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modbus: Fix buffer overrun in RTU RX handler #100548

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modbus: Fix buffer overrun in RTU RX handler #100548

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11 changes: 6 additions & 5 deletions subsys/modbus/modbus_serial.c
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Original file line number Diff line number Diff line change
Expand Up @@ -287,10 +287,11 @@ static int modbus_rtu_rx_adu(struct modbus_context *ctx)
uint16_t calc_crc;
uint16_t crc_idx;
uint8_t *data_ptr;
uint16_t buf_ctr = cfg->uart_buf_ctr;

/* Is the message long enough? */
if ((cfg->uart_buf_ctr < MODBUS_RTU_MIN_MSG_SIZE) ||
(cfg->uart_buf_ctr > CONFIG_MODBUS_BUFFER_SIZE)) {
if ((buf_ctr < MODBUS_RTU_MIN_MSG_SIZE) ||
(buf_ctr > CONFIG_MODBUS_BUFFER_SIZE)) {
LOG_WRN("Frame length error");
return -EMSGSIZE;
}
Expand All @@ -299,16 +300,16 @@ static int modbus_rtu_rx_adu(struct modbus_context *ctx)
ctx->rx_adu.fc = cfg->uart_buf[1];
data_ptr = &cfg->uart_buf[2];
/* Payload length without node address, function code, and CRC */
ctx->rx_adu.length = cfg->uart_buf_ctr - 4;
ctx->rx_adu.length = buf_ctr - 4;
/* CRC index */
crc_idx = cfg->uart_buf_ctr - sizeof(uint16_t);
crc_idx = buf_ctr - sizeof(uint16_t);

memcpy(ctx->rx_adu.data, data_ptr, ctx->rx_adu.length);

ctx->rx_adu.crc = sys_get_le16(&cfg->uart_buf[crc_idx]);
/* Calculate CRC over address, function code, and payload */
calc_crc = crc16_ansi(&cfg->uart_buf[0],
cfg->uart_buf_ctr - sizeof(ctx->rx_adu.crc));
buf_ctr - sizeof(ctx->rx_adu.crc));

if (ctx->rx_adu.crc != calc_crc) {
LOG_WRN("Calculated CRC does not match received CRC");
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