Commit 305853cc authored Sep 15, 2025 by Yunseong Kim Committed by Steve French Sep 30, 2025

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.

Fixes: a1f46c99 ("ksmbd: fix use-after-free in ksmbd_session_rpc_open")
Fixes: b685757c ("ksmbd: Implements sess->rpc_handle_list as xarray")
Cc: stable@vger.kernel.org
Signed-off-by: Yunseong Kim <ysk@kzalloc.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

parent a9401710

fs/smb/server/mgmt/user_session.c

+17 −9

Original line number	Diff line number	Diff line
		@@ -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)