mm: migrate: Close race between migration completion and mprotect

A migration entry is marked as write if pte_write was true at the time the
entry was created. The VMA protections are not double checked when migration
entries are being removed as mprotect marks write-migration-entries as
read. It means that potentially we take a spurious fault to mark PTEs write
again but it's straight-forward. However, there is a race between write
migrations being marked read and migrations finishing. This potentially
allows a PTE to be write that should have been read. Close this race by
double checking the VMA permissions using maybe_mkwrite when migration
completes.

[[email protected]: use maybe_mkwrite]
Cc: [email protected]
Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/migrate.c b/mm/migrate.c
index f78ec9bd454dd04585d208bd065460c92f04c042..2740360cd216ca45054387f481c7c31775d4ebff 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -146,8 +146,11 @@ static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,
        pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
        if (pte_swp_soft_dirty(*ptep))
                pte = pte_mksoft_dirty(pte);
+
+       /* Recheck VMA as permissions can change since migration started  */
        if (is_write_migration_entry(entry))
-               pte = pte_mkwrite(pte);
+               pte = maybe_mkwrite(pte, vma);
+
 #ifdef CONFIG_HUGETLB_PAGE
        if (PageHuge(new)) {
                pte = pte_mkhuge(pte);