UnbalancedBnCtx.md

Unbalanced BN_CTX_start and BN_CTX_end pair

This query detects unbalanced pairs of BN_CTX_start and BN_CTX_end calls in OpenSSL code. These functions must be used in matching pairs to properly manage temporary BIGNUM allocations within a BN_CTX context.

BN_CTX_start marks the beginning of a nested allocation scope, and BN_CTX_end releases all temporary BIGNUMs allocated via BN_CTX_get since the matching BN_CTX_start. Each call to BN_CTX_start must have a corresponding BN_CTX_end call, and vice versa.

Common issues include:

• Calling BN_CTX_start without a corresponding BN_CTX_end (memory leak of temporary allocations)
• Calling BN_CTX_end without a corresponding BN_CTX_start (undefined behavior)
• Missing BN_CTX_end in error paths

The following example would be flagged for missing BN_CTX_end:

int compute(BN_CTX *ctx, BIGNUM *result) {
    BN_CTX_start(ctx);

    BIGNUM *tmp1 = BN_CTX_get(ctx);
    BIGNUM *tmp2 = BN_CTX_get(ctx);

    if (!tmp1 || !tmp2) {
        // ERROR: Missing BN_CTX_end on error path
        return 0;
    }

    // Perform computation

    BN_CTX_end(ctx);
    return 1;
}

The correct version ensures BN_CTX_end is called on all code paths:

int compute(BN_CTX *ctx, BIGNUM *result) {
    BN_CTX_start(ctx);

    BIGNUM *tmp1 = BN_CTX_get(ctx);
    BIGNUM *tmp2 = BN_CTX_get(ctx);

    if (!tmp1 || !tmp2) {
        BN_CTX_end(ctx);  // Properly clean up on error path
        return 0;
    }

    // Perform computation

    BN_CTX_end(ctx);
    return 1;
}