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When make_new_segment() creates an odd-sized segment, the pagemap was only sized based on a number of usable_pages entries, forgetting that a segment also contains metadata pages, and that the FreePageManager uses absolute page indices that cover the entire segment. This miscalculation could cause accesses to pagemap entries to be out of bounds. During subsequent reuse of the allocated segment, allocations landing on pages with indices higher than usable_pages could cause out-of-bounds pagemap reads and/or writes. On write, 'span' pointers are stored into the data area, corrupting the allocated objects. On read (aka during a dsa_free), garbage is interpreted as a span pointer, typically crashing the server in dsa_get_address(). The normal geometric path correctly sizes the pagemap for all pages in the segment. The odd-sized path needs to do the same, but it works forward from usable_pages rather than backward from total_size. This commit fixes the sizing of the odd-sized case by adding pagemap entries for the metadata pages after the initial metadata_bytes calculation, using an integer ceiling division to compute the exact number of additional entries needed in one go, avoiding any iteration in the calculation. An assertion is added in the code path for odd-sized segments, ensuring that the pagemap includes the metadata area, and that the result is appropriately sized. This problem would show up depending on the size requested for the allocation of a DSA segment. The reporter has noticed this issue when a parallel hash join makes a DSA allocation large enough to trigger the odd-sized segment path, but it could happen for anything that does a DSA allocation. A regression test is added to test_dsa, down to v17 where the test module has been introduced. This adds a set of cheap tests to check the problem, the new assertion being useful for this purpose. Sami has proposed a test that took a longer time than what I have done here; the test committed is faster and good enough to check the odd-sized allocation path. Author: Paul Bunn <paul.bunn@icloud.com> Reviewed-by: Sami Imseih <samimseih@gmail.com> Reviewed-by: Chao Li <li.evan.chao@gmail.com> Reviewed-by: Michael Paquier <michael@paquier.xyz> Discussion: https://postgr.es/m/044401dcabac$fe432490$fac96db0$@icloud.com Backpatch-through: 14 |
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PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.