[PATCH v2 1/4] maple_tree: remove mt_clear_meta() to fix a pointer corruption

[Dimitris Charisis]

mt_clear_meta() decides whether the last slot of a maple_range_64 node
holds a child pointer or a struct maple_metadata with the check

        if (unlikely((mte_to_node(next) &&
                      mte_node_type(next))))
                return; /* no metadata, could be node */

The check expects the pointer to be encoded.  But the only callsite of
mt_clear_meta() is mt_destroy_walk() on the RCU destroy path, and by the
time it runs, mte_dead_leaves() has overwritten every slot on a full
node with a raw pointer, stripping the type information. Thus, the check
above never returns early for a full node as it should. It falls through
and then:
        meta->gap = 0;
        meta->end = 0;
zeroes two bytes of a valid child pointer. Later, mt_free_walk()
dereferences the corrupted pointer.

Fix this by removing mt_clear_meta() along with its only callsite.
mt_clear_meta() is only called for the root of each sub-tree destroyed
under RCU. Descendant nodes retain their metadata until they are freed.
RCU readers may use the metadata while traversing a node, but do not use
cleared metadata to detect that a node has been removed. They detect a
dead node via ma_dead_node().

Fixes: 2e5b4921f8ef ("maple_tree: fix freeing of nodes in rcu mode")
Signed-off-by: Dimitris Charisis <dchar at cslab.ece.ntua.gr>
---
 lib/maple_tree.c | 39 ---------------------------------------
 1 file changed, 39 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 1aba6cced71307245cbbca26986e14e74b35a14f..e86eee43aa0ada6963995cd74495d9344f6ccd06 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -763,43 +763,6 @@ static inline void ma_set_meta(struct maple_node *mn, enum maple_type mt,
 	meta->end = end;
 }
 
-/*
- * mt_clear_meta() - clear the metadata information of a node, if it exists
- * @mt: The maple tree
- * @mn: The maple node
- * @type: The maple node type
- */
-static inline void mt_clear_meta(struct maple_tree *mt, struct maple_node *mn,
-				  enum maple_type type)
-{
-	struct maple_metadata *meta;
-	unsigned long *pivots;
-	void __rcu **slots;
-	void *next;
-
-	switch (type) {
-	case maple_range_64:
-		pivots = mn->mr64.pivot;
-		if (unlikely(pivots[MAPLE_RANGE64_SLOTS - 2])) {
-			slots = mn->mr64.slot;
-			next = mt_slot_locked(mt, slots,
-					      MAPLE_RANGE64_SLOTS - 1);
-			if (unlikely((mte_to_node(next) &&
-				      mte_node_type(next))))
-				return; /* no metadata, could be node */
-		}
-		fallthrough;
-	case maple_arange_64:
-		meta = ma_meta(mn, type);
-		break;
-	default:
-		return;
-	}
-
-	meta->gap = 0;
-	meta->end = 0;
-}
-
 /*
  * ma_meta_end() - Get the data end of a node from the metadata
  * @mn: The maple node
@@ -4885,8 +4848,6 @@ static void mt_destroy_walk(struct maple_enode *enode, struct maple_tree *mt,
 free_leaf:
 	if (free)
 		kfree(node);
-	else
-		mt_clear_meta(mt, node, node->type);
 }
 
 /*

-- 
2.47.3