CONFIG_RANDOMIZE_BASE_MAX_OFFSET: Maximum kASLR offset

General informations

The Linux kernel configuration item CONFIG_RANDOMIZE_BASE_MAX_OFFSET has multiple definitions:

Maximum kASLR offset found in arch/mips/Kconfig

The configuration item CONFIG_RANDOMIZE_BASE_MAX_OFFSET:

• prompt: Maximum kASLR offset
• type: hex
• depends on: ( CONFIG_EXPERT ) && ( CONFIG_RANDOMIZE_BASE )
• defined in arch/mips/Kconfig
• found in Linux kernels: 4.7–4.20, 5.0–5.19, 6.0–6.12

Help text

When kASLR is active, this provides the maximum offset that will be applied to the kernel image. It should be set according to the amount of physical RAM available in the target system minus PHYSICAL_START and must be a power of 2.

This is limited by the size of KSEG0, 256Mb on 32-bit or 1Gb with EVA or 64-bit. The default is 16Mb.

Maximum KASLR offset found in arch/loongarch/Kconfig

The configuration item CONFIG_RANDOMIZE_BASE_MAX_OFFSET:

• prompt: Maximum KASLR offset
• type: hex
• depends on: ( CONFIG_EXPERT ) && ( CONFIG_RANDOMIZE_BASE )
• defined in arch/loongarch/Kconfig
• found in Linux kernels: 6.3–6.12

Help text

When KASLR is active, this provides the maximum offset that will be applied to the kernel image. It should be set according to the amount of physical RAM available in the target system.

This is limited by the size of the lower address memory, 256MB.

Maximum kASLR offset allowed found in arch/x86/Kconfig

The configuration item CONFIG_RANDOMIZE_BASE_MAX_OFFSET:

• prompt: Maximum kASLR offset allowed
• type: hex
• depends on: ( CONFIG_EXPERT ) && ( CONFIG_RANDOMIZE_BASE ) && ( CONFIG_X86_32 ) && ( CONFIG_X86_64 )
• defined in arch/x86/Kconfig
• found in Linux kernels: 3.14–3.19, 4.0–4.6

Help text

The lesser of RANDOMIZE_BASE_MAX_OFFSET and available physical memory is used to determine the maximal offset in bytes that will be applied to the kernel when kernel Address Space Layout Randomization (kASLR) is active. This must be a multiple of PHYSICAL_ALIGN.

On 32-bit this is limited to 512MiB by page table layouts. The default is 512MiB.

On 64-bit this is limited by how the kernel fixmap page table is positioned, so this cannot be larger than 1GiB currently. Without RANDOMIZE_BASE, there is a 512MiB to 1.5GiB split between kernel and modules. When RANDOMIZE_BASE_MAX_OFFSET is above 512MiB, the modules area will shrink to compensate, up to the current maximum 1GiB to 1GiB split. The default is 1GiB.

If unsure, leave at the default value.

Hardware

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The data is retrived from:

• Linux kernel
• Linux Kernel Driver DataBase (LKDDb)

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