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Page Table Check

Introduction

Page table check allows to harden the kernel by ensuring that some types of the memory corruptions are prevented.

Page table check performs extra verifications at the time when new pages become accessible from the userspace by getting their page table entries (PTEs PMDs etc.) added into the table.

In case of most detected corruption, the kernel is crashed. There is a small performance and memory overhead associated with the page table check. Therefore, it is disabled by default, but can be optionally enabled on systems where the extra hardening outweighs the performance costs. Also, because page table check is synchronous, it can help with debugging double map memory corruption issues, by crashing kernel at the time wrong mapping occurs instead of later which is often the case with memory corruptions bugs.

It can also be used to do page table entry checks over various flags, dump warnings when illegal combinations of entry flags are detected. Currently, userfaultfd is the only user of such to sanity check wr-protect bit against any writable flags. Illegal flag combinations will not directly cause data corruption in this case immediately, but that will cause read-only data to be writable, leading to corrupt when the page content is later modified.

Double mapping detection logic

Current Mapping

New mapping

Permissions

Rule

Anonymous

Anonymous

Read

Allow

Anonymous

Anonymous

Read / Write

Prohibit

Anonymous

Named

Any

Prohibit

Named

Anonymous

Any

Prohibit

Named

Named

Any

Allow

Enabling Page Table Check

Build kernel with:

  • PAGE_TABLE_CHECK=y Note, it can only be enabled on platforms where ARCH_SUPPORTS_PAGE_TABLE_CHECK is available.

  • Boot with ‘page_table_check=on’ kernel parameter.

Optionally, build kernel with PAGE_TABLE_CHECK_ENFORCED in order to have page table support without extra kernel parameter.

Implementation notes

We specifically decided not to use VMA information in order to avoid relying on MM states (except for limited “struct page” info). The page table check is a separate from Linux-MM state machine that verifies that the user accessible pages are not falsely shared.

PAGE_TABLE_CHECK depends on EXCLUSIVE_SYSTEM_RAM. The reason is that without EXCLUSIVE_SYSTEM_RAM, users are allowed to map arbitrary physical memory regions into the userspace via /dev/mem. At the same time, pages may change their properties (e.g., from anonymous pages to named pages) while they are still being mapped in the userspace, leading to “corruption” detected by the page table check.

Even with EXCLUSIVE_SYSTEM_RAM, I/O pages may be still allowed to be mapped via /dev/mem. However, these pages are always considered as named pages, so they won’t break the logic used in the page table check.