Re: [PATCH] ksmbd: Fix race condition in RPC handle list access

[Namjae Jeon <linkinjeon@kernel.org>]

On Tue, Sep 16, 2025 at 7:44 AM Yunseong Kim <ysk@kzalloc.com> wrote:
>
> 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.
>
> Fixes: a1f46c99d9ea ("ksmbd: fix use-after-free in ksmbd_session_rpc_open")
> Fixes: b685757c7b08 ("ksmbd: Implements sess->rpc_handle_list as xarray")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yunseong Kim <ysk@kzalloc.com>
Applied it to #ksmbd-for-next-next.
Thanks!