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