xref: /linux/Documentation/mm/page_table_check.rst (revision 34dc1baba215b826e454b8d19e4f24adbeb7d00d) 
1.. SPDX-License-Identifier: GPL-2.0
2
3================
4Page Table Check
5================
6
7Introduction
8============
9
10Page table check allows to harden the kernel by ensuring that some types of
11the memory corruptions are prevented.
12
13Page table check performs extra verifications at the time when new pages become
14accessible from the userspace by getting their page table entries (PTEs PMDs
15etc.) added into the table.
16
17In case of detected corruption, the kernel is crashed. There is a small
18performance and memory overhead associated with the page table check. Therefore,
19it is disabled by default, but can be optionally enabled on systems where the
20extra hardening outweighs the performance costs. Also, because page table check
21is synchronous, it can help with debugging double map memory corruption issues,
22by crashing kernel at the time wrong mapping occurs instead of later which is
23often the case with memory corruptions bugs.
24
25Double mapping detection logic
26==============================
27
28+-------------------+-------------------+-------------------+------------------+
29| Current Mapping   | New mapping       | Permissions       | Rule             |
30+===================+===================+===================+==================+
31| Anonymous         | Anonymous         | Read              | Allow            |
32+-------------------+-------------------+-------------------+------------------+
33| Anonymous         | Anonymous         | Read / Write      | Prohibit         |
34+-------------------+-------------------+-------------------+------------------+
35| Anonymous         | Named             | Any               | Prohibit         |
36+-------------------+-------------------+-------------------+------------------+
37| Named             | Anonymous         | Any               | Prohibit         |
38+-------------------+-------------------+-------------------+------------------+
39| Named             | Named             | Any               | Allow            |
40+-------------------+-------------------+-------------------+------------------+
41
42Enabling Page Table Check
43=========================
44
45Build kernel with:
46
47- PAGE_TABLE_CHECK=y
48  Note, it can only be enabled on platforms where ARCH_SUPPORTS_PAGE_TABLE_CHECK
49  is available.
50
51- Boot with 'page_table_check=on' kernel parameter.
52
53Optionally, build kernel with PAGE_TABLE_CHECK_ENFORCED in order to have page
54table support without extra kernel parameter.
55
56Implementation notes
57====================
58
59We specifically decided not to use VMA information in order to avoid relying on
60MM states (except for limited "struct page" info). The page table check is a
61separate from Linux-MM state machine that verifies that the user accessible
62pages are not falsely shared.
63
64PAGE_TABLE_CHECK depends on EXCLUSIVE_SYSTEM_RAM. The reason is that without
65EXCLUSIVE_SYSTEM_RAM, users are allowed to map arbitrary physical memory
66regions into the userspace via /dev/mem. At the same time, pages may change
67their properties (e.g., from anonymous pages to named pages) while they are
68still being mapped in the userspace, leading to "corruption" detected by the
69page table check.
70
71Even with EXCLUSIVE_SYSTEM_RAM, I/O pages may be still allowed to be mapped via
72/dev/mem. However, these pages are always considered as named pages, so they
73won't break the logic used in the page table check.
74