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>

1 files changed, 18 insertions, 11 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 596cd9edeb9c..5c9e19dccf2f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1747,26 +1747,33 @@ config RANDOMIZE_BASE
            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 by page table layouts. The
+          default is 512MiB.
 
-         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 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.
 
-         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.
+          If unsure, leave at the default value.
 
 # Relocation on x86 needs some additional build support
 config X86_NEED_RELOCS