mm: page table check

Check user page table entries at the time they are added and removed.

Allows to synchronously catch memory corruption issues related to double
mapping.

When a pte for an anonymous page is added into page table, we verify
that this pte does not already point to a file backed page, and vice
versa if this is a file backed page that is being added we verify that
this page does not have an anonymous mapping

We also enforce that read-only sharing for anonymous pages is allowed
(i.e.  cow after fork).  All other sharing must be for file pages.

Page table check allows to protect and debug cases where "struct page"
metadata became corrupted for some reason.  For example, when refcnt or
mapcount become invalid.

Link: https://lkml.kernel.org/r/20211221154650.1047963-4-pasha.tatashin@soleen.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Greg Thelen <gthelen@google.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wei Xu <weixugc@google.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 56 insertions, 0 deletions

diff --git a/Documentation/vm/page_table_check.rst b/Documentation/vm/page_table_check.rst
new file mode 100644
index 000000000000..81f521ff7ea7
--- /dev/null
+++ b/Documentation/vm/page_table_check.rst
@@ -0,0 +1,56 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _page_table_check:
+
+================
+Page Table Check
+================
+
+Introduction
+============
+
+Page table check allows to hardern the kernel by ensuring that some types of
+the memory corruptions are prevented.
+
+Page table check performs extra verifications at the time when new pages become
+accessible from the userspace by getting their page table entries (PTEs PMDs
+etc.) added into the table.
+
+In case of detected corruption, the kernel is crashed. There is a small
+performance and memory overhead associated with the page table check. Therefore,
+it is disabled by default, but can be optionally enabled on systems where the
+extra hardening outweighs the performance costs. Also, because page table check
+is synchronous, it can help with debugging double map memory corruption issues,
+by crashing kernel at the time wrong mapping occurs instead of later which is
+often the case with memory corruptions bugs.
+
+Double mapping detection logic
+==============================
+
++-------------------+-------------------+-------------------+------------------+
+| Current Mapping   | New mapping       | Permissions       | Rule             |
++===================+===================+===================+==================+
+| Anonymous         | Anonymous         | Read              | Allow            |
++-------------------+-------------------+-------------------+------------------+
+| Anonymous         | Anonymous         | Read / Write      | Prohibit         |
++-------------------+-------------------+-------------------+------------------+
+| Anonymous         | Named             | Any               | Prohibit         |
++-------------------+-------------------+-------------------+------------------+
+| Named             | Anonymous         | Any               | Prohibit         |
++-------------------+-------------------+-------------------+------------------+
+| Named             | Named             | Any               | Allow            |
++-------------------+-------------------+-------------------+------------------+
+
+Enabling Page Table Check
+=========================
+
+Build kernel with:
+
+- PAGE_TABLE_CHECK=y
+  Note, it can only be enabled on platforms where ARCH_SUPPORTS_PAGE_TABLE_CHECK
+  is available.
+
+- Boot with 'page_table_check=on' kernel parameter.
+
+Optionally, build kernel with PAGE_TABLE_CHECK_ENFORCED in order to have page
+table support without extra kernel parameter.