[PATCH 3/4] ftrace: Protect direct_functions in update_ftrace_direct_mod

[Leon Hwang <leon.hwang@linux.dev>]

Fix accessing the __rcu pointer direct_functions with RCU protection.

Cc: stable@vger.kernel.org
Fixes: e93672f770d7 ("ftrace: Add update_ftrace_direct_mod function")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/trace/ftrace.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 9ea39110927f..414e425c2d80 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -6617,6 +6617,7 @@ int update_ftrace_direct_mod(struct ftrace_ops *ops, struct ftrace_hash *hash, b
 		.func		= ftrace_stub,
 		.flags		= FTRACE_OPS_FL_STUB,
 	};
+	struct ftrace_hash *direct_hash;
 	struct ftrace_hash *orig_hash;
 	unsigned long size, i;
 	int err = -EINVAL;
@@ -6627,8 +6628,6 @@ int update_ftrace_direct_mod(struct ftrace_ops *ops, struct ftrace_hash *hash, b
 		return -EINVAL;
 	if (!(ops->flags & FTRACE_OPS_FL_ENABLED))
 		return -EINVAL;
-	if (direct_functions == EMPTY_HASH)
-		return -EINVAL;
 
 	/*
 	 * We can be called from within ops_func callback with direct_mutex
@@ -6636,6 +6635,12 @@ int update_ftrace_direct_mod(struct ftrace_ops *ops, struct ftrace_hash *hash, b
 	 */
 	if (do_direct_lock)
 		mutex_lock(&direct_mutex);
+	else
+		lockdep_assert_held_once(&direct_mutex);
+
+	direct_hash = rcu_dereference_protected(direct_functions, lockdep_is_held(&direct_mutex));
+	if (direct_hash == EMPTY_HASH)
+		goto unlock;
 
 	orig_hash = ops->func_hash ? ops->func_hash->filter_hash : NULL;
 	if (!orig_hash)
@@ -6667,7 +6672,7 @@ int update_ftrace_direct_mod(struct ftrace_ops *ops, struct ftrace_hash *hash, b
 	size = 1 << hash->size_bits;
 	for (i = 0; i < size; i++) {
 		hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
-			tmp = __ftrace_lookup_ip(direct_functions, entry->ip);
+			tmp = __ftrace_lookup_ip(direct_hash, entry->ip);
 			if (!tmp)
 				continue;
 			tmp->direct = entry->direct;
-- 
2.55.0