From 08378f0314ce30aefe9001a9affbf460dcf6047c Mon Sep 17 00:00:00 2001 From: Ruihan Li Date: Mon, 15 May 2023 21:09:57 +0800 Subject: [PATCH] mm: page_table_check: Make it dependent on EXCLUSIVE_SYSTEM_RAM commit 81a31a860bb61d54eb688af2568d9332ed9b8942 upstream. 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. To avoid these false positives, this patch makes PAGE_TABLE_CHECK depends on EXCLUSIVE_SYSTEM_RAM. This dependency is understandable because PAGE_TABLE_CHECK is a hardening technique but /dev/mem without STRICT_DEVMEM (i.e., !EXCLUSIVE_SYSTEM_RAM) is itself a security problem. 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. Cc: # 5.17 Signed-off-by: Ruihan Li Acked-by: David Hildenbrand Acked-by: Pasha Tatashin Link: https://lore.kernel.org/r/20230515130958.32471-4-lrh2000@pku.edu.cn Signed-off-by: Greg Kroah-Hartman --- Documentation/mm/page_table_check.rst | 19 +++++++++++++++++++ mm/Kconfig.debug | 1 + 2 files changed, 20 insertions(+) diff --git a/Documentation/mm/page_table_check.rst b/Documentation/mm/page_table_check.rst index 1a09472f10a3..d19ca356b7bf 100644 --- a/Documentation/mm/page_table_check.rst +++ b/Documentation/mm/page_table_check.rst @@ -54,3 +54,22 @@ Build kernel with: 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. diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug index ce8dded36de9..32c2df12a5ec 100644 --- a/mm/Kconfig.debug +++ b/mm/Kconfig.debug @@ -98,6 +98,7 @@ config PAGE_OWNER config PAGE_TABLE_CHECK bool "Check for invalid mappings in user page tables" depends on ARCH_SUPPORTS_PAGE_TABLE_CHECK + depends on EXCLUSIVE_SYSTEM_RAM select PAGE_EXTENSION help Check that anonymous page is not being mapped twice with read write