1// SPDX-License-Identifier: GPL-2.0-only
2// Copyright 2022 Google LLC
3// Author: Ard Biesheuvel <ardb@google.com>
4
5// NOTE: code in this file runs *very* early, and is not permitted to use
6// global variables or anything that relies on absolute addressing.
7
8#include <linux/libfdt.h>
9#include <linux/init.h>
10#include <linux/linkage.h>
11#include <linux/types.h>
12#include <linux/sizes.h>
13#include <linux/string.h>
14
15#include <asm/archrandom.h>
16#include <asm/memory.h>
17#include <asm/pgtable.h>
18
19#include "pi.h"
20
21static u64 __init get_kaslr_seed(void *fdt, int node)
22{
23 static char const seed_str[] __initconst = "kaslr-seed";
24 fdt64_t *prop;
25 u64 ret;
26 int len;
27
28 if (node < 0)
29 return 0;
30
31 prop = fdt_getprop_w(fdt, nodeoffset: node, name: seed_str, lenp: &len);
32 if (!prop || len != sizeof(u64))
33 return 0;
34
35 ret = fdt64_to_cpu(*prop);
36 *prop = 0;
37 return ret;
38}
39
40u64 __init kaslr_early_init(void *fdt, int chosen)
41{
42 u64 seed, range;
43
44 if (kaslr_disabled_cmdline())
45 return 0;
46
47 seed = get_kaslr_seed(fdt, node: chosen);
48 if (!seed) {
49 if (!__early_cpu_has_rndr() ||
50 !__arm64_rndr((unsigned long *)&seed))
51 return 0;
52 }
53
54 /*
55 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
56 * kernel image offset from the seed. Let's place the kernel in the
57 * 'middle' half of the VMALLOC area, and stay clear of the lower and
58 * upper quarters to avoid colliding with other allocations.
59 */
60 range = (VMALLOC_END - KIMAGE_VADDR) / 2;
61 return range / 2 + (((__uint128_t)range * seed) >> 64);
62}
63

source code of linux/arch/arm64/kernel/pi/kaslr_early.c