1 | // SPDX-License-Identifier: GPL-2.0 |
2 | // Copyright(c) 2018 Intel Corporation. All rights reserved. |
3 | |
4 | #include <linux/mm.h> |
5 | #include <linux/init.h> |
6 | #include <linux/mmzone.h> |
7 | #include <linux/random.h> |
8 | #include <linux/moduleparam.h> |
9 | #include "internal.h" |
10 | #include "shuffle.h" |
11 | |
12 | DEFINE_STATIC_KEY_FALSE(page_alloc_shuffle_key); |
13 | |
14 | static bool shuffle_param; |
15 | |
16 | static __meminit int shuffle_param_set(const char *val, |
17 | const struct kernel_param *kp) |
18 | { |
19 | if (param_set_bool(val, kp)) |
20 | return -EINVAL; |
21 | if (*(bool *)kp->arg) |
22 | static_branch_enable(&page_alloc_shuffle_key); |
23 | return 0; |
24 | } |
25 | |
26 | static const struct kernel_param_ops shuffle_param_ops = { |
27 | .set = shuffle_param_set, |
28 | .get = param_get_bool, |
29 | }; |
30 | module_param_cb(shuffle, &shuffle_param_ops, &shuffle_param, 0400); |
31 | |
32 | /* |
33 | * For two pages to be swapped in the shuffle, they must be free (on a |
34 | * 'free_area' lru), have the same order, and have the same migratetype. |
35 | */ |
36 | static struct page * __meminit shuffle_valid_page(struct zone *zone, |
37 | unsigned long pfn, int order) |
38 | { |
39 | struct page *page = pfn_to_online_page(pfn); |
40 | |
41 | /* |
42 | * Given we're dealing with randomly selected pfns in a zone we |
43 | * need to ask questions like... |
44 | */ |
45 | |
46 | /* ... is the page managed by the buddy? */ |
47 | if (!page) |
48 | return NULL; |
49 | |
50 | /* ... is the page assigned to the same zone? */ |
51 | if (page_zone(page) != zone) |
52 | return NULL; |
53 | |
54 | /* ...is the page free and currently on a free_area list? */ |
55 | if (!PageBuddy(page)) |
56 | return NULL; |
57 | |
58 | /* |
59 | * ...is the page on the same list as the page we will |
60 | * shuffle it with? |
61 | */ |
62 | if (buddy_order(page) != order) |
63 | return NULL; |
64 | |
65 | return page; |
66 | } |
67 | |
68 | /* |
69 | * Fisher-Yates shuffle the freelist which prescribes iterating through an |
70 | * array, pfns in this case, and randomly swapping each entry with another in |
71 | * the span, end_pfn - start_pfn. |
72 | * |
73 | * To keep the implementation simple it does not attempt to correct for sources |
74 | * of bias in the distribution, like modulo bias or pseudo-random number |
75 | * generator bias. I.e. the expectation is that this shuffling raises the bar |
76 | * for attacks that exploit the predictability of page allocations, but need not |
77 | * be a perfect shuffle. |
78 | */ |
79 | #define SHUFFLE_RETRY 10 |
80 | void __meminit __shuffle_zone(struct zone *z) |
81 | { |
82 | unsigned long i, flags; |
83 | unsigned long start_pfn = z->zone_start_pfn; |
84 | unsigned long end_pfn = zone_end_pfn(zone: z); |
85 | const int order = SHUFFLE_ORDER; |
86 | const int order_pages = 1 << order; |
87 | |
88 | spin_lock_irqsave(&z->lock, flags); |
89 | start_pfn = ALIGN(start_pfn, order_pages); |
90 | for (i = start_pfn; i < end_pfn; i += order_pages) { |
91 | unsigned long j; |
92 | int migratetype, retry; |
93 | struct page *page_i, *page_j; |
94 | |
95 | /* |
96 | * We expect page_i, in the sub-range of a zone being added |
97 | * (@start_pfn to @end_pfn), to more likely be valid compared to |
98 | * page_j randomly selected in the span @zone_start_pfn to |
99 | * @spanned_pages. |
100 | */ |
101 | page_i = shuffle_valid_page(zone: z, pfn: i, order); |
102 | if (!page_i) |
103 | continue; |
104 | |
105 | for (retry = 0; retry < SHUFFLE_RETRY; retry++) { |
106 | /* |
107 | * Pick a random order aligned page in the zone span as |
108 | * a swap target. If the selected pfn is a hole, retry |
109 | * up to SHUFFLE_RETRY attempts find a random valid pfn |
110 | * in the zone. |
111 | */ |
112 | j = z->zone_start_pfn + |
113 | ALIGN_DOWN(get_random_long() % z->spanned_pages, |
114 | order_pages); |
115 | page_j = shuffle_valid_page(zone: z, pfn: j, order); |
116 | if (page_j && page_j != page_i) |
117 | break; |
118 | } |
119 | if (retry >= SHUFFLE_RETRY) { |
120 | pr_debug("%s: failed to swap %#lx\n" , __func__, i); |
121 | continue; |
122 | } |
123 | |
124 | /* |
125 | * Each migratetype corresponds to its own list, make sure the |
126 | * types match otherwise we're moving pages to lists where they |
127 | * do not belong. |
128 | */ |
129 | migratetype = get_pageblock_migratetype(page_i); |
130 | if (get_pageblock_migratetype(page_j) != migratetype) { |
131 | pr_debug("%s: migratetype mismatch %#lx\n" , __func__, i); |
132 | continue; |
133 | } |
134 | |
135 | list_swap(entry1: &page_i->lru, entry2: &page_j->lru); |
136 | |
137 | pr_debug("%s: swap: %#lx -> %#lx\n" , __func__, i, j); |
138 | |
139 | /* take it easy on the zone lock */ |
140 | if ((i % (100 * order_pages)) == 0) { |
141 | spin_unlock_irqrestore(lock: &z->lock, flags); |
142 | cond_resched(); |
143 | spin_lock_irqsave(&z->lock, flags); |
144 | } |
145 | } |
146 | spin_unlock_irqrestore(lock: &z->lock, flags); |
147 | } |
148 | |
149 | /* |
150 | * __shuffle_free_memory - reduce the predictability of the page allocator |
151 | * @pgdat: node page data |
152 | */ |
153 | void __meminit __shuffle_free_memory(pg_data_t *pgdat) |
154 | { |
155 | struct zone *z; |
156 | |
157 | for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++) |
158 | shuffle_zone(z); |
159 | } |
160 | |
161 | bool shuffle_pick_tail(void) |
162 | { |
163 | static u64 rand; |
164 | static u8 rand_bits; |
165 | bool ret; |
166 | |
167 | /* |
168 | * The lack of locking is deliberate. If 2 threads race to |
169 | * update the rand state it just adds to the entropy. |
170 | */ |
171 | if (rand_bits == 0) { |
172 | rand_bits = 64; |
173 | rand = get_random_u64(); |
174 | } |
175 | |
176 | ret = rand & 1; |
177 | |
178 | rand_bits--; |
179 | rand >>= 1; |
180 | |
181 | return ret; |
182 | } |
183 | |