Note: If CONFIG_RELOCATABLE=y, then the kernel runs from the address
it has been loaded at and the compile time physical address
- (CONFIG_PHYSICAL_START) is ignored.
+ (CONFIG_PHYSICAL_START) is used as the minimum location.
-# Relocation on x86-32 needs some additional build support
+config RANDOMIZE_BASE
+ bool "Randomize the address of the kernel image"
+ depends on RELOCATABLE
+ depends on !HIBERNATION
+ 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.
+
+ 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.
+
+config RANDOMIZE_BASE_MAX_OFFSET
+ hex "Maximum ASLR offset allowed"
+ 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---
+ Determines the maximal offset in bytes that will be applied to the
+ kernel when Address Space Layout Randomization (ASLR) is active.
+ Must be less than or equal to the actual physical memory on the
+ system. This must be a multiple of CONFIG_PHYSICAL_ALIGN.
+
+ On 32-bit this is limited to 512MiB.
+
+ On 64-bit this is limited by how the kernel fixmap page table is
+ positioned, so this cannot be larger that 1GiB currently. Normally
+ there is a 512MiB to 1.5GiB split between kernel and modules. When
+ this is raised above the 512MiB default, the modules area will
+ shrink to compensate, up to the current maximum 1GiB to 1GiB split.
+
+# Relocation on x86 needs some additional build support
config X86_NEED_RELOCS
def_bool y
- depends on X86_32 && RELOCATABLE
+ depends on RANDOMIZE_BASE || (X86_32 && RELOCATABLE)
config PHYSICAL_ALIGN
hex "Alignment value to which kernel should be aligned"
- default "0x1000000"
+ default "0x200000"
range 0x2000 0x1000000 if X86_32
range 0x200000 0x1000000 if X86_64
---help---