On Wed, Apr 20, 2016 at 01:55:43PM -0700, Kees Cook wrote:
> From: Baoquan He <b...@redhat.com>
>
> Currently CONFIG_RANDOMIZE_BASE_MAX_OFFSET is used to limit the maximum
> offset for kernel randomization. This limit doesn't need to be a CONFIG
> since it is tied completely to KERNEL_IMAGE_SIZE, and will make no sense
> once physical and virtual offsets are randomized separately. This patch
> removes CONFIG_RANDOMIZE_BASE_MAX_OFFSET and consolidates the Kconfig
> help text.
>
> Signed-off-by: Baoquan He <b...@redhat.com>
> [kees: rewrote changelog, dropped KERNEL_IMAGE_SIZE_DEFAULT, rewrote help]
> Signed-off-by: Kees Cook <keesc...@chromium.org>
> ---
> arch/x86/Kconfig | 72
> ++++++++++++++----------------------
> arch/x86/boot/compressed/kaslr.c | 12 +++---
> arch/x86/include/asm/page_64_types.h | 8 ++--
> arch/x86/mm/init_32.c | 3 --
> 4 files changed, 36 insertions(+), 59 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 2dc18605831f..5892d549596d 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1932,54 +1932,38 @@ 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
Just say "loaded" here.
> + 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,
Does that try to refer to the 1G kernel and 1G fixmap pagetable
mappings? I.e., level2_kernel_pgt and level2_fixmap_pgt in
arch/x86/kernel/head_64.S?
> 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).
...
--
Regards/Gruss,
Boris.
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284
(AG Nürnberg)
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