BnCtxFreeBeforeEnd.md

BN_CTX_free called before BN_CTX_end

This query identifies instances where BN_CTX_free is called before BN_CTX_end, which violates the required lifecycle of OpenSSL's BN_CTX objects.

In OpenSSL, the proper lifecycle for using BN_CTX with nested allocations is:

1. BN_CTX_start(ctx) - marks the start of a nested allocation
2. Use BN_CTX_get(ctx) to get temporary BIGNUMs
3. BN_CTX_end(ctx) - releases the temporary BIGNUMs allocated since the matching BN_CTX_start
4. BN_CTX_free(ctx) - frees the entire context

Calling BN_CTX_free before BN_CTX_end can lead to corrupted state or undefined behavior, as temporary BIGNUMs allocated via BN_CTX_get are not properly released.

The following example would be flagged as an issue by the query:

void compute(BIGNUM *result) {
    BN_CTX *ctx = BN_CTX_new();
    BN_CTX_start(ctx);

    BIGNUM *tmp = BN_CTX_get(ctx);
    // Perform computation using tmp

    // ERROR: BN_CTX_free called without calling BN_CTX_end first
    BN_CTX_free(ctx);
}

The correct version should call BN_CTX_end before BN_CTX_free:

void compute(BIGNUM *result) {
    BN_CTX *ctx = BN_CTX_new();
    BN_CTX_start(ctx);

    BIGNUM *tmp = BN_CTX_get(ctx);
    // Perform computation using tmp

    BN_CTX_end(ctx);  // Properly release temporary BIGNUMs
    BN_CTX_free(ctx); // Now safe to free the context
}