kaslr_early.c

 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, node, seed_str, &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, 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}