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// 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, 0 },
};
/* 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;
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);
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 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;
/*
* 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();
}
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