Skip to content

Commit

Permalink
Documentation/arm64: Add ptdump documentation
Browse files Browse the repository at this point in the history 
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/20230613064845.1882177-1-chaitanyas.prakash@arm.com/

Cc: Will Deacon <will@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
CC: linux-arm-kernel@lists.infradead.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Chaitanya S Prakash <chaitanyas.prakash@arm.com>
Link: https://lore.kernel.org/r/20230619083802.76092-1-chaitanyas.prakash@arm.com
[catalin.marinas@arm.com: various minor fixups; sorted index.rst in alphabetical order]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
  • Loading branch information
Chaitanya S Prakash authored and Catalin Marinas committed Jun 21, 2023
1 parent 44c026a commit a0238ad
Show file tree
Hide file tree
Showing 2 changed files with 97 additions and 0 deletions.
1 change: 1 addition & 0 deletions Documentation/arm64/index.rst
View file
Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,7 @@ ARM64 Architecture
memory-tagging-extension
perf
pointer-authentication
ptdump
silicon-errata
sme
sve
Expand Down
96 changes: 96 additions & 0 deletions Documentation/arm64/ptdump.rst
View file
Original file line number Diff line number Diff line change
@@ -0,0 +1,96 @@
======================
Kernel page table dump
======================

ptdump is a debugfs interface that provides a detailed dump of the
kernel page tables. It offers a comprehensive overview of the kernel
virtual memory layout as well as the attributes associated with the
various regions in a human-readable format. It is useful to dump the
kernel page tables to verify permissions and memory types. Examining the
page table entries and permissions helps identify potential security
vulnerabilities such as mappings with overly permissive access rights or
improper memory protections.

Memory hotplug allows dynamic expansion or contraction of available
memory without requiring a system reboot. To maintain the consistency
and integrity of the memory management data structures, arm64 makes use
of the ``mem_hotplug_lock`` semaphore in write mode. Additionally, in
read mode, ``mem_hotplug_lock`` supports an efficient implementation of
``get_online_mems()`` and ``put_online_mems()``. These protect the
offlining of memory being accessed by the ptdump code.

In order to dump the kernel page tables, enable the following
configurations and mount debugfs::

CONFIG_GENERIC_PTDUMP=y
CONFIG_PTDUMP_CORE=y
CONFIG_PTDUMP_DEBUGFS=y

mount -t debugfs nodev /sys/kernel/debug
cat /sys/kernel/debug/kernel_page_tables

On analysing the output of ``cat /sys/kernel/debug/kernel_page_tables``
one can derive information about the virtual address range of the entry,
followed by size of the memory region covered by this entry, the
hierarchical structure of the page tables and finally the attributes
associated with each page. The page attributes provide information about
access permissions, execution capability, type of mapping such as leaf
level PTE or block level PGD, PMD and PUD, and access status of a page
within the kernel memory. Assessing these attributes can assist in
understanding the memory layout, access patterns and security
characteristics of the kernel pages.

Kernel virtual memory layout example::

start address end address size attributes
+---------------------------------------------------------------------------------------+
| ---[ Linear Mapping start ]---------------------------------------------------------- |
| .................. |
| 0xfff0000000000000-0xfff0000000210000 2112K PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED |
| 0xfff0000000210000-0xfff0000001c00000 26560K PTE ro NX SHD AF UXN MEM/NORMAL |
| .................. |
| ---[ Linear Mapping end ]------------------------------------------------------------ |
+---------------------------------------------------------------------------------------+
| ---[ Modules start ]----------------------------------------------------------------- |
| .................. |
| 0xffff800000000000-0xffff800008000000 128M PTE |
| .................. |
| ---[ Modules end ]------------------------------------------------------------------- |
+---------------------------------------------------------------------------------------+
| ---[ vmalloc() area ]---------------------------------------------------------------- |
| .................. |
| 0xffff800008010000-0xffff800008200000 1984K PTE ro x SHD AF UXN MEM/NORMAL |
| 0xffff800008200000-0xffff800008e00000 12M PTE ro x SHD AF CON UXN MEM/NORMAL |
| .................. |
| ---[ vmalloc() end ]----------------------------------------------------------------- |
+---------------------------------------------------------------------------------------+
| ---[ Fixmap start ]------------------------------------------------------------------ |
| .................. |
| 0xfffffbfffdb80000-0xfffffbfffdb90000 64K PTE ro x SHD AF UXN MEM/NORMAL |
| 0xfffffbfffdb90000-0xfffffbfffdba0000 64K PTE ro NX SHD AF UXN MEM/NORMAL |
| .................. |
| ---[ Fixmap end ]-------------------------------------------------------------------- |
+---------------------------------------------------------------------------------------+
| ---[ PCI I/O start ]----------------------------------------------------------------- |
| .................. |
| 0xfffffbfffe800000-0xfffffbffff800000 16M PTE |
| .................. |
| ---[ PCI I/O end ]------------------------------------------------------------------- |
+---------------------------------------------------------------------------------------+
| ---[ vmemmap start ]----------------------------------------------------------------- |
| .................. |
| 0xfffffc0002000000-0xfffffc0002200000 2M PTE RW NX SHD AF UXN MEM/NORMAL |
| 0xfffffc0002200000-0xfffffc0020000000 478M PTE |
| .................. |
| ---[ vmemmap end ]------------------------------------------------------------------- |
+---------------------------------------------------------------------------------------+

``cat /sys/kernel/debug/kernel_page_tables`` output::

0xfff0000001c00000-0xfff0000080000000 2020M PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED
0xfff0000080000000-0xfff0000800000000 30G PMD
0xfff0000800000000-0xfff0000800700000 7M PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED
0xfff0000800700000-0xfff0000800710000 64K PTE ro NX SHD AF UXN MEM/NORMAL-TAGGED
0xfff0000800710000-0xfff0000880000000 2089920K PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED
0xfff0000880000000-0xfff0040000000000 4062G PMD
0xfff0040000000000-0xffff800000000000 3964T PGD

0 comments on commit a0238ad

Please sign in to comment.