1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
|
// SPDX-License-Identifier: GPL-2.0
// Copyright(c) 2018 Intel Corporation. All rights reserved.
#include <linux/mm.h>
#include <linux/init.h>
#include <linux/mmzone.h>
#include <linux/random.h>
#include <linux/moduleparam.h>
#include "internal.h"
#include "shuffle.h"
DEFINE_STATIC_KEY_FALSE(page_alloc_shuffle_key);
static unsigned long shuffle_state __ro_after_init;
/*
* Depending on the architecture, module parameter parsing may run
* before, or after the cache detection. SHUFFLE_FORCE_DISABLE prevents,
* or reverts the enabling of the shuffle implementation. SHUFFLE_ENABLE
* attempts to turn on the implementation, but aborts if it finds
* SHUFFLE_FORCE_DISABLE already set.
*/
__meminit void page_alloc_shuffle(enum mm_shuffle_ctl ctl)
{
if (ctl == SHUFFLE_FORCE_DISABLE)
set_bit(SHUFFLE_FORCE_DISABLE, &shuffle_state);
if (test_bit(SHUFFLE_FORCE_DISABLE, &shuffle_state)) {
if (test_and_clear_bit(SHUFFLE_ENABLE, &shuffle_state))
static_branch_disable(&page_alloc_shuffle_key);
} else if (ctl == SHUFFLE_ENABLE
&& !test_and_set_bit(SHUFFLE_ENABLE, &shuffle_state))
static_branch_enable(&page_alloc_shuffle_key);
}
static bool shuffle_param;
static int shuffle_show(char *buffer, const struct kernel_param *kp)
{
return sprintf(buffer, "%c\n", test_bit(SHUFFLE_ENABLE, &shuffle_state)
? 'Y' : 'N');
}
static __meminit int shuffle_store(const char *val,
const struct kernel_param *kp)
{
int rc = param_set_bool(val, kp);
if (rc < 0)
return rc;
if (shuffle_param)
page_alloc_shuffle(SHUFFLE_ENABLE);
else
page_alloc_shuffle(SHUFFLE_FORCE_DISABLE);
return 0;
}
module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400);
/*
* For two pages to be swapped in the shuffle, they must be free (on a
* 'free_area' lru), have the same order, and have the same migratetype.
*/
static struct page * __meminit shuffle_valid_page(unsigned long pfn, int order)
{
struct page *page;
/*
* Given we're dealing with randomly selected pfns in a zone we
* need to ask questions like...
*/
/* ...is the pfn even in the memmap? */
if (!pfn_valid_within(pfn))
return NULL;
/* ...is the pfn in a present section or a hole? */
if (!pfn_in_present_section(pfn))
return NULL;
/* ...is the page free and currently on a free_area list? */
page = pfn_to_page(pfn);
if (!PageBuddy(page))
return NULL;
/*
* ...is the page on the same list as the page we will
* shuffle it with?
*/
if (page_order(page) != order)
return NULL;
return page;
}
/*
* Fisher-Yates shuffle the freelist which prescribes iterating through an
* array, pfns in this case, and randomly swapping each entry with another in
* the span, end_pfn - start_pfn.
*
* To keep the implementation simple it does not attempt to correct for sources
* of bias in the distribution, like modulo bias or pseudo-random number
* generator bias. I.e. the expectation is that this shuffling raises the bar
* for attacks that exploit the predictability of page allocations, but need not
* be a perfect shuffle.
*/
#define SHUFFLE_RETRY 10
void __meminit __shuffle_zone(struct zone *z)
{
unsigned long i, flags;
unsigned long start_pfn = z->zone_start_pfn;
unsigned long end_pfn = zone_end_pfn(z);
const int order = SHUFFLE_ORDER;
const int order_pages = 1 << order;
spin_lock_irqsave(&z->lock, flags);
start_pfn = ALIGN(start_pfn, order_pages);
for (i = start_pfn; i < end_pfn; i += order_pages) {
unsigned long j;
int migratetype, retry;
struct page *page_i, *page_j;
/*
* We expect page_i, in the sub-range of a zone being added
* (@start_pfn to @end_pfn), to more likely be valid compared to
* page_j randomly selected in the span @zone_start_pfn to
* @spanned_pages.
*/
page_i = shuffle_valid_page(i, order);
if (!page_i)
continue;
for (retry = 0; retry < SHUFFLE_RETRY; retry++) {
/*
* Pick a random order aligned page in the zone span as
* a swap target. If the selected pfn is a hole, retry
* up to SHUFFLE_RETRY attempts find a random valid pfn
* in the zone.
*/
j = z->zone_start_pfn +
ALIGN_DOWN(get_random_long() % z->spanned_pages,
order_pages);
page_j = shuffle_valid_page(j, order);
if (page_j && page_j != page_i)
break;
}
if (retry >= SHUFFLE_RETRY) {
pr_debug("%s: failed to swap %#lx\n", __func__, i);
continue;
}
/*
* Each migratetype corresponds to its own list, make sure the
* types match otherwise we're moving pages to lists where they
* do not belong.
*/
migratetype = get_pageblock_migratetype(page_i);
if (get_pageblock_migratetype(page_j) != migratetype) {
pr_debug("%s: migratetype mismatch %#lx\n", __func__, i);
continue;
}
list_swap(&page_i->lru, &page_j->lru);
pr_debug("%s: swap: %#lx -> %#lx\n", __func__, i, j);
/* take it easy on the zone lock */
if ((i % (100 * order_pages)) == 0) {
spin_unlock_irqrestore(&z->lock, flags);
cond_resched();
spin_lock_irqsave(&z->lock, flags);
}
}
spin_unlock_irqrestore(&z->lock, flags);
}
/**
* shuffle_free_memory - reduce the predictability of the page allocator
* @pgdat: node page data
*/
void __meminit __shuffle_free_memory(pg_data_t *pgdat)
{
struct zone *z;
for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
shuffle_zone(z);
}
bool shuffle_pick_tail(void)
{
static u64 rand;
static u8 rand_bits;
bool ret;
/*
* The lack of locking is deliberate. If 2 threads race to
* update the rand state it just adds to the entropy.
*/
if (rand_bits == 0) {
rand_bits = 64;
rand = get_random_u64();
}
ret = rand & 1;
rand_bits--;
rand >>= 1;
return ret;
}
|
