memcmp

SGX-Step demo: Building a determinstic memcmp() password oracle

This directory contains a small demo application that illustrates the power of SGX-Step attacks by single-stepping a sample enclave that contains subtle, non-constant-time memcmp password comparison logic.

Description of the vulnerability

The vulnerable enclave function is shown below:

int my_memcmp(char *a, int a_len, char *b, int b_len)
{
    int i;

    /* first check overall len */
    if (a_len != b_len)
        return 0;

    /* now check individual chars */
    for (i=0; i < a_len; i++)
    {
        if (a[i] != b[i])
            return 0;
    }
    return 1;
}

This non-constant-time function is called to compare the attacker-provided password guess with the secret, in-enclave password. The main idea of the exploit is to accurately detect the early-out return 0 cases in the conditional control-flow above to learn which individual password byte was wrong. This allows to brute-force the password in linear (instead of exponential) time one password byte at a time, as illustrated below:

Example exploit

The demo shows that by merely counting the number of instructions executed on the memcmp page per password guess, the password can be trivially brute-forced character-per-character in linear time.

This clearly shows the substantial advantage of an interrupt-driven SGX-Step adversary, as opposed to traditional start-to-end timing attacks, which are notoriously noisy and would need many repetitions to establish the subtle, single-instruction control-flow differences exploited in this example, e.g., as experimentally illustrated below:

Real-world counterpart

This example is loosely based on the real-world CVE-2018-3626 (paper; source code) that similarly exploited a non-constant-time strlen computation to build a deterministic zero-byte oracle with SGX-Step.