[PATCH] ksm: fix rmap_item use after free

[PATCH] ksm: fix rmap_item use after free

[Andrea Arcangeli]

From: Andrea Arcangeli <aarcange@redhat.com>

This avoid crashing with slab debugging enabled by removing a window
for memory corruption if freed slab entries are reused before we read
the next pointer. Against mmotm.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---

diff --git a/mm/ksm.c b/mm/ksm.c
index 74d921b..f060e87 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -892,7 +892,7 @@ static struct rmap_item *stable_tree_search(struct page *page,
 {
 	struct rb_node *node = root_stable_tree.rb_node;
 	struct tree_item *tree_item;
-	struct rmap_item *found_rmap_item;
+	struct rmap_item *found_rmap_item, *next_rmap_item;
 
 	while (node) {
 		int ret;
@@ -907,9 +907,11 @@ static struct rmap_item *stable_tree_search(struct page *page,
 			      found_rmap_item->address == rmap_item->address)) {
 				if (!is_zapped_item(found_rmap_item, page2))
 					break;
+				next_rmap_item = found_rmap_item->next;
 				remove_rmap_item_from_tree(found_rmap_item);
-			}
-			found_rmap_item = found_rmap_item->next;
+				found_rmap_item = next_rmap_item;
+			} else
+				found_rmap_item = found_rmap_item->next;
 		}
 		if (!found_rmap_item)
 			goto out_didnt_find;
@@ -959,7 +961,7 @@ static int stable_tree_insert(struct page *page,
 
 	while (*new) {
 		int ret;
-		struct rmap_item *insert_rmap_item;
+		struct rmap_item *insert_rmap_item, *next_rmap_item;
 
 		tree_item = rb_entry(*new, struct tree_item, node);
 		BUG_ON(!tree_item);
@@ -973,9 +975,11 @@ static int stable_tree_insert(struct page *page,
 			     insert_rmap_item->address == rmap_item->address)) {
 				if (!is_zapped_item(insert_rmap_item, page2))
 					break;
+				next_rmap_item = insert_rmap_item->next;
 				remove_rmap_item_from_tree(insert_rmap_item);
-			}
-			insert_rmap_item = insert_rmap_item->next;
+				insert_rmap_item = next_rmap_item;
+			} else
+				insert_rmap_item = insert_rmap_item->next;
 		}
 		if (!insert_rmap_item)
 			return 1;



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[PATCH] ksm: fix losing visibility of part of rmap_item->next list

[Andrea Arcangeli]

From: Andrea Arcangeli <aarcange@redhat.com>

The tree_item->rmap_item is the head of the list and as such it must
not be overwritten except in the case that the element we removed
(rmap_item) was the previous head of the list, in which case it would
also have rmap_item->prev set to null.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---

diff --git a/mm/ksm.c b/mm/ksm.c
index 74d921b..6d8dfee 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -397,10 +397,10 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
 				free_tree_item(tree_item);
 				nnodes_stable_tree--;
 			} else if (!rmap_item->prev) {
+				BUG_ON(tree_item->rmap_item != rmap_item);
 				tree_item->rmap_item = rmap_item->next;
-			} else {
-				tree_item->rmap_item = rmap_item->prev;
-			}
+			} else
+				BUG_ON(tree_item->rmap_item == rmap_item);
 		} else {
 			/*
 			 * We dont rb_erase(&tree_item->node) here, beacuse

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