Bug 2406744 (CVE-2025-40039)

In the Linux kernel, the following vulnerability has been resolved:

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.