The root cause of the CWE-190 (Integer Overflow or Wraparound) vulnerability at line 108 is the unchecked subtraction operation when calculating the payload_size.

Here's a detailed analysis:

1. The vulnerability occurs because the code is subtracting values from smh->submessageLength without first ensuring that the result won't underflow.

2. smh->submessageLength is likely an unsigned integer (probably uint32_t), as it represents the length of a submessage.

3. The code is attempting to calculate the payload_size by subtracting some value (the rest of the line is cut off) from smh->submessageLength.

4. If the value being subtracted is larger than smh->submessageLength, it will cause an integer underflow, wrapping around to a very large number instead of becoming negative (since it's unsigned).

5. This could lead to allocating a buffer that's much larger than intended, potentially causing memory exhaustion or other issues later in the program.

6. The code doesn't perform any checks to ensure that the subtraction operation is safe and won't result in an underflow.

To fix this vulnerability, the code should:

1. Check that the value being subtracted is not greater than smh->submessageLength before performing the subtraction.
2. Use safe integer arithmetic operations that detect overflow/underflow.
3. Validate input values to ensure they are within expected ranges.

This type of vulnerability is particularly dangerous in network protocol implementations, as it could potentially be exploited by sending specially crafted messages to trigger the underflow.