mm: migrate: add missing flush_dcache_page for non-mapped page migrate

Our MIPS 1004Kc SoCs were seeing random userspace crashes with SIGILL and SIGSEGV that could not be traced back to a userspace code bug. They had all the magic signs of an I/D cache coherency issue.

Now recently we noticed that the /proc/sys/vm/compact_memory interface was quite efficient at provoking this class of userspace crashes.

Studying the code in mm/migrate.c there is a distinction made between migrating a page that is mapped at the instant of migration and one that is not mapped. Our problem turned out to be the non-mapped pages.

For the non-mapped page the code performs a copy of the page content and all relevant meta-data of the page without doing the required D-cache maintenance. This leaves dirty data in the D-cache of the CPU and on the 1004K cores this data is not visible to the I-cache. A subsequent page-fault that triggers a mapping of the page will happily serve the process with potentially stale code.

What about ARM then, this bug should have seen greater exposure? Well ARM became immune to this flaw back in 2010, see commit c01778001a4f ("ARM: 6379/1: Assume new page cache pages have dirty D-cache").

My proposed fix moves the D-cache maintenance inside move_to_new_page to make it common for both cases.

---
v2: Added a Fixes footer and CC for stable. No functional change.

---
 mm/migrate.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

index ac6f4939bb59..663a5449367a 100644
@@ -248,10 +248,8 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma,
 				pte = swp_entry_to_pte(entry);
 			} else if (is_device_public_page(new)) {
 				pte = pte_mkdevmap(pte);
-				flush_dcache_page(new);
 			}
-		} else
-			flush_dcache_page(new);
+		}
 
 #ifdef CONFIG_HUGETLB_PAGE
 		if (PageHuge(new)) {

@@ -995,6 +993,13 @@ static int move_to_new_page(struct page *newpage, struct page *page,
 		 */
 		if (!PageMappingFlags(page))
 			page->mapping = NULL;
+
+		if (unlikely(is_zone_device_page(newpage))) {
+			if (is_device_public_page(newpage))
+				flush_dcache_page(newpage);
+		} else
+			flush_dcache_page(newpage);
+
 	}
 out:
 	return rc;

2.11.0