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>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 2dc1860..5892d54 100644 (file)
--- 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 7d86c5d..3ad71a0 100644 (file)
--- 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 4928cf0..d5c2f8b 100644 (file)
--- 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 bd7a9b9..f2ee42d 100644 (file)
--- 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);