FAILED: patch "[PATCH] ksmbd: Fix race condition in RPC handle list access" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 305853cce379407090a73b38c5de5ba748893aee
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026012822-pennant-explore-e187@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 305853cce379407090a73b38c5de5ba748893aee Mon Sep 17 00:00:00 2001
From: Yunseong Kim <[email protected]>
Date: Mon, 15 Sep 2025 22:44:09 +0000
Subject: [PATCH] ksmbd: Fix race condition in RPC handle list access

The 'sess->rpc_handle_list' XArray manages RPC handles within a ksmbd session. Access to this list is intended to be protected by 'sess->rpc_lock' (an rw_semaphore). However, the locking implementation was flawed, leading to potential race conditions.

In ksmbd_session_rpc_open(), the code incorrectly acquired only a read lock before calling xa_store() and xa_erase(). Since these operations modify the XArray structure, a write lock is required to ensure exclusive access and prevent data corruption from concurrent modifications.

Furthermore, ksmbd_session_rpc_method() accessed the list using xa_load() without holding any lock at all. This could lead to reading inconsistent data or a potential use-after-free if an entry is concurrently removed and the pointer is dereferenced.

Fix these issues by:
1. Using down_write() and up_write() in ksmbd_session_rpc_open() to ensure exclusive access during XArray modification, and ensuring the lock is correctly released on error paths.
2. Adding down_read() and up_read() in ksmbd_session_rpc_method() to safely protect the lookup.

index 9dec4c2940bc..b36d0676dbe5 100644
@@ -104,29 +104,32 @@ int ksmbd_session_rpc_open(struct ksmbd_session *sess, char *rpc_name)
 	if (!entry)
 		return -ENOMEM;
 
-	down_read(&sess->rpc_lock);
 	entry->method = method;
 	entry->id = id = ksmbd_ipc_id_alloc();
 	if (id < 0)
 		goto free_entry;
+
+	down_write(&sess->rpc_lock);
 	old = xa_store(&sess->rpc_handle_list, id, entry, KSMBD_DEFAULT_GFP);
-	if (xa_is_err(old))
+	if (xa_is_err(old)) {
+		up_write(&sess->rpc_lock);
 		goto free_id;
+	}
 
 	resp = ksmbd_rpc_open(sess, id);
-	if (!resp)
-		goto erase_xa;
+	if (!resp) {
+		xa_erase(&sess->rpc_handle_list, entry->id);
+		up_write(&sess->rpc_lock);
+		goto free_id;
+	}
 
-	up_read(&sess->rpc_lock);
+	up_write(&sess->rpc_lock);
 	kvfree(resp);
 	return id;
-erase_xa:
-	xa_erase(&sess->rpc_handle_list, entry->id);
 free_id:
 	ksmbd_rpc_id_free(entry->id);
 free_entry:
 	kfree(entry);
-	up_read(&sess->rpc_lock);
 	return -EINVAL;
 }
 

@@ -144,9 +147,14 @@ void ksmbd_session_rpc_close(struct ksmbd_session *sess, int id)
 int ksmbd_session_rpc_method(struct ksmbd_session *sess, int id)
 {
 	struct ksmbd_session_rpc *entry;
+	int method;
 
+	down_read(&sess->rpc_lock);
 	entry = xa_load(&sess->rpc_handle_list, id);
-	return entry ? entry->method : 0;
+	method = entry ? entry->method : 0;
+	up_read(&sess->rpc_lock);
+
+	return method;
 }
 
 void ksmbd_session_destroy(struct ksmbd_session *sess)