Re: [PATCH] Documentation/arm64: Add ptdump documentation

[Randy Dunlap]

Hi--

On 6/12/23 23:48, Chaitanya S Prakash wrote:
> 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.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> 
> Cc: Will Deacon <will@xxxxxxxxxx> 
> Cc: Jonathan Corbet <corbet@xxxxxxx> 
> CC: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx 
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Chaitanya S Prakash <chaitanyas.prakash@xxxxxxx>
> ---
>  Documentation/arm64/index.rst  |  1 +
>  Documentation/arm64/ptdump.rst | 94 ++++++++++++++++++++++++++++++++++
>  2 files changed, 95 insertions(+)
>  create mode 100644 Documentation/arm64/ptdump.rst
> 
> diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst
> index ae21f8118830..e0bda3ec2090 100644
> --- a/Documentation/arm64/index.rst
> +++ b/Documentation/arm64/index.rst
> @@ -15,6 +15,7 @@ ARM64 Architecture
>      cpu-feature-registers
>      elf_hwcaps
>      hugetlbpage
> +    ptdump
>      legacy_instructions
>      memory
>      memory-tagging-extension
> diff --git a/Documentation/arm64/ptdump.rst b/Documentation/arm64/ptdump.rst
> new file mode 100644
> index 000000000000..296a5fe217b0
> --- /dev/null
> +++ b/Documentation/arm64/ptdump.rst
> @@ -0,0 +1,94 @@
> +======================
> +Kernel page table dump
> +======================
> +
> +ptdump is a debugfs interface that provides a detailed dump of the kernel's
> +page tables. It offers a comprehensive overview of the kernel’s 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 consistency and integrity of
> +memory management data structures, arm64 makes use of mem_hotplug_lock in
> +write mode. Additionally, in read mode mem_hotplug_lock supports efficient
> +implementation of get_online_mems and put_online_mems. This protects the
> +offlining of memory being accessed by code.
> +
> +In order to dump the kernel page tables you will need to have the following
> +configurations enabled and follow it up by mounting 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 you can

Preferably quote the command.
This says kernel_page_tables but below it is 'kernel_page_table'.
Which is correct?

> +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::
> +

Is this memory layout arm64-specific or generic?

> + 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_table::
> +
> + 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

thanks.
-- 
~Randy