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854d0982ee
Fixes: df4e817b71 ("mm: page table check")
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Link: https://lore.kernel.org/r/20220117111338.115455-1-pmenzel@molgen.mpg.de
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
56 lines
2.4 KiB
ReStructuredText
56 lines
2.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. _page_table_check:
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================
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Page Table Check
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================
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Introduction
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============
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Page table check allows to harden the kernel by ensuring that some types of
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the memory corruptions are prevented.
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Page table check performs extra verifications at the time when new pages become
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accessible from the userspace by getting their page table entries (PTEs PMDs
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etc.) added into the table.
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In case of detected corruption, the kernel is crashed. There is a small
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performance and memory overhead associated with the page table check. Therefore,
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it is disabled by default, but can be optionally enabled on systems where the
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extra hardening outweighs the performance costs. Also, because page table check
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is synchronous, it can help with debugging double map memory corruption issues,
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by crashing kernel at the time wrong mapping occurs instead of later which is
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often the case with memory corruptions bugs.
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Double mapping detection logic
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==============================
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+-------------------+-------------------+-------------------+------------------+
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| Current Mapping | New mapping | Permissions | Rule |
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+===================+===================+===================+==================+
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| Anonymous | Anonymous | Read | Allow |
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+-------------------+-------------------+-------------------+------------------+
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| Anonymous | Anonymous | Read / Write | Prohibit |
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+-------------------+-------------------+-------------------+------------------+
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| Anonymous | Named | Any | Prohibit |
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+-------------------+-------------------+-------------------+------------------+
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| Named | Anonymous | Any | Prohibit |
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+-------------------+-------------------+-------------------+------------------+
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| Named | Named | Any | Allow |
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+-------------------+-------------------+-------------------+------------------+
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Enabling Page Table Check
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=========================
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Build kernel with:
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- PAGE_TABLE_CHECK=y
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Note, it can only be enabled on platforms where ARCH_SUPPORTS_PAGE_TABLE_CHECK
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is available.
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- Boot with 'page_table_check=on' kernel parameter.
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Optionally, build kernel with PAGE_TABLE_CHECK_ENFORCED in order to have page
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table support without extra kernel parameter.
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