480 lines
14 KiB
C
480 lines
14 KiB
C
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/*
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* kaslr.c
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*
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* This contains the routines needed to generate a reasonable level of
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* entropy to choose a randomized kernel base address offset in support
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* of Kernel Address Space Layout Randomization (KASLR). Additionally
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* handles walking the physical memory maps (and tracking memory regions
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* to avoid) in order to select a physical memory location that can
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* contain the entire properly aligned running kernel image.
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*
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*/
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#include "misc.h"
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#include "error.h"
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#include <generated/compile.h>
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#include <linux/module.h>
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#include <linux/uts.h>
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#include <linux/utsname.h>
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#include <generated/utsrelease.h>
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/* Simplified build-specific string for starting entropy. */
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static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
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LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
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static unsigned long rotate_xor(unsigned long hash, const void *area,
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size_t size)
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{
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size_t i;
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unsigned long *ptr = (unsigned long *)area;
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for (i = 0; i < size / sizeof(hash); i++) {
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/* Rotate by odd number of bits and XOR. */
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hash = (hash << ((sizeof(hash) * 8) - 7)) | (hash >> 7);
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hash ^= ptr[i];
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}
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return hash;
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}
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/* Attempt to create a simple but unpredictable starting entropy. */
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static unsigned long get_boot_seed(void)
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{
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unsigned long hash = 0;
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hash = rotate_xor(hash, build_str, sizeof(build_str));
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hash = rotate_xor(hash, boot_params, sizeof(*boot_params));
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return hash;
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}
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#define KASLR_COMPRESSED_BOOT
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#include "../../lib/kaslr.c"
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struct mem_vector {
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unsigned long start;
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unsigned long size;
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};
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enum mem_avoid_index {
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MEM_AVOID_ZO_RANGE = 0,
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MEM_AVOID_INITRD,
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MEM_AVOID_CMDLINE,
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MEM_AVOID_BOOTPARAMS,
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MEM_AVOID_MAX,
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};
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static struct mem_vector mem_avoid[MEM_AVOID_MAX];
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static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
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{
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/* Item one is entirely before item two. */
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if (one->start + one->size <= two->start)
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return false;
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/* Item one is entirely after item two. */
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if (one->start >= two->start + two->size)
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return false;
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return true;
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}
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/*
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* In theory, KASLR can put the kernel anywhere in the range of [16M, 64T).
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* The mem_avoid array is used to store the ranges that need to be avoided
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* when KASLR searches for an appropriate random address. We must avoid any
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* regions that are unsafe to overlap with during decompression, and other
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* things like the initrd, cmdline and boot_params. This comment seeks to
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* explain mem_avoid as clearly as possible since incorrect mem_avoid
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* memory ranges lead to really hard to debug boot failures.
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*
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* The initrd, cmdline, and boot_params are trivial to identify for
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* avoiding. They are MEM_AVOID_INITRD, MEM_AVOID_CMDLINE, and
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* MEM_AVOID_BOOTPARAMS respectively below.
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*
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* What is not obvious how to avoid is the range of memory that is used
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* during decompression (MEM_AVOID_ZO_RANGE below). This range must cover
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* the compressed kernel (ZO) and its run space, which is used to extract
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* the uncompressed kernel (VO) and relocs.
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*
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* ZO's full run size sits against the end of the decompression buffer, so
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* we can calculate where text, data, bss, etc of ZO are positioned more
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* easily.
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*
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* For additional background, the decompression calculations can be found
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* in header.S, and the memory diagram is based on the one found in misc.c.
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*
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* The following conditions are already enforced by the image layouts and
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* associated code:
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* - input + input_size >= output + output_size
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* - kernel_total_size <= init_size
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* - kernel_total_size <= output_size (see Note below)
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* - output + init_size >= output + output_size
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*
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* (Note that kernel_total_size and output_size have no fundamental
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* relationship, but output_size is passed to choose_random_location
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* as a maximum of the two. The diagram is showing a case where
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* kernel_total_size is larger than output_size, but this case is
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* handled by bumping output_size.)
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*
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* The above conditions can be illustrated by a diagram:
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*
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* 0 output input input+input_size output+init_size
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* | | | | |
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* | | | | |
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* |-----|--------|--------|--------------|-----------|--|-------------|
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* | | |
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* | | |
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* output+init_size-ZO_INIT_SIZE output+output_size output+kernel_total_size
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*
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* [output, output+init_size) is the entire memory range used for
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* extracting the compressed image.
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*
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* [output, output+kernel_total_size) is the range needed for the
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* uncompressed kernel (VO) and its run size (bss, brk, etc).
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*
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* [output, output+output_size) is VO plus relocs (i.e. the entire
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* uncompressed payload contained by ZO). This is the area of the buffer
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* written to during decompression.
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*
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* [output+init_size-ZO_INIT_SIZE, output+init_size) is the worst-case
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* range of the copied ZO and decompression code. (i.e. the range
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* covered backwards of size ZO_INIT_SIZE, starting from output+init_size.)
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*
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* [input, input+input_size) is the original copied compressed image (ZO)
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* (i.e. it does not include its run size). This range must be avoided
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* because it contains the data used for decompression.
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*
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* [input+input_size, output+init_size) is [_text, _end) for ZO. This
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* range includes ZO's heap and stack, and must be avoided since it
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* performs the decompression.
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*
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* Since the above two ranges need to be avoided and they are adjacent,
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* they can be merged, resulting in: [input, output+init_size) which
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* becomes the MEM_AVOID_ZO_RANGE below.
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*/
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static void mem_avoid_init(unsigned long input, unsigned long input_size,
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unsigned long output)
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{
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unsigned long init_size = boot_params->hdr.init_size;
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u64 initrd_start, initrd_size;
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u64 cmd_line, cmd_line_size;
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char *ptr;
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/*
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* Avoid the region that is unsafe to overlap during
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* decompression.
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*/
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mem_avoid[MEM_AVOID_ZO_RANGE].start = input;
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mem_avoid[MEM_AVOID_ZO_RANGE].size = (output + init_size) - input;
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add_identity_map(mem_avoid[MEM_AVOID_ZO_RANGE].start,
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mem_avoid[MEM_AVOID_ZO_RANGE].size);
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/* Avoid initrd. */
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initrd_start = (u64)boot_params->ext_ramdisk_image << 32;
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initrd_start |= boot_params->hdr.ramdisk_image;
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initrd_size = (u64)boot_params->ext_ramdisk_size << 32;
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initrd_size |= boot_params->hdr.ramdisk_size;
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mem_avoid[MEM_AVOID_INITRD].start = initrd_start;
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mem_avoid[MEM_AVOID_INITRD].size = initrd_size;
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/* No need to set mapping for initrd, it will be handled in VO. */
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/* Avoid kernel command line. */
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cmd_line = (u64)boot_params->ext_cmd_line_ptr << 32;
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cmd_line |= boot_params->hdr.cmd_line_ptr;
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/* Calculate size of cmd_line. */
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ptr = (char *)(unsigned long)cmd_line;
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for (cmd_line_size = 0; ptr[cmd_line_size++]; )
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;
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mem_avoid[MEM_AVOID_CMDLINE].start = cmd_line;
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mem_avoid[MEM_AVOID_CMDLINE].size = cmd_line_size;
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add_identity_map(mem_avoid[MEM_AVOID_CMDLINE].start,
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mem_avoid[MEM_AVOID_CMDLINE].size);
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/* Avoid boot parameters. */
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mem_avoid[MEM_AVOID_BOOTPARAMS].start = (unsigned long)boot_params;
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mem_avoid[MEM_AVOID_BOOTPARAMS].size = sizeof(*boot_params);
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add_identity_map(mem_avoid[MEM_AVOID_BOOTPARAMS].start,
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mem_avoid[MEM_AVOID_BOOTPARAMS].size);
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/* We don't need to set a mapping for setup_data. */
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#ifdef CONFIG_X86_VERBOSE_BOOTUP
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/* Make sure video RAM can be used. */
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add_identity_map(0, PMD_SIZE);
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#endif
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}
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/*
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* Does this memory vector overlap a known avoided area? If so, record the
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* overlap region with the lowest address.
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*/
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static bool mem_avoid_overlap(struct mem_vector *img,
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struct mem_vector *overlap)
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{
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int i;
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struct setup_data *ptr;
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unsigned long earliest = img->start + img->size;
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bool is_overlapping = false;
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for (i = 0; i < MEM_AVOID_MAX; i++) {
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if (mem_overlaps(img, &mem_avoid[i]) &&
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mem_avoid[i].start < earliest) {
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*overlap = mem_avoid[i];
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earliest = overlap->start;
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is_overlapping = true;
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}
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}
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/* Avoid all entries in the setup_data linked list. */
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ptr = (struct setup_data *)(unsigned long)boot_params->hdr.setup_data;
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while (ptr) {
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struct mem_vector avoid;
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avoid.start = (unsigned long)ptr;
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avoid.size = sizeof(*ptr) + ptr->len;
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if (mem_overlaps(img, &avoid) && (avoid.start < earliest)) {
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*overlap = avoid;
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earliest = overlap->start;
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is_overlapping = true;
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}
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ptr = (struct setup_data *)(unsigned long)ptr->next;
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}
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return is_overlapping;
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}
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struct slot_area {
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unsigned long addr;
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int num;
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};
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#define MAX_SLOT_AREA 100
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static struct slot_area slot_areas[MAX_SLOT_AREA];
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static unsigned long slot_max;
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static unsigned long slot_area_index;
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static void store_slot_info(struct mem_vector *region, unsigned long image_size)
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{
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struct slot_area slot_area;
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if (slot_area_index == MAX_SLOT_AREA)
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return;
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slot_area.addr = region->start;
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slot_area.num = (region->size - image_size) /
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CONFIG_PHYSICAL_ALIGN + 1;
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if (slot_area.num > 0) {
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slot_areas[slot_area_index++] = slot_area;
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slot_max += slot_area.num;
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}
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}
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static unsigned long slots_fetch_random(void)
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{
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unsigned long slot;
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int i;
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/* Handle case of no slots stored. */
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if (slot_max == 0)
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return 0;
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slot = kaslr_get_random_long("Physical") % slot_max;
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for (i = 0; i < slot_area_index; i++) {
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if (slot >= slot_areas[i].num) {
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slot -= slot_areas[i].num;
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continue;
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}
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return slot_areas[i].addr + slot * CONFIG_PHYSICAL_ALIGN;
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}
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if (i == slot_area_index)
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debug_putstr("slots_fetch_random() failed!?\n");
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return 0;
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}
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static void process_e820_entry(struct e820entry *entry,
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unsigned long minimum,
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unsigned long image_size)
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{
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struct mem_vector region, overlap;
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struct slot_area slot_area;
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unsigned long start_orig;
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/* Skip non-RAM entries. */
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if (entry->type != E820_RAM)
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return;
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/* On 32-bit, ignore entries entirely above our maximum. */
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if (IS_ENABLED(CONFIG_X86_32) && entry->addr >= KERNEL_IMAGE_SIZE)
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return;
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/* Ignore entries entirely below our minimum. */
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if (entry->addr + entry->size < minimum)
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return;
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region.start = entry->addr;
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region.size = entry->size;
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/* Give up if slot area array is full. */
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while (slot_area_index < MAX_SLOT_AREA) {
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start_orig = region.start;
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/* Potentially raise address to minimum location. */
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if (region.start < minimum)
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region.start = minimum;
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/* Potentially raise address to meet alignment needs. */
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region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
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/* Did we raise the address above this e820 region? */
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if (region.start > entry->addr + entry->size)
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return;
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/* Reduce size by any delta from the original address. */
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region.size -= region.start - start_orig;
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/* On 32-bit, reduce region size to fit within max size. */
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if (IS_ENABLED(CONFIG_X86_32) &&
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region.start + region.size > KERNEL_IMAGE_SIZE)
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region.size = KERNEL_IMAGE_SIZE - region.start;
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/* Return if region can't contain decompressed kernel */
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if (region.size < image_size)
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return;
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/* If nothing overlaps, store the region and return. */
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if (!mem_avoid_overlap(®ion, &overlap)) {
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store_slot_info(®ion, image_size);
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return;
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}
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/* Store beginning of region if holds at least image_size. */
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if (overlap.start > region.start + image_size) {
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struct mem_vector beginning;
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beginning.start = region.start;
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beginning.size = overlap.start - region.start;
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store_slot_info(&beginning, image_size);
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}
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||
|
|
/* 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;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
static unsigned long find_random_phys_addr(unsigned long minimum,
|
||
|
|
unsigned long image_size)
|
||
|
|
{
|
||
|
|
int i;
|
||
|
|
unsigned long addr;
|
||
|
|
|
||
|
|
/* Make sure minimum is aligned. */
|
||
|
|
minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
|
||
|
|
|
||
|
|
/* Verify potential e820 positions, appending to slots list. */
|
||
|
|
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();
|
||
|
|
}
|
||
|
|
|
||
|
|
static unsigned long find_random_virt_addr(unsigned long minimum,
|
||
|
|
unsigned long image_size)
|
||
|
|
{
|
||
|
|
unsigned long slots, random_addr;
|
||
|
|
|
||
|
|
/* Make sure minimum is aligned. */
|
||
|
|
minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
|
||
|
|
/* Align image_size for easy slot calculations. */
|
||
|
|
image_size = ALIGN(image_size, CONFIG_PHYSICAL_ALIGN);
|
||
|
|
|
||
|
|
/*
|
||
|
|
* There are how many CONFIG_PHYSICAL_ALIGN-sized slots
|
||
|
|
* that can hold image_size within the range of minimum to
|
||
|
|
* KERNEL_IMAGE_SIZE?
|
||
|
|
*/
|
||
|
|
slots = (KERNEL_IMAGE_SIZE - minimum - image_size) /
|
||
|
|
CONFIG_PHYSICAL_ALIGN + 1;
|
||
|
|
|
||
|
|
random_addr = kaslr_get_random_long("Virtual") % slots;
|
||
|
|
|
||
|
|
return random_addr * CONFIG_PHYSICAL_ALIGN + minimum;
|
||
|
|
}
|
||
|
|
|
||
|
|
/*
|
||
|
|
* Since this function examines addresses much more numerically,
|
||
|
|
* it takes the input and output pointers as 'unsigned long'.
|
||
|
|
*/
|
||
|
|
void choose_random_location(unsigned long input,
|
||
|
|
unsigned long input_size,
|
||
|
|
unsigned long *output,
|
||
|
|
unsigned long output_size,
|
||
|
|
unsigned long *virt_addr)
|
||
|
|
{
|
||
|
|
unsigned long random_addr, min_addr;
|
||
|
|
|
||
|
|
if (cmdline_find_option_bool("nokaslr")) {
|
||
|
|
warn("KASLR disabled: 'nokaslr' on cmdline.");
|
||
|
|
return;
|
||
|
|
}
|
||
|
|
|
||
|
|
boot_params->hdr.loadflags |= KASLR_FLAG;
|
||
|
|
|
||
|
|
/* Prepare to add new identity pagetables on demand. */
|
||
|
|
initialize_identity_maps();
|
||
|
|
|
||
|
|
/* Record the various known unsafe memory ranges. */
|
||
|
|
mem_avoid_init(input, input_size, *output);
|
||
|
|
|
||
|
|
/*
|
||
|
|
* Low end of the randomization range should be the
|
||
|
|
* smaller of 512M or the initial kernel image
|
||
|
|
* location:
|
||
|
|
*/
|
||
|
|
min_addr = min(*output, 512UL << 20);
|
||
|
|
|
||
|
|
/* Walk e820 and find a random address. */
|
||
|
|
random_addr = find_random_phys_addr(min_addr, output_size);
|
||
|
|
if (!random_addr) {
|
||
|
|
warn("KASLR disabled: could not find suitable E820 region!");
|
||
|
|
} else {
|
||
|
|
/* Update the new physical address location. */
|
||
|
|
if (*output != random_addr) {
|
||
|
|
add_identity_map(random_addr, output_size);
|
||
|
|
*output = random_addr;
|
||
|
|
}
|
||
|
|
|
||
|
|
/*
|
||
|
|
* This loads the identity mapping page table.
|
||
|
|
* This should only be done if a new physical address
|
||
|
|
* is found for the kernel, otherwise we should keep
|
||
|
|
* the old page table to make it be like the "nokaslr"
|
||
|
|
* case.
|
||
|
|
*/
|
||
|
|
finalize_identity_maps();
|
||
|
|
}
|
||
|
|
|
||
|
|
|
||
|
|
/* Pick random virtual address starting from LOAD_PHYSICAL_ADDR. */
|
||
|
|
if (IS_ENABLED(CONFIG_X86_64))
|
||
|
|
random_addr = find_random_virt_addr(LOAD_PHYSICAL_ADDR, output_size);
|
||
|
|
*virt_addr = random_addr;
|
||
|
|
}
|