[PATCH 2/5] x86, KASLR: Drop CONFIG_RANDOMIZE_BASE_MAX_OFFSET
From: Kees Cook
Date: Wed Apr 20 2016 - 16:56:40 EST
From: Baoquan He <bhe@xxxxxxxxxx>
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 <bhe@xxxxxxxxxx>
[kees: rewrote changelog, dropped KERNEL_IMAGE_SIZE_DEFAULT, rewrote help]
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
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
+ 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
diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index 7d86c5dd8e99..3ad71a0afa24 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -217,15 +217,13 @@ static bool mem_avoid_overlap(struct mem_vector *img)
return false;
}
-static unsigned long slots[CONFIG_RANDOMIZE_BASE_MAX_OFFSET /
- CONFIG_PHYSICAL_ALIGN];
+static unsigned long slots[KERNEL_IMAGE_SIZE / CONFIG_PHYSICAL_ALIGN];
static unsigned long slot_max;
static void slots_append(unsigned long addr)
{
/* Overflowing the slots list should be impossible. */
- if (slot_max >= CONFIG_RANDOMIZE_BASE_MAX_OFFSET /
- CONFIG_PHYSICAL_ALIGN)
+ if (slot_max >= KERNEL_IMAGE_SIZE / CONFIG_PHYSICAL_ALIGN)
return;
slots[slot_max++] = addr;
@@ -251,7 +249,7 @@ static void process_e820_entry(struct e820entry *entry,
return;
/* Ignore entries entirely above our maximum. */
- if (entry->addr >= CONFIG_RANDOMIZE_BASE_MAX_OFFSET)
+ if (entry->addr >= KERNEL_IMAGE_SIZE)
return;
/* Ignore entries entirely below our minimum. */
@@ -276,8 +274,8 @@ static void process_e820_entry(struct e820entry *entry,
region.size -= region.start - entry->addr;
/* Reduce maximum size to fit end of image within maximum limit. */
- if (region.start + region.size > CONFIG_RANDOMIZE_BASE_MAX_OFFSET)
- region.size = CONFIG_RANDOMIZE_BASE_MAX_OFFSET - region.start;
+ if (region.start + region.size > KERNEL_IMAGE_SIZE)
+ region.size = KERNEL_IMAGE_SIZE - region.start;
/* Walk each aligned slot and check for avoided areas. */
for (img.start = region.start, img.size = image_size ;
diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
index 4928cf0d5af0..d5c2f8b40faa 100644
--- a/arch/x86/include/asm/page_64_types.h
+++ b/arch/x86/include/asm/page_64_types.h
@@ -47,12 +47,10 @@
* are fully set up. If kernel ASLR is configured, it can extend the
* kernel page table mapping, reducing the size of the modules area.
*/
-#define KERNEL_IMAGE_SIZE_DEFAULT (512 * 1024 * 1024)
-#if defined(CONFIG_RANDOMIZE_BASE) && \
- CONFIG_RANDOMIZE_BASE_MAX_OFFSET > KERNEL_IMAGE_SIZE_DEFAULT
-#define KERNEL_IMAGE_SIZE CONFIG_RANDOMIZE_BASE_MAX_OFFSET
+#if defined(CONFIG_RANDOMIZE_BASE)
+#define KERNEL_IMAGE_SIZE (1024 * 1024 * 1024)
#else
-#define KERNEL_IMAGE_SIZE KERNEL_IMAGE_SIZE_DEFAULT
+#define KERNEL_IMAGE_SIZE (512 * 1024 * 1024)
#endif
#endif /* _ASM_X86_PAGE_64_DEFS_H */
diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
index bd7a9b9e2e14..f2ee42d61894 100644
--- a/arch/x86/mm/init_32.c
+++ b/arch/x86/mm/init_32.c
@@ -804,9 +804,6 @@ void __init mem_init(void)
BUILD_BUG_ON(VMALLOC_START >= VMALLOC_END);
#undef high_memory
#undef __FIXADDR_TOP
-#ifdef CONFIG_RANDOMIZE_BASE
- BUILD_BUG_ON(CONFIG_RANDOMIZE_BASE_MAX_OFFSET > KERNEL_IMAGE_SIZE);
-#endif
#ifdef CONFIG_HIGHMEM
BUG_ON(PKMAP_BASE + LAST_PKMAP*PAGE_SIZE > FIXADDR_START);
--
2.6.3