5 ptdump is a debugfs interface that provides a detailed dump of the
6 kernel page tables. It offers a comprehensive overview of the kernel
7 virtual memory layout as well as the attributes associated with the
8 various regions in a human-readable format. It is useful to dump the
9 kernel page tables to verify permissions and memory types. Examining the
10 page table entries and permissions helps identify potential security
11 vulnerabilities such as mappings with overly permissive access rights or
12 improper memory protections.
14 Memory hotplug allows dynamic expansion or contraction of available
15 memory without requiring a system reboot. To maintain the consistency
16 and integrity of the memory management data structures, arm64 makes use
17 of the ``mem_hotplug_lock`` semaphore in write mode. Additionally, in
18 read mode, ``mem_hotplug_lock`` supports an efficient implementation of
19 ``get_online_mems()`` and ``put_online_mems()``. These protect the
20 offlining of memory being accessed by the ptdump code.
22 In order to dump the kernel page tables, enable the following
23 configurations and mount debugfs::
25 CONFIG_GENERIC_PTDUMP=y
27 CONFIG_PTDUMP_DEBUGFS=y
29 mount -t debugfs nodev /sys/kernel/debug
30 cat /sys/kernel/debug/kernel_page_tables
32 On analysing the output of ``cat /sys/kernel/debug/kernel_page_tables``
33 one can derive information about the virtual address range of the entry,
34 followed by size of the memory region covered by this entry, the
35 hierarchical structure of the page tables and finally the attributes
36 associated with each page. The page attributes provide information about
37 access permissions, execution capability, type of mapping such as leaf
38 level PTE or block level PGD, PMD and PUD, and access status of a page
39 within the kernel memory. Assessing these attributes can assist in
40 understanding the memory layout, access patterns and security
41 characteristics of the kernel pages.
43 Kernel virtual memory layout example::
45 start address end address size attributes
46 +---------------------------------------------------------------------------------------+
47 | ---[ Linear Mapping start ]---------------------------------------------------------- |
48 | .................. |
49 | 0xfff0000000000000-0xfff0000000210000 2112K PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED |
50 | 0xfff0000000210000-0xfff0000001c00000 26560K PTE ro NX SHD AF UXN MEM/NORMAL |
51 | .................. |
52 | ---[ Linear Mapping end ]------------------------------------------------------------ |
53 +---------------------------------------------------------------------------------------+
54 | ---[ Modules start ]----------------------------------------------------------------- |
55 | .................. |
56 | 0xffff800000000000-0xffff800008000000 128M PTE |
57 | .................. |
58 | ---[ Modules end ]------------------------------------------------------------------- |
59 +---------------------------------------------------------------------------------------+
60 | ---[ vmalloc() area ]---------------------------------------------------------------- |
61 | .................. |
62 | 0xffff800008010000-0xffff800008200000 1984K PTE ro x SHD AF UXN MEM/NORMAL |
63 | 0xffff800008200000-0xffff800008e00000 12M PTE ro x SHD AF CON UXN MEM/NORMAL |
64 | .................. |
65 | ---[ vmalloc() end ]----------------------------------------------------------------- |
66 +---------------------------------------------------------------------------------------+
67 | ---[ Fixmap start ]------------------------------------------------------------------ |
68 | .................. |
69 | 0xfffffbfffdb80000-0xfffffbfffdb90000 64K PTE ro x SHD AF UXN MEM/NORMAL |
70 | 0xfffffbfffdb90000-0xfffffbfffdba0000 64K PTE ro NX SHD AF UXN MEM/NORMAL |
71 | .................. |
72 | ---[ Fixmap end ]-------------------------------------------------------------------- |
73 +---------------------------------------------------------------------------------------+
74 | ---[ PCI I/O start ]----------------------------------------------------------------- |
75 | .................. |
76 | 0xfffffbfffe800000-0xfffffbffff800000 16M PTE |
77 | .................. |
78 | ---[ PCI I/O end ]------------------------------------------------------------------- |
79 +---------------------------------------------------------------------------------------+
80 | ---[ vmemmap start ]----------------------------------------------------------------- |
81 | .................. |
82 | 0xfffffc0002000000-0xfffffc0002200000 2M PTE RW NX SHD AF UXN MEM/NORMAL |
83 | 0xfffffc0002200000-0xfffffc0020000000 478M PTE |
84 | .................. |
85 | ---[ vmemmap end ]------------------------------------------------------------------- |
86 +---------------------------------------------------------------------------------------+
88 ``cat /sys/kernel/debug/kernel_page_tables`` output::
90 0xfff0000001c00000-0xfff0000080000000 2020M PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED
91 0xfff0000080000000-0xfff0000800000000 30G PMD
92 0xfff0000800000000-0xfff0000800700000 7M PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED
93 0xfff0000800700000-0xfff0000800710000 64K PTE ro NX SHD AF UXN MEM/NORMAL-TAGGED
94 0xfff0000800710000-0xfff0000880000000 2089920K PTE RW NX SHD AF UXN MEM/NORMAL-TAGGED
95 0xfff0000880000000-0xfff0040000000000 4062G PMD
96 0xfff0040000000000-0xffff800000000000 3964T PGD