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/random.h>
#include <linux/memblock.h>

#include <asm/pgalloc.h>
#include <asm/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 size_tb;
} kaslr_regions[] = {
	{ &page_offset_base, 0 },
	{ &vmalloc_base, 0 },
	{ &vmemmap_base, 1 },
};

/* 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);
}

/*
 * Apply no randomization if KASLR was disabled at boot or if KASAN
 * is enabled. KASAN shadow mappings rely on regions being PGD aligned.
 */
static inline bool kaslr_memory_enabled(void)
{
	return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
}

/* 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;

	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);

	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 phyiscal memory region size based on available memory */
	if (memory_tb < kaslr_regions[0].size_tb)
		kaslr_regions[0].size_tb = memory_tb;

	/* 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;

		/*
		 * Jump the region and add a minimum padding based on
		 * randomization alignment.
		 */
		vaddr += get_padding(&kaslr_regions[i]);
		vaddr = round_up(vaddr + 1, PUD_SIZE);
		remain_entropy -= entropy;
	}
}

static void __meminit init_trampoline_pud(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)));

		set_pgd(&trampoline_pgd_entry,
			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
	} else {
		set_pgd(&trampoline_pgd_entry,
			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
	}
}

/*
 * The real mode trampoline, which is required for bootstrapping CPUs
 * occupies only a small area under the low 1MB.  See reserve_real_mode()
 * for details.
 *
 * If KASLR is disabled the first PGD entry of the direct mapping is copied
 * to map the real mode trampoline.
 *
 * If KASLR is enabled, copy only the PUD which covers the low 1MB
 * area. This limits the randomization granularity to 1GB for both 4-level
 * and 5-level paging.
 */
void __meminit init_trampoline(void)
{
	if (!kaslr_memory_enabled()) {
		init_trampoline_default();
		return;
	}

	init_trampoline_pud();
}