ksmbd: ipc: fix use-after-free in ipc_msg_send_request

ipc_msg_send_request() waits for a generic netlink reply using an
ipc_msg_table_entry on the stack. The generic netlink handler
(handle_generic_event()/handle_response()) fills entry->response under
ipc_msg_table_lock, but ipc_msg_send_request() used to validate and free
entry->response without holding the same lock.

Under high concurrency this allows a race where handle_response() is
copying data into entry->response while ipc_msg_send_request() has just
freed it, leading to a slab-use-after-free reported by KASAN in
handle_generic_event():

  BUG: KASAN: slab-use-after-free in handle_generic_event+0x3c4/0x5f0 [ksmbd]
  Write of size 12 at addr ffff888198ee6e20 by task pool/109349
  ...
  Freed by task:
    kvfree
    ipc_msg_send_request [ksmbd]
    ksmbd_rpc_open -> ksmbd_session_rpc_open [ksmbd]

Fix by:
- Taking ipc_msg_table_lock in ipc_msg_send_request() while validating
  entry->response, freeing it when invalid, and removing the entry from
  ipc_msg_table.
- Returning the final entry->response pointer to the caller only after
  the hash entry is removed under the lock.
- Returning NULL in the error path, preserving the original API
  semantics.

This makes all accesses to entry->response consistent with
handle_response(), which already updates and fills the response buffer
under ipc_msg_table_lock, and closes the race that allowed the UAF.

Cc: stable@vger.kernel.org
Reported-by: Qianchang Zhao <pioooooooooip@gmail.com>
Reported-by: Zhitong Liu <liuzhitong1993@gmail.com>
Signed-off-by: Qianchang Zhao <pioooooooooip@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: llllIIIllll

fs/smb/server/transport_ipc.c

@@ -553,12 +553,16 @@ static void *ipc_msg_send_request(struct ksmbd_ipc_msg *msg, unsigned int handle
 	up_write(&ipc_msg_table_lock);
 
 	ret = ipc_msg_send(msg);
-	if (ret)
+	if (ret) {
+		down_write(&ipc_msg_table_lock);
 		goto out;
+	}
 
 	ret = wait_event_interruptible_timeout(entry.wait,
 					       entry.response != NULL,
 					       IPC_WAIT_TIMEOUT);
+
+	down_write(&ipc_msg_table_lock);
 	if (entry.response) {
 		ret = ipc_validate_msg(&entry);
 		if (ret) {
@@ -567,7 +571,6 @@ static void *ipc_msg_send_request(struct ksmbd_ipc_msg *msg, unsigned int handle
 		}
 	}
 out:
-	down_write(&ipc_msg_table_lock);
 	hash_del(&entry.ipc_table_hlist);
 	up_write(&ipc_msg_table_lock);
 	return entry.response;