mm: migrate: Close race between migration completion and mprotect

commit d3cb8bf6081b8b7a2dabb1264fe968fd870fa595 upstream.

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.

[torvalds@linux-foundation.org: use maybe_mkwrite]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/mm/migrate.c b/mm/migrate.c
index 09d6a9d..7d26ea5 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -141,8 +141,11 @@ static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,
 
        get_page(new);
        pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
+
+       /* 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);