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x86/KASLR: Drop CONFIG_RANDOMIZE_BASE_MAX_OFFSET 

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.

[kees: rewrote changelog, dropped KERNEL_IMAGE_SIZE_DEFAULT, rewrote help]
Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: H.J. Lu <hjl.tools@gmail.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1461185746-8017-3-git-send-email-keescook@chromium.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Baoquan He 2016-04-20 13:55:43 -07:00 committed by Ingo Molnar
parent 4252db1055
commit e8581e3d67
4 changed files with 32 additions and 55 deletions
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64
arch/x86/Kconfig
View File

@ -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.
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 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.)
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.
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.
If unsure, say N.
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.
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.
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).
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

12
arch/x86/boot/compressed/kaslr.c
View File

@ -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 ;

8
arch/x86/include/asm/page_64_types.h
View File

@ -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 */

3
arch/x86/mm/init_32.c
View File

@ -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);