1.. SPDX-License-Identifier: GPL-2.0 2 3.. _page_table_check: 4 5================ 6Page Table Check 7================ 8 9Introduction 10============ 11 12Page table check allows to harden the kernel by ensuring that some types of 13the memory corruptions are prevented. 14 15Page table check performs extra verifications at the time when new pages become 16accessible from the userspace by getting their page table entries (PTEs PMDs 17etc.) added into the table. 18 19In case of detected corruption, the kernel is crashed. There is a small 20performance and memory overhead associated with the page table check. Therefore, 21it is disabled by default, but can be optionally enabled on systems where the 22extra hardening outweighs the performance costs. Also, because page table check 23is synchronous, it can help with debugging double map memory corruption issues, 24by crashing kernel at the time wrong mapping occurs instead of later which is 25often the case with memory corruptions bugs. 26 27Double mapping detection logic 28============================== 29 30+-------------------+-------------------+-------------------+------------------+ 31| Current Mapping | New mapping | Permissions | Rule | 32+===================+===================+===================+==================+ 33| Anonymous | Anonymous | Read | Allow | 34+-------------------+-------------------+-------------------+------------------+ 35| Anonymous | Anonymous | Read / Write | Prohibit | 36+-------------------+-------------------+-------------------+------------------+ 37| Anonymous | Named | Any | Prohibit | 38+-------------------+-------------------+-------------------+------------------+ 39| Named | Anonymous | Any | Prohibit | 40+-------------------+-------------------+-------------------+------------------+ 41| Named | Named | Any | Allow | 42+-------------------+-------------------+-------------------+------------------+ 43 44Enabling Page Table Check 45========================= 46 47Build kernel with: 48 49- PAGE_TABLE_CHECK=y 50 Note, it can only be enabled on platforms where ARCH_SUPPORTS_PAGE_TABLE_CHECK 51 is available. 52 53- Boot with 'page_table_check=on' kernel parameter. 54 55Optionally, build kernel with PAGE_TABLE_CHECK_ENFORCED in order to have page 56table support without extra kernel parameter. 57 58Implementation notes 59==================== 60 61We specifically decided not to use VMA information in order to avoid relying on 62MM states (except for limited "struct page" info). The page table check is a 63separate from Linux-MM state machine that verifies that the user accessible 64pages are not falsely shared. 65 66PAGE_TABLE_CHECK depends on EXCLUSIVE_SYSTEM_RAM. The reason is that without 67EXCLUSIVE_SYSTEM_RAM, users are allowed to map arbitrary physical memory 68regions into the userspace via /dev/mem. At the same time, pages may change 69their properties (e.g., from anonymous pages to named pages) while they are 70still being mapped in the userspace, leading to "corruption" detected by the 71page table check. 72 73Even with EXCLUSIVE_SYSTEM_RAM, I/O pages may be still allowed to be mapped via 74/dev/mem. However, these pages are always considered as named pages, so they 75won't break the logic used in the page table check. 76