[PATCH 3.2 075/102] mm: migrate: Close race between migration completion and mprotect

[Ben Hutchings]

3.2.64-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Mel Gorman <mgorman@xxxxxxx>

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@xxxxxxxxxxxxxxxxxxxx: use maybe_mkwrite]
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/migrate.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -141,8 +141,11 @@ static int remove_migration_pte(struct p
 
 	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);

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