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authorRuihan Li <lrh2000@pku.edu.cn>2023-05-15 16:09:57 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2023-05-29 18:14:28 +0300
commit81a31a860bb61d54eb688af2568d9332ed9b8942 (patch)
treea34c1ac8fb3f3814f3b596d74e4561bac35c99b4 /Documentation/mm
parentd0b861653f8c16839c3035875b556afc4472f941 (diff)
downloadlinux-81a31a860bb61d54eb688af2568d9332ed9b8942.tar.xz
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. 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>
Diffstat (limited to 'Documentation/mm')
-rw-r--r--Documentation/mm/page_table_check.rst19
1 files changed, 19 insertions, 0 deletions
diff --git a/Documentation/mm/page_table_check.rst b/Documentation/mm/page_table_check.rst
index cfd8f4117cf3..c12838ce6b8d 100644
--- a/Documentation/mm/page_table_check.rst
+++ b/Documentation/mm/page_table_check.rst
@@ -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.