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Message-ID: <20160506082746.GB6240@x1.redhat.com>
Date: Fri, 6 May 2016 16:27:46 +0800
From: Baoquan He <bhe@...hat.com>
To: Kees Cook <keescook@...omium.org>
Cc: Ingo Molnar <mingo@...nel.org>, Ingo Molnar <mingo@...hat.com>,
Yinghai Lu <yinghai@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
Borislav Petkov <bp@...en8.de>, Vivek Goyal <vgoyal@...hat.com>,
Andy Lutomirski <luto@...nel.org>, lasse.collin@...aani.org,
Andrew Morton <akpm@...ux-foundation.org>,
Dave Young <dyoung@...hat.com>, kernel-hardening@...ts.openwall.com,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 10/11] x86/KASLR: Add physical address randomization
>4G
Hi Kees,
On 05/05/16 at 03:13pm, Kees Cook wrote:
> From: Baoquan He <bhe@...hat.com>
>
> This patch exchanges the prior slots[] array for the new slot_areas[]
> array, and lifts the limitation of KERNEL_IMAGE_SIZE on the physical
> address offset for 64-bit. As before, process_e820_entry() walks
Sorry, I didn't get what you said about lifting the limitation for
64-bit. Do you mean 32-bit?
Thanks
Baoquan
> memory and populates slot_areas[], splitting on any detected mem_avoid
> collisions.
>
> Finally, since the slots[] array and its associated functions are not
> needed any more, so they are removed.
>
> Signed-off-by: Baoquan He <bhe@...hat.com>
> [kees: rewrote changelog, refactored goto into while]
> [kees: limit 32-bit to KERNEL_IMAGE_SIZE]
> [kees: squash dead-code removal into this patch]
> [kees: refactored to use new mem_overlap code, renamed variables]
> [kees: updated Kconfig to reflect updated implementation]
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> arch/x86/Kconfig | 27 +++++----
> arch/x86/boot/compressed/kaslr.c | 115 +++++++++++++++++++++++----------------
> 2 files changed, 85 insertions(+), 57 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 5892d549596d..39be0f7b49ef 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1943,21 +1943,26 @@ config RANDOMIZE_BASE
> 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.)
> + On 64-bit, the kernel physical and virtual addresses are
> + randomized separately. The physical address will be anywhere
> + between 16MB and the top of physical memory (up to 64TB). The
> + virtual address will be randomized from 16MB up to 1GB (9 bits
> + of entropy). Note that this also reduces the memory space
> + available to kernel modules from 1.5GB to 1GB.
> +
> + On 32-bit, the kernel physical and virtual addresses are
> + randomized together. They will be randomized from 16MB up to
> + 512MB (8 bits of entropy).
>
> 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.
> + supported, then entropy is read from the i8254 timer. The
> + usable entropy is limited by the kernel being built using
> + 2GB addressing, and that PHYSICAL_ALIGN must be at a
> + minimum of 2MB. As a result, only 10 bits of entropy is
> + theoretically possible, but the implementations are further
> + limited due to memory layouts.
>
> If CONFIG_HIBERNATE is also enabled, KASLR is disabled at boot
> time. To enable it, boot with "kaslr" on the kernel command
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 58d0871be037..8cf705826bdc 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -125,17 +125,6 @@ struct mem_vector {
> #define MEM_AVOID_MAX 4
> static struct mem_vector mem_avoid[MEM_AVOID_MAX];
>
> -static bool mem_contains(struct mem_vector *region, struct mem_vector *item)
> -{
> - /* Item at least partially before region. */
> - if (item->start < region->start)
> - return false;
> - /* Item at least partially after region. */
> - if (item->start + item->size > region->start + region->size)
> - return false;
> - return true;
> -}
> -
> static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
> {
> /* Item one is entirely before item two. */
> @@ -286,8 +275,6 @@ static bool mem_avoid_overlap(struct mem_vector *img,
> return is_overlapping;
> }
>
> -static unsigned long slots[KERNEL_IMAGE_SIZE / CONFIG_PHYSICAL_ALIGN];
> -
> struct slot_area {
> unsigned long addr;
> int num;
> @@ -318,36 +305,44 @@ static void store_slot_info(struct mem_vector *region, unsigned long image_size)
> }
> }
>
> -static void slots_append(unsigned long addr)
> -{
> - /* Overflowing the slots list should be impossible. */
> - if (slot_max >= KERNEL_IMAGE_SIZE / CONFIG_PHYSICAL_ALIGN)
> - return;
> -
> - slots[slot_max++] = addr;
> -}
> -
> static unsigned long slots_fetch_random(void)
> {
> + unsigned long slot;
> + int i;
> +
> /* Handle case of no slots stored. */
> if (slot_max == 0)
> return 0;
>
> - return slots[get_random_long("Physical") % slot_max];
> + slot = get_random_long("Physical") % slot_max;
> +
> + for (i = 0; i < slot_area_index; i++) {
> + if (slot >= slot_areas[i].num) {
> + slot -= slot_areas[i].num;
> + continue;
> + }
> + return slot_areas[i].addr + slot * CONFIG_PHYSICAL_ALIGN;
> + }
> +
> + if (i == slot_area_index)
> + debug_putstr("slots_fetch_random() failed!?\n");
> + return 0;
> }
>
> static void process_e820_entry(struct e820entry *entry,
> unsigned long minimum,
> unsigned long image_size)
> {
> - struct mem_vector region, img, overlap;
> + struct mem_vector region, overlap;
> + struct slot_area slot_area;
> + unsigned long start_orig;
>
> /* Skip non-RAM entries. */
> if (entry->type != E820_RAM)
> return;
>
> - /* Ignore entries entirely above our maximum. */
> - if (entry->addr >= KERNEL_IMAGE_SIZE)
> + /* On 32-bit, ignore entries entirely above our maximum. */
> + if (IS_ENABLED(CONFIG_X86_32) && entry->addr >= KERNEL_IMAGE_SIZE)
> return;
>
> /* Ignore entries entirely below our minimum. */
> @@ -357,31 +352,55 @@ static void process_e820_entry(struct e820entry *entry,
> region.start = entry->addr;
> region.size = entry->size;
>
> - /* Potentially raise address to minimum location. */
> - if (region.start < minimum)
> - region.start = minimum;
> + /* Give up if slot area array is full. */
> + while (slot_area_index < MAX_SLOT_AREA) {
> + start_orig = region.start;
>
> - /* Potentially raise address to meet alignment requirements. */
> - region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
> + /* Potentially raise address to minimum location. */
> + if (region.start < minimum)
> + region.start = minimum;
>
> - /* Did we raise the address above the bounds of this e820 region? */
> - if (region.start > entry->addr + entry->size)
> - return;
> + /* Potentially raise address to meet alignment needs. */
> + region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
>
> - /* Reduce size by any delta from the original address. */
> - region.size -= region.start - entry->addr;
> + /* Did we raise the address above this e820 region? */
> + if (region.start > entry->addr + entry->size)
> + return;
>
> - /* Reduce maximum size to fit end of image within maximum limit. */
> - if (region.start + region.size > KERNEL_IMAGE_SIZE)
> - region.size = KERNEL_IMAGE_SIZE - region.start;
> + /* Reduce size by any delta from the original address. */
> + region.size -= region.start - start_orig;
>
> - /* Walk each aligned slot and check for avoided areas. */
> - for (img.start = region.start, img.size = image_size ;
> - mem_contains(®ion, &img) ;
> - img.start += CONFIG_PHYSICAL_ALIGN) {
> - if (mem_avoid_overlap(&img, &overlap))
> - continue;
> - slots_append(img.start);
> + /* On 32-bit, reduce region size to fit within max size. */
> + if (IS_ENABLED(CONFIG_X86_32) &&
> + region.start + region.size > KERNEL_IMAGE_SIZE)
> + region.size = KERNEL_IMAGE_SIZE - region.start;
> +
> + /* Return if region can't contain decompressed kernel */
> + if (region.size < image_size)
> + return;
> +
> + /* If nothing overlaps, store the region and return. */
> + if (!mem_avoid_overlap(®ion, &overlap)) {
> + store_slot_info(®ion, image_size);
> + return;
> + }
> +
> + /* Store beginning of region if holds at least image_size. */
> + if (overlap.start > region.start + image_size) {
> + struct mem_vector beginning;
> +
> + beginning.start = region.start;
> + beginning.size = overlap.start - region.start;
> + store_slot_info(&beginning, image_size);
> + }
> +
> + /* Return if overlap extends to or past end of region. */
> + if (overlap.start + overlap.size >= region.start + region.size)
> + return;
> +
> + /* Clip off the overlapping region and start over. */
> + region.size -= overlap.start - region.start + overlap.size;
> + region.start = overlap.start + overlap.size;
> }
> }
>
> @@ -398,6 +417,10 @@ static unsigned long find_random_phys_addr(unsigned long minimum,
> for (i = 0; i < boot_params->e820_entries; i++) {
> process_e820_entry(&boot_params->e820_map[i], minimum,
> image_size);
> + if (slot_area_index == MAX_SLOT_AREA) {
> + debug_putstr("Aborted e820 scan (slot_areas full)!\n");
> + break;
> + }
> }
>
> return slots_fetch_random();
> --
> 2.6.3
>
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