x86, kaslr: Clarify RANDOMIZE_BASE_MAX_OFFSET
The help text for RANDOMIZE_BASE_MAX_OFFSET was confusing. This has been
clarified, and updated to be an export-only tunable.
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/20131210202745.GA2961@www.outflux.net
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 596cd9e..5c9e19d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1747,26 +1747,33 @@
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 ASLR offset allowed"
+ 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---
- 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.
+ 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.
+ 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 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.
+ 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.
# Relocation on x86 needs some additional build support
config X86_NEED_RELOCS
