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: <stable@vger.kernel.org> # 5.17
Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/r/20230515130958.32471-4-lrh2000@pku.edu.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 0 deletions

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