1 | // SPDX-License-Identifier: GPL-2.0 |
2 | /* |
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. |
7 | * |
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). |
16 | * |
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. |
21 | */ |
22 | |
23 | #include <linux/kernel.h> |
24 | #include <linux/init.h> |
25 | #include <linux/random.h> |
26 | #include <linux/memblock.h> |
27 | #include <linux/pgtable.h> |
28 | |
29 | #include <asm/setup.h> |
30 | #include <asm/kaslr.h> |
31 | |
32 | #include "mm_internal.h" |
33 | |
34 | #define TB_SHIFT 40 |
35 | |
36 | /* |
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. |
40 | */ |
41 | static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE; |
42 | |
43 | /* |
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. |
47 | */ |
48 | static __initdata struct kaslr_memory_region { |
49 | unsigned long *base; |
50 | unsigned long size_tb; |
51 | } kaslr_regions[] = { |
52 | { &page_offset_base, 0 }, |
53 | { &vmalloc_base, 0 }, |
54 | { &vmemmap_base, 0 }, |
55 | }; |
56 | |
57 | /* Get size in bytes used by the memory region */ |
58 | static inline unsigned long get_padding(struct kaslr_memory_region *region) |
59 | { |
60 | return (region->size_tb << TB_SHIFT); |
61 | } |
62 | |
63 | /* Initialize base and padding for each memory region randomized with KASLR */ |
64 | void __init kernel_randomize_memory(void) |
65 | { |
66 | size_t i; |
67 | unsigned long vaddr_start, vaddr; |
68 | unsigned long rand, memory_tb; |
69 | struct rnd_state rand_state; |
70 | unsigned long remain_entropy; |
71 | unsigned long vmemmap_size; |
72 | |
73 | vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4; |
74 | vaddr = vaddr_start; |
75 | |
76 | /* |
77 | * These BUILD_BUG_ON checks ensure the memory layout is consistent |
78 | * with the vaddr_start/vaddr_end variables. These checks are very |
79 | * limited.... |
80 | */ |
81 | BUILD_BUG_ON(vaddr_start >= vaddr_end); |
82 | BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE); |
83 | BUILD_BUG_ON(vaddr_end > __START_KERNEL_map); |
84 | |
85 | if (!kaslr_memory_enabled()) |
86 | return; |
87 | |
88 | kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT); |
89 | kaslr_regions[1].size_tb = VMALLOC_SIZE_TB; |
90 | |
91 | /* |
92 | * Update Physical memory mapping to available and |
93 | * add padding if needed (especially for memory hotplug support). |
94 | */ |
95 | BUG_ON(kaslr_regions[0].base != &page_offset_base); |
96 | memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) + |
97 | CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING; |
98 | |
99 | /* Adapt physical memory region size based on available memory */ |
100 | if (memory_tb < kaslr_regions[0].size_tb) |
101 | kaslr_regions[0].size_tb = memory_tb; |
102 | |
103 | /* |
104 | * Calculate the vmemmap region size in TBs, aligned to a TB |
105 | * boundary. |
106 | */ |
107 | vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) * |
108 | sizeof(struct page); |
109 | kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT); |
110 | |
111 | /* Calculate entropy available between regions */ |
112 | remain_entropy = vaddr_end - vaddr_start; |
113 | for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) |
114 | remain_entropy -= get_padding(region: &kaslr_regions[i]); |
115 | |
116 | prandom_seed_state(state: &rand_state, seed: kaslr_get_random_long(purpose: "Memory" )); |
117 | |
118 | for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) { |
119 | unsigned long entropy; |
120 | |
121 | /* |
122 | * Select a random virtual address using the extra entropy |
123 | * available. |
124 | */ |
125 | entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i); |
126 | prandom_bytes_state(state: &rand_state, buf: &rand, nbytes: sizeof(rand)); |
127 | entropy = (rand % (entropy + 1)) & PUD_MASK; |
128 | vaddr += entropy; |
129 | *kaslr_regions[i].base = vaddr; |
130 | |
131 | /* |
132 | * Jump the region and add a minimum padding based on |
133 | * randomization alignment. |
134 | */ |
135 | vaddr += get_padding(region: &kaslr_regions[i]); |
136 | vaddr = round_up(vaddr + 1, PUD_SIZE); |
137 | remain_entropy -= entropy; |
138 | } |
139 | } |
140 | |
141 | void __meminit init_trampoline_kaslr(void) |
142 | { |
143 | pud_t *pud_page_tramp, *pud, *pud_tramp; |
144 | p4d_t *p4d_page_tramp, *p4d, *p4d_tramp; |
145 | unsigned long paddr, vaddr; |
146 | pgd_t *pgd; |
147 | |
148 | pud_page_tramp = alloc_low_page(); |
149 | |
150 | /* |
151 | * There are two mappings for the low 1MB area, the direct mapping |
152 | * and the 1:1 mapping for the real mode trampoline: |
153 | * |
154 | * Direct mapping: virt_addr = phys_addr + PAGE_OFFSET |
155 | * 1:1 mapping: virt_addr = phys_addr |
156 | */ |
157 | paddr = 0; |
158 | vaddr = (unsigned long)__va(paddr); |
159 | pgd = pgd_offset_k(vaddr); |
160 | |
161 | p4d = p4d_offset(pgd, address: vaddr); |
162 | pud = pud_offset(p4d, address: vaddr); |
163 | |
164 | pud_tramp = pud_page_tramp + pud_index(address: paddr); |
165 | *pud_tramp = *pud; |
166 | |
167 | if (pgtable_l5_enabled()) { |
168 | p4d_page_tramp = alloc_low_page(); |
169 | |
170 | p4d_tramp = p4d_page_tramp + p4d_index(address: paddr); |
171 | |
172 | set_p4d(p4dp: p4d_tramp, |
173 | p4d: __p4d(_KERNPG_TABLE | __pa(pud_page_tramp))); |
174 | |
175 | trampoline_pgd_entry = |
176 | __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)); |
177 | } else { |
178 | trampoline_pgd_entry = |
179 | __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)); |
180 | } |
181 | } |
182 | |