/* * kaslr.c * * This contains the routines needed to generate a reasonable level of * entropy to choose a randomized kernel base address offset in support * of Kernel Address Space Layout Randomization (KASLR). Additionally * handles walking the physical memory maps (and tracking memory regions * to avoid) in order to select a physical memory location that can * contain the entire properly aligned running kernel image. * */ #include "misc.h" #include "error.h" #include "../boot.h" #include #include #include #include #include /* Simplified build-specific string for starting entropy. */ static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@" LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION; static unsigned long rotate_xor(unsigned long hash, const void *area, size_t size) { size_t i; unsigned long *ptr = (unsigned long *)area; for (i = 0; i < size / sizeof(hash); i++) { /* Rotate by odd number of bits and XOR. */ hash = (hash << ((sizeof(hash) * 8) - 7)) | (hash >> 7); hash ^= ptr[i]; } return hash; } /* Attempt to create a simple but unpredictable starting entropy. */ static unsigned long get_boot_seed(void) { unsigned long hash = 0; hash = rotate_xor(hash, build_str, sizeof(build_str)); hash = rotate_xor(hash, boot_params, sizeof(*boot_params)); return hash; } #define KASLR_COMPRESSED_BOOT #include "../../lib/kaslr.c" struct mem_vector { unsigned long long start; unsigned long long size; }; /* Only supporting at most 4 unusable memmap regions with kaslr */ #define MAX_MEMMAP_REGIONS 4 static bool memmap_too_large; enum mem_avoid_index { MEM_AVOID_ZO_RANGE = 0, MEM_AVOID_INITRD, MEM_AVOID_CMDLINE, MEM_AVOID_BOOTPARAMS, MEM_AVOID_MEMMAP_BEGIN, MEM_AVOID_MEMMAP_END = MEM_AVOID_MEMMAP_BEGIN + MAX_MEMMAP_REGIONS - 1, MEM_AVOID_MAX, }; static struct mem_vector mem_avoid[MEM_AVOID_MAX]; static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two) { /* Item one is entirely before item two. */ if (one->start + one->size <= two->start) return false; /* Item one is entirely after item two. */ if (one->start >= two->start + two->size) return false; return true; } /** * _memparse - Parse a string with mem suffixes into a number * @ptr: Where parse begins * @retptr: (output) Optional pointer to next char after parse completes * * Parses a string into a number. The number stored at @ptr is * potentially suffixed with K, M, G, T, P, E. */ static unsigned long long _memparse(const char *ptr, char **retptr) { char *endptr; /* Local pointer to end of parsed string */ unsigned long long ret = simple_strtoull(ptr, &endptr, 0); switch (*endptr) { case 'E': case 'e': ret <<= 10; case 'P': case 'p': ret <<= 10; case 'T': case 't': ret <<= 10; case 'G': case 'g': ret <<= 10; case 'M': case 'm': ret <<= 10; case 'K': case 'k': ret <<= 10; endptr++; default: break; } if (retptr) *retptr = endptr; return ret; } static int parse_memmap(char *p, unsigned long long *start, unsigned long long *size) { char *oldp; if (!p) return -EINVAL; /* We don't care about this option here */ if (!strncmp(p, "exactmap", 8)) return -EINVAL; oldp = p; *size = _memparse(p, &p); if (p == oldp) return -EINVAL; switch (*p) { case '@': /* Skip this region, usable */ *start = 0; *size = 0; return 0; case '#': case '$': case '!': *start = _memparse(p + 1, &p); return 0; } return -EINVAL; } static void mem_avoid_memmap(void) { char arg[128]; int rc; int i; char *str; /* See if we have any memmap areas */ rc = cmdline_find_option("memmap", arg, sizeof(arg)); if (rc <= 0) return; i = 0; str = arg; while (str && (i < MAX_MEMMAP_REGIONS)) { int rc; unsigned long long start, size; char *k = strchr(str, ','); if (k) *k++ = 0; rc = parse_memmap(str, &start, &size); if (rc < 0) break; str = k; /* A usable region that should not be skipped */ if (size == 0) continue; mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].start = start; mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].size = size; i++; } /* More than 4 memmaps, fail kaslr */ if ((i >= MAX_MEMMAP_REGIONS) && str) memmap_too_large = true; } /* * In theory, KASLR can put the kernel anywhere in the range of [16M, 64T). * The mem_avoid array is used to store the ranges that need to be avoided * when KASLR searches for an appropriate random address. We must avoid any * regions that are unsafe to overlap with during decompression, and other * things like the initrd, cmdline and boot_params. This comment seeks to * explain mem_avoid as clearly as possible since incorrect mem_avoid * memory ranges lead to really hard to debug boot failures. * * The initrd, cmdline, and boot_params are trivial to identify for * avoiding. They are MEM_AVOID_INITRD, MEM_AVOID_CMDLINE, and * MEM_AVOID_BOOTPARAMS respectively below. * * What is not obvious how to avoid is the range of memory that is used * during decompression (MEM_AVOID_ZO_RANGE below). This range must cover * the compressed kernel (ZO) and its run space, which is used to extract * the uncompressed kernel (VO) and relocs. * * ZO's full run size sits against the end of the decompression buffer, so * we can calculate where text, data, bss, etc of ZO are positioned more * easily. * * For additional background, the decompression calculations can be found * in header.S, and the memory diagram is based on the one found in misc.c. * * The following conditions are already enforced by the image layouts and * associated code: * - input + input_size >= output + output_size * - kernel_total_size <= init_size * - kernel_total_size <= output_size (see Note below) * - output + init_size >= output + output_size * * (Note that kernel_total_size and output_size have no fundamental * relationship, but output_size is passed to choose_random_location * as a maximum of the two. The diagram is showing a case where * kernel_total_size is larger than output_size, but this case is * handled by bumping output_size.) * * The above conditions can be illustrated by a diagram: * * 0 output input input+input_size output+init_size * | | | | | * | | | | | * |-----|--------|--------|--------------|-----------|--|-------------| * | | | * | | | * output+init_size-ZO_INIT_SIZE output+output_size output+kernel_total_size * * [output, output+init_size) is the entire memory range used for * extracting the compressed image. * * [output, output+kernel_total_size) is the range needed for the * uncompressed kernel (VO) and its run size (bss, brk, etc). * * [output, output+output_size) is VO plus relocs (i.e. the entire * uncompressed payload contained by ZO). This is the area of the buffer * written to during decompression. * * [output+init_size-ZO_INIT_SIZE, output+init_size) is the worst-case * range of the copied ZO and decompression code. (i.e. the range * covered backwards of size ZO_INIT_SIZE, starting from output+init_size.) * * [input, input+input_size) is the original copied compressed image (ZO) * (i.e. it does not include its run size). This range must be avoided * because it contains the data used for decompression. * * [input+input_size, output+init_size) is [_text, _end) for ZO. This * range includes ZO's heap and stack, and must be avoided since it * performs the decompression. * * Since the above two ranges need to be avoided and they are adjacent, * they can be merged, resulting in: [input, output+init_size) which * becomes the MEM_AVOID_ZO_RANGE below. */ static void mem_avoid_init(unsigned long input, unsigned long input_size, unsigned long output) { unsigned long init_size = boot_params->hdr.init_size; u64 initrd_start, initrd_size; u64 cmd_line, cmd_line_size; char *ptr; /* * Avoid the region that is unsafe to overlap during * decompression. */ mem_avoid[MEM_AVOID_ZO_RANGE].start = input; mem_avoid[MEM_AVOID_ZO_RANGE].size = (output + init_size) - input; add_identity_map(mem_avoid[MEM_AVOID_ZO_RANGE].start, mem_avoid[MEM_AVOID_ZO_RANGE].size); /* Avoid initrd. */ initrd_start = (u64)boot_params->ext_ramdisk_image << 32; initrd_start |= boot_params->hdr.ramdisk_image; initrd_size = (u64)boot_params->ext_ramdisk_size << 32; initrd_size |= boot_params->hdr.ramdisk_size; mem_avoid[MEM_AVOID_INITRD].start = initrd_start; mem_avoid[MEM_AVOID_INITRD].size = initrd_size; /* No need to set mapping for initrd, it will be handled in VO. */ /* Avoid kernel command line. */ cmd_line = (u64)boot_params->ext_cmd_line_ptr << 32; cmd_line |= boot_params->hdr.cmd_line_ptr; /* Calculate size of cmd_line. */ ptr = (char *)(unsigned long)cmd_line; for (cmd_line_size = 0; ptr[cmd_line_size++]; ) ; mem_avoid[MEM_AVOID_CMDLINE].start = cmd_line; mem_avoid[MEM_AVOID_CMDLINE].size = cmd_line_size; add_identity_map(mem_avoid[MEM_AVOID_CMDLINE].start, mem_avoid[MEM_AVOID_CMDLINE].size); /* Avoid boot parameters. */ mem_avoid[MEM_AVOID_BOOTPARAMS].start = (unsigned long)boot_params; mem_avoid[MEM_AVOID_BOOTPARAMS].size = sizeof(*boot_params); add_identity_map(mem_avoid[MEM_AVOID_BOOTPARAMS].start, mem_avoid[MEM_AVOID_BOOTPARAMS].size); /* We don't need to set a mapping for setup_data. */ /* Mark the memmap regions we need to avoid */ mem_avoid_memmap(); #ifdef CONFIG_X86_VERBOSE_BOOTUP /* Make sure video RAM can be used. */ add_identity_map(0, PMD_SIZE); #endif } /* * Does this memory vector overlap a known avoided area? If so, record the * overlap region with the lowest address. */ static bool mem_avoid_overlap(struct mem_vector *img, struct mem_vector *overlap) { int i; struct setup_data *ptr; unsigned long earliest = img->start + img->size; bool is_overlapping = false; for (i = 0; i < MEM_AVOID_MAX; i++) { if (mem_overlaps(img, &mem_avoid[i]) && mem_avoid[i].start < earliest) { *overlap = mem_avoid[i]; earliest = overlap->start; is_overlapping = true; } } /* Avoid all entries in the setup_data linked list. */ ptr = (struct setup_data *)(unsigned long)boot_params->hdr.setup_data; while (ptr) { struct mem_vector avoid; avoid.start = (unsigned long)ptr; avoid.size = sizeof(*ptr) + ptr->len; if (mem_overlaps(img, &avoid) && (avoid.start < earliest)) { *overlap = avoid; earliest = overlap->start; is_overlapping = true; } ptr = (struct setup_data *)(unsigned long)ptr->next; } return is_overlapping; } struct slot_area { unsigned long addr; int num; }; #define MAX_SLOT_AREA 100 static struct slot_area slot_areas[MAX_SLOT_AREA]; static unsigned long slot_max; static unsigned long slot_area_index; static void store_slot_info(struct mem_vector *region, unsigned long image_size) { struct slot_area slot_area; if (slot_area_index == MAX_SLOT_AREA) return; slot_area.addr = region->start; slot_area.num = (region->size - image_size) / CONFIG_PHYSICAL_ALIGN + 1; if (slot_area.num > 0) { slot_areas[slot_area_index++] = slot_area; slot_max += slot_area.num; } } static unsigned long slots_fetch_random(void) { unsigned long slot; int i; /* Handle case of no slots stored. */ if (slot_max == 0) return 0; slot = kaslr_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 boot_e820_entry *entry, unsigned long minimum, unsigned long image_size) { struct mem_vector region, overlap; struct slot_area slot_area; unsigned long start_orig; /* Skip non-RAM entries. */ if (entry->type != E820_TYPE_RAM) return; /* 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. */ if (entry->addr + entry->size < minimum) return; region.start = entry->addr; region.size = entry->size; /* Give up if slot area array is full. */ while (slot_area_index < MAX_SLOT_AREA) { start_orig = region.start; /* Potentially raise address to minimum location. */ if (region.start < minimum) region.start = minimum; /* Potentially raise address to meet alignment needs. */ region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN); /* Did we raise the address above this e820 region? */ if (region.start > entry->addr + entry->size) return; /* Reduce size by any delta from the original address. */ region.size -= region.start - start_orig; /* 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; } } static unsigned long find_random_phys_addr(unsigned long minimum, unsigned long image_size) { int i; unsigned long addr; /* Check if we had too many memmaps. */ if (memmap_too_large) { debug_putstr("Aborted e820 scan (more than 4 memmap= args)!\n"); return 0; } /* 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_table[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("Physical KASLR disabled: no suitable memory 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; }