From f80fb3a3d50843a401dac4b566b3b131da8077a2 Mon Sep 17 00:00:00 2001 From: Ard Biesheuvel Date: Tue, 26 Jan 2016 14:12:01 +0100 Subject: arm64: add support for kernel ASLR This adds support for KASLR is implemented, based on entropy provided by the bootloader in the /chosen/kaslr-seed DT property. Depending on the size of the address space (VA_BITS) and the page size, the entropy in the virtual displacement is up to 13 bits (16k/2 levels) and up to 25 bits (all 4 levels), with the sidenote that displacements that result in the kernel image straddling a 1GB/32MB/512MB alignment boundary (for 4KB/16KB/64KB granule kernels, respectively) are not allowed, and will be rounded up to an acceptable value. If CONFIG_RANDOMIZE_MODULE_REGION_FULL is enabled, the module region is randomized independently from the core kernel. This makes it less likely that the location of core kernel data structures can be determined by an adversary, but causes all function calls from modules into the core kernel to be resolved via entries in the module PLTs. If CONFIG_RANDOMIZE_MODULE_REGION_FULL is not enabled, the module region is randomized by choosing a page aligned 128 MB region inside the interval [_etext - 128 MB, _stext + 128 MB). This gives between 10 and 14 bits of entropy (depending on page size), independently of the kernel randomization, but still guarantees that modules are within the range of relative branch and jump instructions (with the caveat that, since the module region is shared with other uses of the vmalloc area, modules may need to be loaded further away if the module region is exhausted) Signed-off-by: Ard Biesheuvel Signed-off-by: Catalin Marinas --- arch/arm64/Kconfig | 29 +++++++++++++++++++++++++++++ 1 file changed, 29 insertions(+) (limited to 'arch/arm64/Kconfig') diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index fef3dc6e21b9..968fa13cc25b 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -798,6 +798,35 @@ config RELOCATABLE relocation pass at runtime even if the kernel is loaded at the same address it was linked at. +config RANDOMIZE_BASE + bool "Randomize the address of the kernel image" + select ARM64_MODULE_PLTS + select RELOCATABLE + help + Randomizes the virtual address at which the kernel image is + loaded, as a security feature that deters exploit attempts + relying on knowledge of the location of kernel internals. + + It is the bootloader's job to provide entropy, by passing a + random u64 value in /chosen/kaslr-seed at kernel entry. + + If unsure, say N. + +config RANDOMIZE_MODULE_REGION_FULL + bool "Randomize the module region independently from the core kernel" + depends on RANDOMIZE_BASE + default y + help + Randomizes the location of the module region without considering the + location of the core kernel. This way, it is impossible for modules + to leak information about the location of core kernel data structures + but it does imply that function calls between modules and the core + kernel will need to be resolved via veneers in the module PLT. + + When this option is not set, the module region will be randomized over + a limited range that contains the [_stext, _etext] interval of the + core kernel, so branch relocations are always in range. + endmenu menu "Boot options" -- cgit v1.2.3