[PATCH] maple_tree: Add dead node check in mas_dup_alloc()

[Boudewijn van der Heide <boudewijn@delta-utec.com>]

The __mt_dup() function is exported and can be called without internal
locking, relying on the caller to provide appropriate synchronization.
If a caller fails to hold proper locks, the source tree may be modified
concurrently, potentially resulting in dead nodes during traversal.

The call stack is:
  __mt_dup()
    → mas_dup_build()
      → mas_dup_alloc()  [accesses node->slot[]]

The mas_dup_alloc() function may access node slots without first
verifying that the node is still alive. If a dead node is encountered,
its memory layout may have been switched to the RCU union member, making
slot array access undefined behavior as we would be reading from the
rcu_head structure instead.

Add an explicit dead node check to detect concurrent modification during
duplication. When a dead node is detected, return -EBUSY to indicate that
the tree is undergoing concurrent modification.

Signed-off-by: Boudewijn van der Heide <boudewijn@delta-utec.com>

---

Build-tested and boot-tested with QEMU with Buildroot on x86_64. The
kernel booted and basic commandline operations work correctly. The race
condition this patch addresses is difficult to reproduce in testing, as
it requires concurrent tree modifications without proper locking.
---
 lib/maple_tree.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 5aa4c9500018..f623a7aabd53 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -6251,6 +6251,11 @@ static inline void mas_dup_alloc(struct ma_state *mas, struct ma_state *new_mas,
 	/* Allocate memory for child nodes. */
 	type = mte_node_type(mas->node);
 	new_slots = ma_slots(new_node, type);
+	if (unlikely(ma_dead_node(node))) {
+		mas_set_err(mas, -EBUSY);
+		return;
+	}
+
 	count = mas->node_request = mas_data_end(mas) + 1;
 	mas_alloc_nodes(mas, gfp);
 	if (unlikely(mas_is_err(mas)))
-- 
2.47.3