patch "mm: page_table_check: Make it dependent on EXCLUSIVE_SYSTEM_RAM" added to usb-linus

This is a note to let you know that I've just added the patch titled

mm: page_table_check: Make it dependent on EXCLUSIVE_SYSTEM_RAM

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 81a31a860bb61d54eb688af2568d9332ed9b8942 Mon Sep 17 00:00:00 2001
From: Ruihan Li <[email protected]>
Date: Mon, 15 May 2023 21:09:57 +0800
Subject: mm: page_table_check: Make it dependent on EXCLUSIVE_SYSTEM_RAM

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.

---
 Documentation/mm/page_table_check.rst | 19 +++++++++++++++++++
 mm/Kconfig.debug                      |  1 +
 2 files changed, 20 insertions(+)

index cfd8f4117cf3..c12838ce6b8d 100644
@@ -52,3 +52,22 @@ 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.

index a925415b4d10..018a5bd2f576 100644
@@ -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

2.40.1