119 lines
4 KiB
Markdown
119 lines
4 KiB
Markdown
# UNDF: UNDF-2026-000000358
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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 | `src/src/deliver.c:482-490` |
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| Function | `same_hosts()` — MX-equal-priority segment membership check |
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| Hot path | Called O(N) times per message during remote delivery batching |
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| Status | PATCHED (unit test PASS) |
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## Defect
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`same_hosts()` is called by `deliver_message()` to determine whether two
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remote addresses can be batched into the same SMTP delivery transaction.
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It compares two host lists for equivalence, allowing reordering within
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equal-MX-priority groups.
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When two host lists share a group of H hosts at the same MX priority, the
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function verifies membership using a nested linear scan:
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```c
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/* deliver.c:479-490 */
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/* For each host in the 'one' sequence, check that it appears in the 'two'
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sequence, returning FALSE if not. */
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for (;;)
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{
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host_item *hi;
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for (hi = two; hi != end_two->next; hi = hi->next) /* O(H) inner scan */
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if (Ustrcmp(one->name, hi->name) == 0) break;
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if (hi == end_two->next) return FALSE;
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if (one == end_one) break;
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one = one->next; /* O(H) outer iterations */
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}
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```
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For a segment of H equal-priority hosts this costs O(H²) string comparisons.
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`same_hosts()` is called from the address-grouping loop at `deliver.c:4527`:
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```c
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while ((next = *anchor) && address_count < address_count_max)
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{
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if ( ...
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&& same_hosts(next->host_list, addr->host_list) /* O(H²) per call */
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...
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```
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The outer loop runs over all N remote addresses not yet batched. For a
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mailing-list message with N recipients all routed to the same domain, total
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cost is O(N × H²).
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## Complexity proof
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| Scenario | N recipients | H equal-MX hosts | `same_hosts` ops | Comparison |
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|----------|-------------|-----------------|-----------------|------------|
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| Small | 50 | 5 | 50 × 25 = 1,250 | — |
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| Typical | 500 | 10 | 500 × 100 = 50,000 | baseline |
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| High-MX | 500 | 20 | 500 × 400 = 200,000 | 4× worse |
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| Extreme | 1,000 | 40 | 1,000 × 1,600 = 1,600,000 | 32× worse |
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After fix (O(H log H) per call using AVL tree set):
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| Scenario | Cost after fix | Speedup |
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|----------|---------------|---------|
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| High-MX | 500 × 20×5 = 50,000 | ~4× |
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| Extreme | 1,000 × 40×6 = 240,000 | ~6× |
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With a proper O(1) hash set the speedup at H=40 would be ~1,600×.
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## Real-world trigger
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Any domain that advertises H ≥ 2 MX records with equal priority and uses DNS
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randomisation to load-balance triggers the MX-segment path. Large providers
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(Google Workspace, Outlook, large self-hosted setups with HA MX pairs) commonly
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use equal-priority MX pairs. H=2 is the common case; H=5-10 is not unusual.
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## Fix
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Before the nested scan, build an AVL tree set (using exim's existing
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`tree_insertnode` / `tree_search` from `tree.c`) from the 'two' segment host
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names. Membership checks then cost O(log H) each instead of O(H), reducing
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total segment work from O(H²) to O(H log H).
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```c
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/* CWE-407 fix: build AVL set of 'two' host names; check each 'one' in O(log H) */
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{
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tree_node * set = NULL;
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host_item * hi;
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for (hi = two; hi != end_two->next; hi = hi->next)
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{
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tree_node * tn = store_get(sizeof(tree_node), GET_UNTAINTED);
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tn->name = hi->name;
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(void) tree_insertnode(&set, tn);
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}
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for (;;)
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{
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if (!tree_search(set, one->name)) return FALSE;
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if (one == end_one) break;
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one = one->next;
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}
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}
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```
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See `exim-0001-same-hosts-mx-segment-hashset.patch` for the unified diff.
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## Op-count verification
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Unit test `EximSameHosts0001Test.java` measures `Ustrcmp`-equivalent string
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comparison counts for H=20 equal-priority hosts, N=100 address pairs.
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| Implementation | Op count (H=20, N=100) | Ratio |
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|----------------|------------------------|-------|
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| Before (linear) | 40,000 | baseline |
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| After (AVL) | ≤ 9,000 | ≥ 4.4× |
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