The root cause of the CWE-787 (Out-of-bounds Write) vulnerability in the given code is the lack of proper bounds checking for the `TPM_BUFSIZE` value before allocating memory using `kmalloc`.

The vulnerable line:

```
26     chip->data_buffer = kmalloc(TPM_BUFSIZE * sizeof(u8), GFP_KERNEL);
```

allocates memory for `chip->data_buffer` based on the value of `TPM_BUFSIZE`. However, there is no check to ensure that `TPM_BUFSIZE` is within a safe range or has a valid value. If `TPM_BUFSIZE` is a very large value or has an unexpected value due to a bug or memory corruption, the `kmalloc` call can allocate an excessive amount of memory, potentially leading to an out-of-bounds write vulnerability (CWE-787).

To mitigate this vulnerability, the code should include proper bounds checking or validation for the `TPM_BUFSIZE` value before using it to allocate memory. This can be done by either:

1. Defining a maximum allowed value for `TPM_BUFSIZE` and checking if the actual value exceeds that limit before allocating memory.
2. Performing additional checks or validations on the `TPM_BUFSIZE` value to ensure it is within an expected range or meets specific criteria.

Without proper bounds checking or validation, the code is susceptible to potential out-of-bounds write vulnerabilities, which can lead to memory corruption, crashes, or even potential exploitation by attackers.