SurrealDB: Field-level SELECT permissions bypassed via indexed COUNT fast paths

Summary

A record user could learn the value of a hidden field by counting how many records match a guess. When `DEFINE FIELD ... PERMISSIONS FOR select WHERE ...` hides a field's contents from a caller, and that field is indexed, running `SELECT count() FROM t WHERE hidden_field = "guess" GROUP ALL` returned a count greater than zero whenever a record actually had that value — even though the caller was never allowed to read the field directly. The query planner used an indexed-COUNT shortcut (`Index::Count`, `IndexCountScan`, or the legacy `Iterate Index Count` / `Iterate Index Keys` paths) that counts matching index entries and skips the permission check that would normally hide the value. The same query with `WITH NOINDEX` correctly returned `[]`, confirming the gap. By repeating the count query with different guesses, an attacker can confirm or recover the contents of any restricted field they could not read through a normal `SELECT`. ### Impact What an attacker **can** do: - Confirm or recover values of a field protected by field-level SELECT permissions on any table they hold table-level SELECT on, provided the field is indexed. - Repeat the query with different guesses to read restricted field contents one value at a time. What it **can't** do: - Read fields that are not indexed (the shortcut only fires when an index covers the predicate column). - Cross table, database or namespace isolation boundaries. - Modify data, escalate privileges, or affect availability. ### Patches The legacy planner (`surrealdb/core/src/idx/planner/tree.rs`) and the streaming planner (`surrealdb/core/src/exec/planner/select/mod.rs`) now both refuse the indexed fast path when the WHERE / ORDER tree references a field governed by a non-`Full` SELECT permission: - `resolve_indexes` skips any B-tree / unique index whose columns are governed by such a permission. - A new `cond_touches_restricted_field` flag is propagated; `eval_count` refuses a dedicated `Index::Count` when set. - The streaming planner adds `cond_touches_restricted_select_field`, a `RestrictedIdiomChecker` visitor that matches each idiom against the table's field-permission prefixes (loaded via the plan-time txn), and gates `IndexCountScan` emission on it. - The fast paths are preserved for root / owner sessions via `should_check_perms_for_view`. Versions 3.1.0 and later are not affected. ### Workarounds Users unable to patch are advised to consider the following workarounds: - Avoid `DEFINE INDEX` on fields whose values are protected by field-level SELECT permissions. The class of attack is specific to the indexed fast paths. - Restrict the ability of record users to issue arbitrary `SELECT count() … GROUP ALL` queries against tables containing field-protected columns. - Use namespace / database isolation as the primary boundary where feasible.