linux/arch/x86/mm/kaslr.c

// SPDX-License-Identifier: GPL-2.0
/*
 * This file implements KASLR memory randomization for x86_64. It randomizes
 * the virtual address space of kernel memory regions (physical memory
 * mapping, vmalloc & vmemmap) for x86_64. This security feature mitigates
 * exploits relying on predictable kernel addresses.
 *
 * Entropy is generated using the KASLR early boot functions now shared in
 * the lib directory (originally written by Kees Cook). Randomization is
 * done on PGD & P4D/PUD page table levels to increase possible addresses.
 * The physical memory mapping code was adapted to support P4D/PUD level
 * virtual addresses. This implementation on the best configuration provides
 * 30,000 possible virtual addresses in average for each memory region.
 * An additional low memory page is used to ensure each CPU can start with
 * a PGD aligned virtual address (for realmode).
 *
 * The order of each memory region is not changed. The feature looks at
 * the available space for the regions based on different configuration
 * options and randomizes the base and space between each. The size of the
 * physical memory mapping is the available physical memory.
 */

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/prandom.h>
#include <linux/memblock.h>
#include <linux/pgtable.h>

#include <asm/setup.h>
#include <asm/kaslr.h>

#include "mm_internal.h"

#define TB_SHIFT 40

/*
 * The end address could depend on more configuration options to make the
 * highest amount of space for randomization available, but that's too hard
 * to keep straight and caused issues already.
 */
static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE;

/*
 * Memory regions randomized by KASLR (except modules that use a separate logic
 * earlier during boot). The list is ordered based on virtual addresses. This
 * order is kept after randomization.
 */
static __initdata struct kaslr_memory_region {
	unsigned long *base;
	unsigned long *end;
	unsigned long size_tb;
} kaslr_regions[] = {
	{
		.base	= &page_offset_base,
		.end	= &direct_map_physmem_end,
	},
	{
		.base	= &vmalloc_base,
	},
	{
		.base	= &vmemmap_base,
	},
};

/*
 * The end of the physical address space that can be mapped directly by the
 * kernel. This starts out at (1<<MAX_PHYSMEM_BITS) - 1), but KASLR may reduce
 * that in order to increase the available entropy for mapping other regions.
 */
unsigned long direct_map_physmem_end __ro_after_init;

/* Get size in bytes used by the memory region */
static inline unsigned long get_padding(struct kaslr_memory_region *region)
{
	return (region->size_tb << TB_SHIFT);
}

/* Initialize base and padding for each memory region randomized with KASLR */
void __init kernel_randomize_memory(void)
{
	size_t i;
	unsigned long vaddr_start, vaddr;
	unsigned long rand, memory_tb;
	struct rnd_state rand_state;
	unsigned long remain_entropy;
	unsigned long vmemmap_size;

	vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
	vaddr = vaddr_start;

	/*
	 * These BUILD_BUG_ON checks ensure the memory layout is consistent
	 * with the vaddr_start/vaddr_end variables. These checks are very
	 * limited....
	 */
	BUILD_BUG_ON(vaddr_start >= vaddr_end);
	BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE);
	BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);

	/* Preset the end of the possible address space for physical memory */
	direct_map_physmem_end = ((1ULL << MAX_PHYSMEM_BITS) - 1);
	if (!kaslr_memory_enabled())
		return;

	kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
	kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;

	/*
	 * Update Physical memory mapping to available and
	 * add padding if needed (especially for memory hotplug support).
	 */
	BUG_ON(kaslr_regions[0].base != &page_offset_base);
	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;

	/*
	 * Adapt physical memory region size based on available memory,
	 * except when CONFIG_ZONE_DEVICE is enabled. ZONE_DEVICE wants to map
	 * any physical address into the direct-map. KASLR wants to reliably
	 * steal some physical address bits. Those design choices are in direct
	 * conflict.
	 */
	if (!IS_ENABLED(CONFIG_ZONE_DEVICE) && (memory_tb < kaslr_regions[0].size_tb))
		kaslr_regions[0].size_tb = memory_tb;

	/*
	 * Calculate the vmemmap region size in TBs, aligned to a TB
	 * boundary.
	 */
	vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) *
			sizeof(struct page);
	kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);

	/* Calculate entropy available between regions */
	remain_entropy = vaddr_end - vaddr_start;
	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
		remain_entropy -= get_padding(&kaslr_regions[i]);

	prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));

	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
		unsigned long entropy;

		/*
		 * Select a random virtual address using the extra entropy
		 * available.
		 */
		entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
		prandom_bytes_state(&rand_state, &rand, sizeof(rand));
		entropy = (rand % (entropy + 1)) & PUD_MASK;
		vaddr += entropy;
		*kaslr_regions[i].base = vaddr;

		/* Calculate the end of the region */
		vaddr += get_padding(&kaslr_regions[i]);
		/*
		 * KASLR trims the maximum possible size of the
		 * direct-map. Update the direct_map_physmem_end boundary.
		 * No rounding required as the region starts
		 * PUD aligned and size is in units of TB.
		 */
		if (kaslr_regions[i].end)
			*kaslr_regions[i].end = __pa_nodebug(vaddr - 1);

		/* Add a minimum padding based on randomization alignment. */
		vaddr = round_up(vaddr + 1, PUD_SIZE);
		remain_entropy -= entropy;
	}
}

void __meminit init_trampoline_kaslr(void)
{
	pud_t *pud_page_tramp, *pud, *pud_tramp;
	p4d_t *p4d_page_tramp, *p4d, *p4d_tramp;
	unsigned long paddr, vaddr;
	pgd_t *pgd;

	pud_page_tramp = alloc_low_page();

	/*
	 * There are two mappings for the low 1MB area, the direct mapping
	 * and the 1:1 mapping for the real mode trampoline:
	 *
	 * Direct mapping: virt_addr = phys_addr + PAGE_OFFSET
	 * 1:1 mapping:    virt_addr = phys_addr
	 */
	paddr = 0;
	vaddr = (unsigned long)__va(paddr);
	pgd = pgd_offset_k(vaddr);

	p4d = p4d_offset(pgd, vaddr);
	pud = pud_offset(p4d, vaddr);

	pud_tramp = pud_page_tramp + pud_index(paddr);
	*pud_tramp = *pud;

	if (pgtable_l5_enabled()) {
		p4d_page_tramp = alloc_low_page();

		p4d_tramp = p4d_page_tramp + p4d_index(paddr);

		set_p4d(p4d_tramp,
			__p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));

		trampoline_pgd_entry =
			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
	} else {
		trampoline_pgd_entry =
			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
	}
}