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Chaitanya S Prakashctmarinas
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Documentation/arm64: Add ptdump documentation
ptdump is a debugfs interface used to dump the kernel page tables. It provides a comprehensive overview about the kernel's virtual memory layout, page table entries and associated page attributes. A document detailing how to enable ptdump in the kernel and analyse its output has been added. Changes in V2: - Corrected command to cat /sys/kernel/debug/kernel_page_tables Changes in V1: https://lore.kernel.org/all/[email protected]/ Cc: Will Deacon <[email protected]> Cc: Jonathan Corbet <[email protected]> CC: [email protected] Cc: [email protected] Cc: [email protected] Signed-off-by: Chaitanya S Prakash <[email protected]> Link: https://lore.kernel.org/r/[email protected] [[email protected]: various minor fixups; sorted index.rst in alphabetical order] Signed-off-by: Catalin Marinas <[email protected]>
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‎Documentation/arm64/index.rst

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@@ -20,6 +20,7 @@ ARM64 Architecture
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memory-tagging-extension
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perf
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pointer-authentication
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ptdump
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silicon-errata
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sme
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sve

‎Documentation/arm64/ptdump.rst

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======================
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Kernel page table dump
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======================
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ptdump is a debugfs interface that provides a detailed dump of the
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kernel page tables. It offers a comprehensive overview of the kernel
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virtual memory layout as well as the attributes associated with the
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various regions in a human-readable format. It is useful to dump the
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kernel page tables to verify permissions and memory types. Examining the
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page table entries and permissions helps identify potential security
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vulnerabilities such as mappings with overly permissive access rights or
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improper memory protections.
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Memory hotplug allows dynamic expansion or contraction of available
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memory without requiring a system reboot. To maintain the consistency
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and integrity of the memory management data structures, arm64 makes use
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of the ``mem_hotplug_lock`` semaphore in write mode. Additionally, in
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read mode, ``mem_hotplug_lock`` supports an efficient implementation of
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``get_online_mems()`` and ``put_online_mems()``. These protect the
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offlining of memory being accessed by the ptdump code.
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In order to dump the kernel page tables, enable the following
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configurations and mount debugfs::
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CONFIG_GENERIC_PTDUMP=y
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CONFIG_PTDUMP_CORE=y
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CONFIG_PTDUMP_DEBUGFS=y
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mount -t debugfs nodev /sys/kernel/debug
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cat /sys/kernel/debug/kernel_page_tables
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On analysing the output of ``cat /sys/kernel/debug/kernel_page_tables``
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one can derive information about the virtual address range of the entry,
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followed by size of the memory region covered by this entry, the
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hierarchical structure of the page tables and finally the attributes
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associated with each page. The page attributes provide information about
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access permissions, execution capability, type of mapping such as leaf
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level PTE or block level PGD, PMD and PUD, and access status of a page
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within the kernel memory. Assessing these attributes can assist in
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understanding the memory layout, access patterns and security
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characteristics of the kernel pages.
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Kernel virtual memory layout example::
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start address end address size attributes
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+---------------------------------------------------------------------------------------+
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| ---[ Linear Mapping start ]---------------------------------------------------------- |
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| .................. |
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| 0xfff0000000000000-0xfff0000000210000 2112K PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED |
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| 0xfff0000000210000-0xfff0000001c00000 26560K PTE ro NX SHD AF UXN MEM/NORMAL |
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| .................. |
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| ---[ Linear Mapping end ]------------------------------------------------------------ |
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+---------------------------------------------------------------------------------------+
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| ---[ Modules start ]----------------------------------------------------------------- |
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| .................. |
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| 0xffff800000000000-0xffff800008000000 128M PTE |
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| .................. |
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| ---[ Modules end ]------------------------------------------------------------------- |
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+---------------------------------------------------------------------------------------+
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| ---[ vmalloc() area ]---------------------------------------------------------------- |
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| .................. |
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| 0xffff800008010000-0xffff800008200000 1984K PTE ro x SHD AF UXN MEM/NORMAL |
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| 0xffff800008200000-0xffff800008e00000 12M PTE ro x SHD AF CON UXN MEM/NORMAL |
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| .................. |
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| ---[ vmalloc() end ]----------------------------------------------------------------- |
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+---------------------------------------------------------------------------------------+
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| ---[ Fixmap start ]------------------------------------------------------------------ |
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| .................. |
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| 0xfffffbfffdb80000-0xfffffbfffdb90000 64K PTE ro x SHD AF UXN MEM/NORMAL |
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| 0xfffffbfffdb90000-0xfffffbfffdba0000 64K PTE ro NX SHD AF UXN MEM/NORMAL |
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| .................. |
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| ---[ Fixmap end ]-------------------------------------------------------------------- |
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+---------------------------------------------------------------------------------------+
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| ---[ PCI I/O start ]----------------------------------------------------------------- |
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| .................. |
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| 0xfffffbfffe800000-0xfffffbffff800000 16M PTE |
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| .................. |
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| ---[ PCI I/O end ]------------------------------------------------------------------- |
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+---------------------------------------------------------------------------------------+
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| ---[ vmemmap start ]----------------------------------------------------------------- |
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| .................. |
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| 0xfffffc0002000000-0xfffffc0002200000 2M PTE RW NX SHD AF UXN MEM/NORMAL |
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| 0xfffffc0002200000-0xfffffc0020000000 478M PTE |
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| .................. |
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| ---[ vmemmap end ]------------------------------------------------------------------- |
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+---------------------------------------------------------------------------------------+
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``cat /sys/kernel/debug/kernel_page_tables`` output::
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0xfff0000001c00000-0xfff0000080000000 2020M PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED
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0xfff0000080000000-0xfff0000800000000 30G PMD
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0xfff0000800000000-0xfff0000800700000 7M PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED
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0xfff0000800700000-0xfff0000800710000 64K PTE ro NX SHD AF UXN MEM/NORMAL-TAGGED
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0xfff0000800710000-0xfff0000880000000 2089920K PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED
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0xfff0000880000000-0xfff0040000000000 4062G PMD
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0xfff0040000000000-0xffff800000000000 3964T PGD

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