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// SPDX-License-Identifier: GPL-2.0-only
/*
* Copyright (C) 2016 Linaro Ltd <ard.biesheuvel@linaro.org>
*/
#include <linux/cache.h>
#include <linux/crc32.h>
#include <linux/init.h>
#include <linux/libfdt.h>
#include <linux/mm_types.h>
#include <linux/sched.h>
#include <linux/types.h>
#include <linux/pgtable.h>
#include <linux/random.h>
#include <asm/cacheflush.h>
#include <asm/fixmap.h>
#include <asm/kernel-pgtable.h>
#include <asm/memory.h>
#include <asm/mmu.h>
#include <asm/sections.h>
enum kaslr_status {
KASLR_ENABLED,
KASLR_DISABLED_CMDLINE,
KASLR_DISABLED_NO_SEED,
KASLR_DISABLED_FDT_REMAP,
};
static enum kaslr_status __initdata kaslr_status;
u64 __ro_after_init module_alloc_base;
u16 __initdata memstart_offset_seed;
static __init u64 get_kaslr_seed(void *fdt)
{
int node, len;
fdt64_t *prop;
u64 ret;
node = fdt_path_offset(fdt, "/chosen");
if (node < 0)
return 0;
prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
if (!prop || len != sizeof(u64))
return 0;
ret = fdt64_to_cpu(*prop);
*prop = 0;
return ret;
}
static __init bool cmdline_contains_nokaslr(const u8 *cmdline)
{
const u8 *str;
str = strstr(cmdline, "nokaslr");
return str == cmdline || (str > cmdline && *(str - 1) == ' ');
}
static __init bool is_kaslr_disabled_cmdline(void *fdt)
{
if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
int node;
const u8 *prop;
node = fdt_path_offset(fdt, "/chosen");
if (node < 0)
goto out;
prop = fdt_getprop(fdt, node, "bootargs", NULL);
if (!prop)
goto out;
if (cmdline_contains_nokaslr(prop))
return true;
if (IS_ENABLED(CONFIG_CMDLINE_EXTEND))
goto out;
return false;
}
out:
return cmdline_contains_nokaslr(CONFIG_CMDLINE);
}
/*
* This routine will be executed with the kernel mapped at its default virtual
* address, and if it returns successfully, the kernel will be remapped, and
* start_kernel() will be executed from a randomized virtual offset. The
* relocation will result in all absolute references (e.g., static variables
* containing function pointers) to be reinitialized, and zero-initialized
* .bss variables will be reset to 0.
*/
u64 __init kaslr_early_init(u64 dt_phys)
{
void *fdt;
u64 seed, offset, mask, module_range;
unsigned long raw;
int size;
/*
* Set a reasonable default for module_alloc_base in case
* we end up running with module randomization disabled.
*/
module_alloc_base = (u64)_etext - MODULES_VSIZE;
__flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
/*
* Try to map the FDT early. If this fails, we simply bail,
* and proceed with KASLR disabled. We will make another
* attempt at mapping the FDT in setup_machine()
*/
early_fixmap_init();
fdt = fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
if (!fdt) {
kaslr_status = KASLR_DISABLED_FDT_REMAP;
return 0;
}
/*
* Retrieve (and wipe) the seed from the FDT
*/
seed = get_kaslr_seed(fdt);
/*
* Check if 'nokaslr' appears on the command line, and
* return 0 if that is the case.
*/
if (is_kaslr_disabled_cmdline(fdt)) {
kaslr_status = KASLR_DISABLED_CMDLINE;
return 0;
}
/*
* Mix in any entropy obtainable architecturally if enabled
* and supported.
*/
if (arch_get_random_seed_long_early(&raw))
seed ^= raw;
if (!seed) {
kaslr_status = KASLR_DISABLED_NO_SEED;
return 0;
}
/*
* OK, so we are proceeding with KASLR enabled. Calculate a suitable
* kernel image offset from the seed. Let's place the kernel in the
* middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
* the lower and upper quarters to avoid colliding with other
* allocations.
* Even if we could randomize at page granularity for 16k and 64k pages,
* let's always round to 2 MB so we don't interfere with the ability to
* map using contiguous PTEs
*/
mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
/* use the top 16 bits to randomize the linear region */
memstart_offset_seed = seed >> 48;
if (IS_ENABLED(CONFIG_KASAN))
/*
* KASAN does not expect the module region to intersect the
* vmalloc region, since shadow memory is allocated for each
* module at load time, whereas the vmalloc region is shadowed
* by KASAN zero pages. So keep modules out of the vmalloc
* region if KASAN is enabled, and put the kernel well within
* 4 GB of the module region.
*/
return offset % SZ_2G;
if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
/*
* Randomize the module region over a 2 GB window covering the
* kernel. This reduces the risk of modules leaking information
* about the address of the kernel itself, but results in
* branches between modules and the core kernel that are
* resolved via PLTs. (Branches between modules will be
* resolved normally.)
*/
module_range = SZ_2G - (u64)(_end - _stext);
module_alloc_base = max((u64)_end + offset - SZ_2G,
(u64)MODULES_VADDR);
} else {
/*
* Randomize the module region by setting module_alloc_base to
* a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
* _stext) . This guarantees that the resulting region still
* covers [_stext, _etext], and that all relative branches can
* be resolved without veneers.
*/
module_range = MODULES_VSIZE - (u64)(_etext - _stext);
module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
}
/* use the lower 21 bits to randomize the base of the module region */
module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
module_alloc_base &= PAGE_MASK;
__flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
__flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));
return offset;
}
static int __init kaslr_init(void)
{
switch (kaslr_status) {
case KASLR_ENABLED:
pr_info("KASLR enabled\n");
break;
case KASLR_DISABLED_CMDLINE:
pr_info("KASLR disabled on command line\n");
break;
case KASLR_DISABLED_NO_SEED:
pr_warn("KASLR disabled due to lack of seed\n");
break;
case KASLR_DISABLED_FDT_REMAP:
pr_warn("KASLR disabled due to FDT remapping failure\n");
break;
}
return 0;
}
core_initcall(kaslr_init)
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