The fix for CVE-2017-7484 disallowed use of pg_statistic data for planning purposes if the user would not be able to select the associated column and a non-leakproof function is to be applied to the statistics
values. That turns out to disable use of pg_statistic data in some common cases involving inheritance/partitioning, where the user does have permission to select from the parent table that was actually named in the query, but not from a child table whose stats are needed. Since, in non-corner cases, the user *can* select the child table's data via the parent, this restriction is not actually useful from a security standpoint. Improve the logic so that we also check the permissions of the originally-named table, and allow access if select permission exists for that.
When checking access to stats for a simple child column, we can map the child column number back to the parent, and perform this test exactly (including not allowing access if the child column isn't exposed by the parent). For expression indexes, the current logic just insists on whole-table select access, and this patch allows access if the user can select the whole parent table. In principle, if the child table has extra columns, this might allow access to stats on columns the user can't read. In practice, it's unlikely that the planner is going to do any stats calculations involving expressions that are not visible to the query, so we'll ignore that fine point for now. Perhaps someday we'll improve that logic to detect exactly which columns are used by an expression index ... but today is not that day.
Back-patch to v11. The issue was created in 9.2 and up by the CVE-2017-7484 fix, but this patch depends on the append_rel_array planner data structure which only exists in v11 and up. In practice the issue is most urgent with partitioned tables, so fixing v11 and later should satisfy much of the practical need.
Dilip Kumar and Amit Langote, with some kibitzing by me
553d2ec271 Allow access to child table statistics if user can read parent table.
src/backend/utils/adt/selfuncs.c | 128 ++++++++++++++++++++++++++++++++++
src/test/regress/expected/inherit.out | 74 ++++++++++++++++++++
src/test/regress/sql/inherit.sql | 38 ++++++++++
3 files changed, 240 insertions(+)