diff options
Diffstat (limited to 'arch/x86/Kconfig')
| -rw-r--r-- | arch/x86/Kconfig | 72 |
1 files changed, 28 insertions, 44 deletions
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index a494fa34713a..7bb15747fea2 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -1921,54 +1921,38 @@ config RELOCATABLE (CONFIG_PHYSICAL_START) is used as the minimum location. config RANDOMIZE_BASE - bool "Randomize the address of the kernel image" + bool "Randomize the address of the kernel image (KASLR)" depends on RELOCATABLE default n ---help--- - Randomizes the physical and virtual address at which the - kernel image is decompressed, as a security feature that - deters exploit attempts relying on knowledge of the location - of kernel internals. + In support of Kernel Address Space Layout Randomization (KASLR), + this randomizes the physical address at which the kernel image + is decompressed and the virtual address where the kernel + image is mapped, as a security feature that deters exploit + attempts relying on knowledge of the location of kernel + code internals. + + The kernel physical and virtual address can be randomized + from 16MB up to 1GB on 64-bit and 512MB on 32-bit. (Note that + using RANDOMIZE_BASE reduces the memory space available to + kernel modules from 1.5GB to 1GB.) + + Entropy is generated using the RDRAND instruction if it is + supported. If RDTSC is supported, its value is mixed into + the entropy pool as well. If neither RDRAND nor RDTSC are + supported, then entropy is read from the i8254 timer. + + Since the kernel is built using 2GB addressing, and + PHYSICAL_ALIGN must be at a minimum of 2MB, only 10 bits of + entropy is theoretically possible. Currently, with the + default value for PHYSICAL_ALIGN and due to page table + layouts, 64-bit uses 9 bits of entropy and 32-bit uses 8 bits. + + If CONFIG_HIBERNATE is also enabled, KASLR is disabled at boot + time. To enable it, boot with "kaslr" on the kernel command + line (which will also disable hibernation). - Entropy is generated using the RDRAND instruction if it is - supported. If RDTSC is supported, it is used as well. If - neither RDRAND nor RDTSC are supported, then randomness is - read from the i8254 timer. - - The kernel will be offset by up to RANDOMIZE_BASE_MAX_OFFSET, - and aligned according to PHYSICAL_ALIGN. Since the kernel is - built using 2GiB addressing, and PHYSICAL_ALGIN must be at a - minimum of 2MiB, only 10 bits of entropy is theoretically - possible. At best, due to page table layouts, 64-bit can use - 9 bits of entropy and 32-bit uses 8 bits. - - If unsure, say N. - -config RANDOMIZE_BASE_MAX_OFFSET - hex "Maximum kASLR offset allowed" if EXPERT - depends on RANDOMIZE_BASE - range 0x0 0x20000000 if X86_32 - default "0x20000000" if X86_32 - range 0x0 0x40000000 if X86_64 - default "0x40000000" if X86_64 - ---help--- - 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. + If unsure, say N. # Relocation on x86 needs some additional build support config X86_NEED_RELOCS |