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 <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: H.J. Lu <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Yinghai Lu <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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

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 ;

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 */

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);