Re: [PATCH 2/2] x86/KASLR/64: Determine kernel text mapping size at runtime

From: Alexnader Kuleshov
Date: Thu Dec 08 2016 - 03:18:05 EST

Hello,

On 12-08-16, Baoquan He wrote:
> X86 64 kernel takes KERNEL_IMAGE_SIZE as the kernel text mapping size,
> and it's fixed as compiling time, changing from 512M to 1G as long as
> CONFIG_RANDOMIZE_BASE is enabled, though people specify kernel option
> "nokaslr" explicitly.
>
> This could be a wrong behaviour. CONFIG_RANDOMIZE_BASE should only decide
> if the KASLR code need be compiled in. If user specify "nokaslr", the
> kernel should behave as no KASLR code compiled in at all.
>
> So in this patch, define a new MACRO KERNEL_MAPPING_SIZE to represent the
> size of kernel text mapping area, and let KERNEL_IMAGE_SIZE limit the size
> of kernel runtime space. And change to determine the size of kernel text
> mapping area at runtime. Though KASLR code compiled in, if "nokaslr" specified,
> still set kernel mapping size to be 512M.
>
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> ---
> arch/x86/boot/compressed/kaslr.c | 15 ++++++++++-----
> arch/x86/include/asm/kaslr.h | 1 +
> arch/x86/include/asm/page_64_types.h | 19 +++++++++++--------
> arch/x86/include/asm/pgtable_64_types.h | 2 +-
> arch/x86/kernel/head64.c | 11 ++++++-----
> arch/x86/kernel/head_64.S | 3 ++-
> arch/x86/mm/dump_pagetables.c | 3 ++-
> arch/x86/mm/physaddr.c | 6 +++---
> 8 files changed, 36 insertions(+), 24 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index a66854d..3b73c76 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -22,6 +22,8 @@
> static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
> LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
>
> +unsigned long kernel_mapping_size = KERNEL_IMAGE_SIZE;
> +
> static unsigned long rotate_xor(unsigned long hash, const void *area,
> size_t size)
> {
> @@ -311,7 +313,7 @@ static void process_e820_entry(struct e820entry *entry,
> return;
>
> /* On 32-bit, ignore entries entirely above our maximum. */
> - if (IS_ENABLED(CONFIG_X86_32) && entry->addr >= KERNEL_IMAGE_SIZE)
> + if (IS_ENABLED(CONFIG_X86_32) && entry->addr >= kernel_mapping_size)
> return;
>
> /* Ignore entries entirely below our minimum. */
> @@ -341,8 +343,8 @@ static void process_e820_entry(struct e820entry *entry,
>
> /* 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;
> + region.start + region.size > kernel_mapping_size)
> + region.size = kernel_mapping_size - region.start;
>
> /* Return if region can't contain decompressed kernel */
> if (region.size < image_size)
> @@ -408,9 +410,9 @@ static unsigned long find_random_virt_addr(unsigned long minimum,
> /*
> * There are how many CONFIG_PHYSICAL_ALIGN-sized slots
> * that can hold image_size within the range of minimum to
> - * KERNEL_IMAGE_SIZE?
> + * kernel_mapping_size?
> */
> - slots = (KERNEL_IMAGE_SIZE - minimum - image_size) /
> + slots = (kernel_mapping_size - minimum - image_size) /
> CONFIG_PHYSICAL_ALIGN + 1;
>
> random_addr = kaslr_get_random_long("Virtual") % slots;
> @@ -438,6 +440,9 @@ void choose_random_location(unsigned long input,
> return;
> }
>
> + if (IS_ENABLED(CONFIG_X86_64))
> + kernel_mapping_size = KERNEL_MAPPING_SIZE_EXT;
> +
> boot_params->hdr.loadflags |= KASLR_FLAG;
>
> /* Prepare to add new identity pagetables on demand. */
> diff --git a/arch/x86/include/asm/kaslr.h b/arch/x86/include/asm/kaslr.h
> index 1052a79..c4f5728 100644
> --- a/arch/x86/include/asm/kaslr.h
> +++ b/arch/x86/include/asm/kaslr.h
> @@ -7,6 +7,7 @@ unsigned long kaslr_get_random_long(const char *purpose);
> extern unsigned long page_offset_base;
> extern unsigned long vmalloc_base;
> extern unsigned long vmemmap_base;
> +extern unsigned long kernel_mapping_size;
>
> void kernel_randomize_memory(void);
> #else
> diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
> index 62a20ea..b8e79d7 100644
> --- a/arch/x86/include/asm/page_64_types.h
> +++ b/arch/x86/include/asm/page_64_types.h
> @@ -49,18 +49,21 @@
> #define __PHYSICAL_MASK_SHIFT 46
> #define __VIRTUAL_MASK_SHIFT 47
>
> +
> +/*
> + * Kernel image size is limited to 512 MB. The kernel code+data+bss
> + * must not be bigger than that.
> + */
> +#define KERNEL_IMAGE_SIZE (512 * 1024 * 1024)
> +
> /*
> - * Kernel image size is limited to 1GiB due to the fixmap living in the
> - * next 1GiB (see level2_kernel_pgt in arch/x86/kernel/head_64.S). Use
> - * 512MiB by default, leaving 1.5GiB for modules once the page tables
> + * Kernel mapping size is limited to 1GiB due to the fixmap living in
> + * the next 1GiB (see level2_kernel_pgt in arch/x86/kernel/head_64.S).
> + * Use 512MiB by default, leaving 1.5GiB for modules once the page tables
> * 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_MAPPING_SIZE_EXT (1024 * 1024 * 1024)
> -#if defined(CONFIG_RANDOMIZE_BASE)
> -#define KERNEL_IMAGE_SIZE KERNEL_MAPPING_SIZE_EXT
> -#else
> -#define KERNEL_IMAGE_SIZE (512 * 1024 * 1024)
> -#endif
> +#define KERNEL_MAPPING_SIZE kernel_mapping_size

After applying these patches I'm getting:

CC arch/x86/kernel/setup.o
In file included from ./arch/x86/include/asm/page_types.h:47:0,
from ./arch/x86/include/asm/page.h:8,
from ./arch/x86/include/asm/thread_info.h:11,
from ./include/linux/thread_info.h:58,
from ./arch/x86/include/asm/preempt.h:6,
from ./include/linux/preempt.h:59,
from ./include/linux/spinlock.h:50,
from ./include/linux/seqlock.h:35,
from ./include/linux/time.h:5,
from ./include/uapi/linux/timex.h:56,
from ./include/linux/timex.h:56,
from ./include/linux/sched.h:19,
from arch/x86/kernel/setup.c:24:
arch/x86/kernel/setup.c: In function âdump_kernel_offsetâ:
./arch/x86/include/asm/page_64_types.h:67:29: error: âkernel_mapping_sizeâ undeclared (first use in this function)
#define KERNEL_MAPPING_SIZE kernel_mapping_size
^
./arch/x86/include/asm/pgtable_64_types.h:69:48: note: in expansion of macro âKERNEL_MAPPING_SIZEâ
#define MODULES_VADDR (__START_KERNEL_map + KERNEL_MAPPING_SIZE)
^~~~~~~~~~~~~~~~~~~
./include/linux/printk.h:271:35: note: in expansion of macro âMODULES_VADDRâ
printk(KERN_EMERG pr_fmt(fmt), ##__VA_ARGS__)
^~~~~~~~~~~
arch/x86/kernel/setup.c:826:3: note: in expansion of macro âpr_emergâ
pr_emerg("Kernel Offset: 0x%lx from 0x%lx (relocation range: 0x%lx-0x%lx)\n",
^~~~~~~~
./arch/x86/include/asm/page_64_types.h:67:29: note: each undeclared identifier is reported only once for each function it appears in
#define KERNEL_MAPPING_SIZE kernel_mapping_size
^
./arch/x86/include/asm/pgtable_64_types.h:69:48: note: in expansion of macro âKERNEL_MAPPING_SIZEâ
#define MODULES_VADDR (__START_KERNEL_map + KERNEL_MAPPING_SIZE)
^~~~~~~~~~~~~~~~~~~
./include/linux/printk.h:271:35: note: in expansion of macro âMODULES_VADDRâ
printk(KERN_EMERG pr_fmt(fmt), ##__VA_ARGS__)
^~~~~~~~~~~
arch/x86/kernel/setup.c:826:3: note: in expansion of macro âpr_emergâ
pr_emerg("Kernel Offset: 0x%lx from 0x%lx (relocation range: 0x%lx-0x%lx)\n",
^~~~~~~~
scripts/Makefile.build:293: recipe for target 'arch/x86/kernel/setup.o' failed
make[2]: *** [arch/x86/kernel/setup.o] Error 1
scripts/Makefile.build:544: recipe for target 'arch/x86/kernel' failed
make[1]: *** [arch/x86/kernel] Error 2
Makefile:988: recipe for target 'arch/x86' failed
make: *** [arch/x86] Error 2

here with disabled CONFIG_RANDOMIZE_MEMORY.