// 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 bool shuffle_param; static int shuffle_show(char *buffer, const struct kernel_param *kp) { return sprintf(buffer, "%c\n", shuffle_param ? '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) static_branch_enable(&page_alloc_shuffle_key); 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(struct zone *zone, unsigned long pfn, int order) { struct page *page = pfn_to_online_page(pfn); /* * Given we're dealing with randomly selected pfns in a zone we * need to ask questions like... */ /* ... is the page managed by the buddy? */ if (!page) return NULL; /* ... is the page assigned to the same zone? */ if (page_zone(page) != zone) return NULL; /* ...is the page free and currently on a free_area list? */ if (!PageBuddy(page)) return NULL; /* * ...is the page on the same list as the page we will * shuffle it with? */ if (buddy_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(z, 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(z, 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; }