sync_tags(pte_t old_pte, pte_t pte)
 		return;
 
 	/* if PG_mte_tagged is set, tags have already been initialised */
-	for (i = 0; i < nr_pages; i++, page++) {
-		if (!page_mte_tagged(page)) {
+	for (i = 0; i < nr_pages; i++, page++)
+		if (!page_mte_tagged(page))
 			mte_sync_page_tags(page, old_pte, check_swap,
 					   pte_is_tagged);
-			set_page_mte_tagged(page);
-		}
-	}
 
 	/* ensure the tags are visible before the PTE is set */
 	smp_wmb();