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mm: page_table_check: Make it dependent on EXCLUSIVE_SYSTEM_RAM

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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>
This commit is contained in:
Ruihan Li 2023-05-15 21:09:57 +08:00 committed by Greg Kroah-Hartman
parent d0b861653f
commit 81a31a860b
2 changed files with 20 additions and 0 deletions
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19
Documentation/mm/page_table_check.rst
View File

@ -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.

1
mm/Kconfig.debug
View File

@ -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