1 // SPDX-License-Identifier: GPL-2.0
3 * This file implements KASLR memory randomization for x86_64. It randomizes
4 * the virtual address space of kernel memory regions (physical memory
5 * mapping, vmalloc & vmemmap) for x86_64. This security feature mitigates
6 * exploits relying on predictable kernel addresses.
8 * Entropy is generated using the KASLR early boot functions now shared in
9 * the lib directory (originally written by Kees Cook). Randomization is
10 * done on PGD & P4D/PUD page table levels to increase possible addresses.
11 * The physical memory mapping code was adapted to support P4D/PUD level
12 * virtual addresses. This implementation on the best configuration provides
13 * 30,000 possible virtual addresses in average for each memory region.
14 * An additional low memory page is used to ensure each CPU can start with
15 * a PGD aligned virtual address (for realmode).
17 * The order of each memory region is not changed. The feature looks at
18 * the available space for the regions based on different configuration
19 * options and randomizes the base and space between each. The size of the
20 * physical memory mapping is the available physical memory.
23 #include <linux/kernel.h>
24 #include <linux/init.h>
25 #include <linux/random.h>
27 #include <asm/pgalloc.h>
28 #include <asm/pgtable.h>
29 #include <asm/setup.h>
30 #include <asm/kaslr.h>
32 #include "mm_internal.h"
37 * The end address could depend on more configuration options to make the
38 * highest amount of space for randomization available, but that's too hard
39 * to keep straight and caused issues already.
41 static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE;
44 * Memory regions randomized by KASLR (except modules that use a separate logic
45 * earlier during boot). The list is ordered based on virtual addresses. This
46 * order is kept after randomization.
48 static __initdata struct kaslr_memory_region {
50 unsigned long size_tb;
52 { &page_offset_base, 0 },
57 /* Get size in bytes used by the memory region */
58 static inline unsigned long get_padding(struct kaslr_memory_region *region)
60 return (region->size_tb << TB_SHIFT);
64 * Apply no randomization if KASLR was disabled at boot or if KASAN
65 * is enabled. KASAN shadow mappings rely on regions being PGD aligned.
67 static inline bool kaslr_memory_enabled(void)
69 return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
72 /* Initialize base and padding for each memory region randomized with KASLR */
73 void __init kernel_randomize_memory(void)
76 unsigned long vaddr_start, vaddr;
77 unsigned long rand, memory_tb;
78 struct rnd_state rand_state;
79 unsigned long remain_entropy;
80 unsigned long vmemmap_size;
82 vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
86 * These BUILD_BUG_ON checks ensure the memory layout is consistent
87 * with the vaddr_start/vaddr_end variables. These checks are very
90 BUILD_BUG_ON(vaddr_start >= vaddr_end);
91 BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE);
92 BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
94 if (!kaslr_memory_enabled())
97 kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
98 kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
101 * Update Physical memory mapping to available and
102 * add padding if needed (especially for memory hotplug support).
104 BUG_ON(kaslr_regions[0].base != &page_offset_base);
105 memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
106 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
108 /* Adapt phyiscal memory region size based on available memory */
109 if (memory_tb < kaslr_regions[0].size_tb)
110 kaslr_regions[0].size_tb = memory_tb;
113 * Calculate the vmemmap region size in TBs, aligned to a TB
116 vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) *
118 kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);
120 /* Calculate entropy available between regions */
121 remain_entropy = vaddr_end - vaddr_start;
122 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
123 remain_entropy -= get_padding(&kaslr_regions[i]);
125 prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
127 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
128 unsigned long entropy;
131 * Select a random virtual address using the extra entropy
134 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
135 prandom_bytes_state(&rand_state, &rand, sizeof(rand));
136 if (pgtable_l5_enabled())
137 entropy = (rand % (entropy + 1)) & P4D_MASK;
139 entropy = (rand % (entropy + 1)) & PUD_MASK;
141 *kaslr_regions[i].base = vaddr;
144 * Jump the region and add a minimum padding based on
145 * randomization alignment.
147 vaddr += get_padding(&kaslr_regions[i]);
148 if (pgtable_l5_enabled())
149 vaddr = round_up(vaddr + 1, P4D_SIZE);
151 vaddr = round_up(vaddr + 1, PUD_SIZE);
152 remain_entropy -= entropy;
156 static void __meminit init_trampoline_pud(void)
158 unsigned long paddr, paddr_next;
160 pud_t *pud_page, *pud_page_tramp;
163 pud_page_tramp = alloc_low_page();
166 pgd = pgd_offset_k((unsigned long)__va(paddr));
167 pud_page = (pud_t *) pgd_page_vaddr(*pgd);
169 for (i = pud_index(paddr); i < PTRS_PER_PUD; i++, paddr = paddr_next) {
170 pud_t *pud, *pud_tramp;
171 unsigned long vaddr = (unsigned long)__va(paddr);
173 pud_tramp = pud_page_tramp + pud_index(paddr);
174 pud = pud_page + pud_index(vaddr);
175 paddr_next = (paddr & PUD_MASK) + PUD_SIZE;
180 set_pgd(&trampoline_pgd_entry,
181 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
184 static void __meminit init_trampoline_p4d(void)
186 unsigned long paddr, paddr_next;
188 p4d_t *p4d_page, *p4d_page_tramp;
191 p4d_page_tramp = alloc_low_page();
194 pgd = pgd_offset_k((unsigned long)__va(paddr));
195 p4d_page = (p4d_t *) pgd_page_vaddr(*pgd);
197 for (i = p4d_index(paddr); i < PTRS_PER_P4D; i++, paddr = paddr_next) {
198 p4d_t *p4d, *p4d_tramp;
199 unsigned long vaddr = (unsigned long)__va(paddr);
201 p4d_tramp = p4d_page_tramp + p4d_index(paddr);
202 p4d = p4d_page + p4d_index(vaddr);
203 paddr_next = (paddr & P4D_MASK) + P4D_SIZE;
208 set_pgd(&trampoline_pgd_entry,
209 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
213 * Create PGD aligned trampoline table to allow real mode initialization
214 * of additional CPUs. Consume only 1 low memory page.
216 void __meminit init_trampoline(void)
219 if (!kaslr_memory_enabled()) {
220 init_trampoline_default();
224 if (pgtable_l5_enabled())
225 init_trampoline_p4d();
227 init_trampoline_pud();