[Core] guard RPC invocation by checking RPC info against crc checksum (#17840)

quantum/split_common/transactions.c

@@ -694,7 +694,7 @@ split_transaction_desc_t split_transaction_table[NUM_TOTAL_TRANSACTIONS] = {
 #if defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
         [PUT_RPC_INFO]  = trans_initiator2target_initializer_cb(rpc_info, slave_rpc_info_callback),
     [PUT_RPC_REQ_DATA]  = trans_initiator2target_initializer(rpc_m2s_buffer),
-    [EXECUTE_RPC]       = trans_initiator2target_initializer_cb(rpc_info.transaction_id, slave_rpc_exec_callback),
+    [EXECUTE_RPC]       = trans_initiator2target_initializer_cb(rpc_info.payload.transaction_id, slave_rpc_exec_callback),
     [GET_RPC_RESP_DATA] = trans_target2initiator_initializer(rpc_s2m_buffer),
 #endif // defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
 };
@@ -760,7 +760,8 @@ bool transaction_rpc_exec(int8_t transaction_id, uint8_t initiator2target_buffer
     if (target2initiator_buffer_size > RPC_S2M_BUFFER_SIZE) return false;
 
     // Prepare the metadata block
-    rpc_sync_info_t info = {.transaction_id = transaction_id, .m2s_length = initiator2target_buffer_size, .s2m_length = target2initiator_buffer_size};
+    rpc_sync_info_t info = {.payload = {.transaction_id = transaction_id, .m2s_length = initiator2target_buffer_size, .s2m_length = target2initiator_buffer_size}};
+    info.checksum        = crc8(&info.payload, sizeof(info.payload));
 
     // Make sure the local side knows that we're not sending the full block of data
     split_transaction_table[PUT_RPC_REQ_DATA].initiator2target_buffer_size  = initiator2target_buffer_size;
@@ -791,18 +792,23 @@ void slave_rpc_info_callback(uint8_t initiator2target_buffer_size, const void *i
     // Ignore the args -- the `split_shmem` already has the info, we just need to act upon it.
     // We must keep the `split_transaction_table` non-const, so that it is able to be modified at runtime.
 
-    split_transaction_table[PUT_RPC_REQ_DATA].initiator2target_buffer_size  = split_shmem->rpc_info.m2s_length;
-    split_transaction_table[GET_RPC_RESP_DATA].target2initiator_buffer_size = split_shmem->rpc_info.s2m_length;
+    split_transaction_table[PUT_RPC_REQ_DATA].initiator2target_buffer_size  = split_shmem->rpc_info.payload.m2s_length;
+    split_transaction_table[GET_RPC_RESP_DATA].target2initiator_buffer_size = split_shmem->rpc_info.payload.s2m_length;
 }
 
 void slave_rpc_exec_callback(uint8_t initiator2target_buffer_size, const void *initiator2target_buffer, uint8_t target2initiator_buffer_size, void *target2initiator_buffer) {
     // We can assume that the buffer lengths are correctly set, now, given that sequentially the rpc_info callback was already executed.
     // Go through the rpc_info and execute _that_ transaction's callback, with the scratch buffers as inputs.
-    int8_t transaction_id = split_shmem->rpc_info.transaction_id;
+    // As a safety precaution we check that the received payload matches its checksum first.
+    if (crc8(&split_shmem->rpc_info.payload, sizeof(split_shmem->rpc_info.payload)) != split_shmem->rpc_info.checksum) {
+        return;
+    }
+
+    int8_t transaction_id = split_shmem->rpc_info.payload.transaction_id;
     if (transaction_id < NUM_TOTAL_TRANSACTIONS) {
         split_transaction_desc_t *trans = &split_transaction_table[transaction_id];
         if (trans->slave_callback) {
-            trans->slave_callback(split_shmem->rpc_info.m2s_length, split_shmem->rpc_m2s_buffer, split_shmem->rpc_info.s2m_length, split_shmem->rpc_s2m_buffer);
+            trans->slave_callback(split_shmem->rpc_info.payload.m2s_length, split_shmem->rpc_m2s_buffer, split_shmem->rpc_info.payload.s2m_length, split_shmem->rpc_s2m_buffer);
         }
     }
 }

quantum/split_common/transport.h

@@ -116,9 +116,12 @@ typedef struct _split_slave_pointing_sync_t {
 
 #if defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
 typedef struct _rpc_sync_info_t {
-    int8_t  transaction_id;
-    uint8_t m2s_length;
-    uint8_t s2m_length;
+    uint8_t checksum;
+    struct {
+        int8_t  transaction_id;
+        uint8_t m2s_length;
+        uint8_t s2m_length;
+    } payload;
 } rpc_sync_info_t;
 #endif // defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)