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authorLinus Torvalds <torvalds@linux-foundation.org>2016-05-16 15:54:01 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2016-05-16 15:54:01 -0700
commit9a45f036af363aec1efec08827c825d69c115a9a (patch)
treed9a81016dacbbcdf87d8e2ec3dcebed6b5029870 /arch/x86/Kconfig
parent168f1a7163b37294a0ef33829e1ed54d41e33c42 (diff)
parentd2d3462f9f08da364c8fbd41e8e32229d610d49d (diff)
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Merge branch 'x86-boot-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull x86 boot updates from Ingo Molnar: "The biggest changes in this cycle were: - prepare for more KASLR related changes, by restructuring, cleaning up and fixing the existing boot code. (Kees Cook, Baoquan He, Yinghai Lu) - simplifly/concentrate subarch handling code, eliminate paravirt_enabled() usage. (Luis R Rodriguez)" * 'x86-boot-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (50 commits) x86/KASLR: Clarify purpose of each get_random_long() x86/KASLR: Add virtual address choosing function x86/KASLR: Return earliest overlap when avoiding regions x86/KASLR: Add 'struct slot_area' to manage random_addr slots x86/boot: Add missing file header comments x86/KASLR: Initialize mapping_info every time x86/boot: Comment what finalize_identity_maps() does x86/KASLR: Build identity mappings on demand x86/boot: Split out kernel_ident_mapping_init() x86/boot: Clean up indenting for asm/boot.h x86/KASLR: Improve comments around the mem_avoid[] logic x86/boot: Simplify pointer casting in choose_random_location() x86/KASLR: Consolidate mem_avoid[] entries x86/boot: Clean up pointer casting x86/boot: Warn on future overlapping memcpy() use x86/boot: Extract error reporting functions x86/boot: Correctly bounds-check relocations x86/KASLR: Clean up unused code from old 'run_size' and rename it to 'kernel_total_size' x86/boot: Fix "run_size" calculation x86/boot: Calculate decompression size during boot not build ...
Diffstat (limited to 'arch/x86/Kconfig')
-rw-r--r--arch/x86/Kconfig72
1 files changed, 28 insertions, 44 deletions
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index a494fa3..7bb1574 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
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