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 the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mm-page_table_check-make-it-dependent-on-exclusive_system_ram.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 81a31a860bb61d54eb688af2568d9332ed9b8942 Mon Sep 17 00:00:00 2001
From: Ruihan Li <lrh2000@xxxxxxxxxx>
Date: Mon, 15 May 2023 21:09:57 +0800
Subject: mm: page_table_check: Make it dependent on EXCLUSIVE_SYSTEM_RAM
From: Ruihan Li <lrh2000@xxxxxxxxxx>
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: <stable@xxxxxxxxxxxxxxx> # 5.17
Signed-off-by: Ruihan Li <lrh2000@xxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Acked-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230515130958.32471-4-lrh2000@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
Documentation/mm/page_table_check.rst | 19 +++++++++++++++++++
mm/Kconfig.debug | 1 +
2 files changed, 20 insertions(+)
--- 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.
--- 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
Patches currently in stable-queue which might be from lrh2000@xxxxxxxxxx are
queue-6.1/usb-usbfs-enforce-page-requirements-for-mmap.patch
queue-6.1/mm-page_table_check-make-it-dependent-on-exclusive_system_ram.patch
queue-6.1/usb-usbfs-use-consistent-mmap-functions.patch
queue-6.1/mm-page_table_check-ensure-user-pages-are-not-slab-pages.patch
