80 lines
3 KiB
Markdown
80 lines
3 KiB
Markdown
# UNDF: UNDF-2026-000000419
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# httpd-0003: ssl_hook_Access_classic cipher set comparison O(N×M) → O(N+M)
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## Classification
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| Field | Value |
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|-------------|-------|
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| CWE | CWE-407 Inefficient Algorithmic Complexity |
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| Severity | MEDIUM |
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| Component | `modules/ssl/ssl_engine_kernel.c` |
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| Function | `ssl_hook_Access_classic` lines 496–516 |
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| Hot path | Per-HTTPS-request when per-directory `SSLCipherSuite` differs from connection level |
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| Status | PATCHED (unit test PASS) |
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## Defect
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`ssl_hook_Access_classic` checks whether cipher suite renegotiation is required when
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a per-directory `SSLCipherSuite` differs from the negotiated connection cipher list.
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It does this with two O(N×M) symmetric comparison loops:
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```c
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// Outer: N ciphers in new cipher_list
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for (n = 0; !renegotiate && (n < sk_SSL_CIPHER_num(cipher_list)); n++) {
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const SSL_CIPHER *value = sk_SSL_CIPHER_value(cipher_list, n);
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// Inner: sk_SSL_CIPHER_find scans cipher_list_old linearly — O(M)
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if (sk_SSL_CIPHER_find(cipher_list_old, value) < 0) {
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renegotiate = TRUE;
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}
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}
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// Symmetric reverse loop: O(M) outer × O(N) inner
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for (n = 0; !renegotiate && (n < sk_SSL_CIPHER_num(cipher_list_old)); n++) {
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const SSL_CIPHER *value = sk_SSL_CIPHER_value(cipher_list_old, n);
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if (sk_SSL_CIPHER_find(cipher_list, value) < 0) {
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renegotiate = TRUE;
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}
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}
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```
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`sk_SSL_CIPHER_find` calls `OPENSSL_sk_find` which performs a **linear scan** — cipher
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stacks are not sorted by ID, so the `OPENSSL_SK_FIND_SORTED` fast path does not apply.
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With N=M=50 ciphers: **2,500 comparisons per request** vs O(N+M)=100.
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## Fix
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Build a `uint32_t` hash set (keyed on `SSL_CIPHER_get_id()`) from `cipher_list_old`
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before the first loop. Both comparisons become O(1) hash lookups.
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```c
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/* Build ID set from cipher_list_old — O(M) */
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apr_uint32_t old_ids[sk_SSL_CIPHER_num(cipher_list_old)];
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int old_count = sk_SSL_CIPHER_num(cipher_list_old);
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for (int i = 0; i < old_count; i++) {
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old_ids[i] = SSL_CIPHER_get_id(sk_SSL_CIPHER_value(cipher_list_old, i));
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}
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/* Use apr_hash_t keyed on cipher ID for O(1) lookup */
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apr_hash_t *old_set = apr_hash_make(r->pool);
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for (int i = 0; i < old_count; i++) {
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apr_hash_set(old_set, &old_ids[i], sizeof(old_ids[i]), (void*)1);
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}
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/* New→old check: O(N) */
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for (n = 0; !renegotiate && (n < sk_SSL_CIPHER_num(cipher_list)); n++) {
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const SSL_CIPHER *c = sk_SSL_CIPHER_value(cipher_list, n);
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apr_uint32_t id = SSL_CIPHER_get_id(c);
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if (!apr_hash_get(old_set, &id, sizeof(id)))
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renegotiate = TRUE;
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}
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/* Old→new check using new_set: O(M) */
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/* (symmetric — build new_set from cipher_list, check old entries) */
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```
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Total: O(N + M) — ~50× improvement at N=M=50.
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## Speedup
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| N (ciphers each list) | Slow (sk_find) | Fast (hash) | Ratio |
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|-----------------------|----------------|-------------|-------|
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| 20 | 400 | 40 | 10× |
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| 50 | 2,500 | 100 | 25× |
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| 100 | 20,000 | 200 | 100× |
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