Let's analyze the code step by step to identify the potential CWE-787 (Out-of-bounds Write) vulnerability:

1. Input analysis:
   - load_addr: intptr_t (destination address)
   - data_addr: uint32_t (source data address)
   - data_len: uint32_t (length of data to process)
   - do_load: bool
   - sha_handle: bootloader_sha256_handle_t
   - checksum: uint32_t*

2. Data mapping:
   - Line 10: data is mapped using bootloader_mmap(data_addr, data_len)
   - This creates a pointer 'data' to the mapped memory

3. Loop setup:
   - Line 38: for loop iterates from 0 to data_len in steps of 4
   - Line 39: w_i is calculated as i / 4 (word index)

4. Potential vulnerability:
   - Line 40: w = src[w_i]
   - src is aliased to 'data' (the mapped memory)
   - The loop runs for data_len bytes, but accesses are made in 4-byte words

5. Vulnerability analysis:
   - If data_len is not a multiple of 4, the last iteration may attempt to read beyond the mapped area
   - Example: If data_len = 10, the loop will run for i = 0, 4, 8
   - When i = 8, w_i = 2, but this might be beyond the actual mapped data

6. CWE-787 consideration:
   - While the vulnerability is more of a read operation (potential out-of-bounds read), it could lead to an out-of-bounds write if the read value is used to write elsewhere in the code (not shown in this snippet)

Conclusion:
The vulnerability at line 40 is a potential out-of-bounds read, which could lead to undefined behavior or system crashes. While it doesn't directly demonstrate CWE-787 (Out-of-bounds Write) in this snippet, it could potentially cause such a vulnerability if the read value is used for writing elsewhere in the code. To fix this, the code should ensure that the last read operation doesn't go beyond the mapped data, possibly by adjusting the loop condition or adding bounds checking.