[PATCH bpf-next] bpf: Avoid kfree_rcu() under lock in bpf_lpm_trie.

[Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>]

From: Alexei Starovoitov <ast@xxxxxxxxxx>

syzbot reported the following lock sequence:
cpu 2:
  grabs timer_base lock
    spins on bpf_lpm lock

cpu 1:
  grab rcu krcp lock
    spins on timer_base lock

cpu 0:
  grab bpf_lpm lock
    spins on rcu krcp lock

bpf_lpm lock can be the same.
timer_base lock can also be the same due to timer migration.
but rcu krcp lock is always per-cpu, so it cannot be the same lock.
Hence it's a false positive.
To avoid lockdep complain move kfree_rcu() after spin_unlock.

Reported-by: syzbot+1fa663a2100308ab6eab@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
 kernel/bpf/lpm_trie.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
index 050fe1ebf0f7..d0febf07051e 100644
--- a/kernel/bpf/lpm_trie.c
+++ b/kernel/bpf/lpm_trie.c
@@ -308,6 +308,7 @@ static long trie_update_elem(struct bpf_map *map,
 {
 	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
 	struct lpm_trie_node *node, *im_node = NULL, *new_node = NULL;
+	struct lpm_trie_node *free_node = NULL;
 	struct lpm_trie_node __rcu **slot;
 	struct bpf_lpm_trie_key_u8 *key = _key;
 	unsigned long irq_flags;
@@ -382,7 +383,7 @@ static long trie_update_elem(struct bpf_map *map,
 			trie->n_entries--;
 
 		rcu_assign_pointer(*slot, new_node);
-		kfree_rcu(node, rcu);
+		free_node = node;
 
 		goto out;
 	}
@@ -429,6 +430,7 @@ static long trie_update_elem(struct bpf_map *map,
 	}
 
 	spin_unlock_irqrestore(&trie->lock, irq_flags);
+	kfree_rcu(free_node, rcu);
 
 	return ret;
 }
@@ -437,6 +439,7 @@ static long trie_update_elem(struct bpf_map *map,
 static long trie_delete_elem(struct bpf_map *map, void *_key)
 {
 	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
+	struct lpm_trie_node *free_node = NULL, *free_parent = NULL;
 	struct bpf_lpm_trie_key_u8 *key = _key;
 	struct lpm_trie_node __rcu **trim, **trim2;
 	struct lpm_trie_node *node, *parent;
@@ -506,8 +509,8 @@ static long trie_delete_elem(struct bpf_map *map, void *_key)
 		else
 			rcu_assign_pointer(
 				*trim2, rcu_access_pointer(parent->child[0]));
-		kfree_rcu(parent, rcu);
-		kfree_rcu(node, rcu);
+		free_parent = parent;
+		free_node = node;
 		goto out;
 	}
 
@@ -521,10 +524,12 @@ static long trie_delete_elem(struct bpf_map *map, void *_key)
 		rcu_assign_pointer(*trim, rcu_access_pointer(node->child[1]));
 	else
 		RCU_INIT_POINTER(*trim, NULL);
-	kfree_rcu(node, rcu);
+	free_node = node;
 
 out:
 	spin_unlock_irqrestore(&trie->lock, irq_flags);
+	kfree_rcu(free_parent, rcu);
+	kfree_rcu(free_node, rcu);
 
 	return ret;
 }
-- 
2.43.0