Pasha Tatashin [off-list ref] writes:

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.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 Documentation/vm/page_table_check.rst |  53 ++++++

Thanks for documenting this feature!  When you add a new RST file,
though, you need to add it to the index.rst file as well so that it is
included in the docs build.

Do you need this label for anything?  As-is it's just added visual
clutter and could come out.

+================
+Page Table Check
+================
+
+Page table check allows to hardern the kernel by ensuring that some types of
+memory corruptions are prevented.
+
+Page table check performs extra verifications at the time when new pages become
+accessible from 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 page table check. Thereofre, it
+is disabled by default but can be optionally enabled on systems where extra
+hardening outweighs the 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
+==============================

I'd use subsection markup (single "==========" line underneath) for the
subsections.

++-------------------+-------------------+-------------------+------------------+
+| 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.

Thanks,

jon