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@xxxxxxx/ 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..c7689d14e744 --- /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 +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:: + + 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:: + + 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 -- 2.25.1
