drivers/firmware/efi/libstub/random.c

/*
 * Copyright (C) 2016 Linaro Ltd;  <ard.biesheuvel@linaro.org>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 */

#include <linux/efi.h>
#include <asm/efi.h>

#include "efistub.h"

struct efi_rng_protocol {
	efi_status_t (*get_info)(struct efi_rng_protocol *,
				 unsigned long *, efi_guid_t *);
	efi_status_t (*get_rng)(struct efi_rng_protocol *,
				efi_guid_t *, unsigned long, u8 *out);
};

efi_status_t efi_get_random_bytes(efi_system_table_t *sys_table_arg,
				  unsigned long size, u8 *out)
{
	efi_guid_t rng_proto = EFI_RNG_PROTOCOL_GUID;
	efi_status_t status;
	struct efi_rng_protocol *rng;

	status = efi_call_early(locate_protocol, &rng_proto, NULL,
				(void **)&rng);
	if (status != EFI_SUCCESS)
		return status;

	return rng->get_rng(rng, NULL, size, out);
}

/*
 * Return the number of slots covered by this entry, i.e., the number of
 * addresses it covers that are suitably aligned and supply enough room
 * for the allocation.
 */
static unsigned long get_entry_num_slots(efi_memory_desc_t *md,
					 unsigned long size,
					 unsigned long align)
{
	u64 start, end;

	if (md->type != EFI_CONVENTIONAL_MEMORY)
		return 0;

	start = round_up(md->phys_addr, align);
	end = round_down(md->phys_addr + md->num_pages * EFI_PAGE_SIZE - size,
			 align);

	if (start > end)
		return 0;

	return (end - start + 1) / align;
}

/*
 * The UEFI memory descriptors have a virtual address field that is only used
 * when installing the virtual mapping using SetVirtualAddressMap(). Since it
 * is unused here, we can reuse it to keep track of each descriptor's slot
 * count.
 */
#define MD_NUM_SLOTS(md)	((md)->virt_addr)

efi_status_t efi_random_alloc(efi_system_table_t *sys_table_arg,
			      unsigned long size,
			      unsigned long align,
			      unsigned long *addr,
			      unsigned long random_seed)
{
	unsigned long map_size, desc_size, total_slots = 0, target_slot;
	efi_status_t status;
	efi_memory_desc_t *memory_map;
	int map_offset;

	status = efi_get_memory_map(sys_table_arg, &memory_map, &map_size,
				    &desc_size, NULL, NULL);
	if (status != EFI_SUCCESS)
		return status;

	if (align < EFI_ALLOC_ALIGN)
		align = EFI_ALLOC_ALIGN;

	/* count the suitable slots in each memory map entry */
	for (map_offset = 0; map_offset < map_size; map_offset += desc_size) {
		efi_memory_desc_t *md = (void *)memory_map + map_offset;
		unsigned long slots;

		slots = get_entry_num_slots(md, size, align);
		MD_NUM_SLOTS(md) = slots;
		total_slots += slots;
	}

	/* find a random number between 0 and total_slots */
	target_slot = (total_slots * (u16)random_seed) >> 16;

	/*
	 * target_slot is now a value in the range [0, total_slots), and so
	 * it corresponds with exactly one of the suitable slots we recorded
	 * when iterating over the memory map the first time around.
	 *
	 * So iterate over the memory map again, subtracting the number of
	 * slots of each entry at each iteration, until we have found the entry
	 * that covers our chosen slot. Use the residual value of target_slot
	 * to calculate the randomly chosen address, and allocate it directly
	 * using EFI_ALLOCATE_ADDRESS.
	 */
	for (map_offset = 0; map_offset < map_size; map_offset += desc_size) {
		efi_memory_desc_t *md = (void *)memory_map + map_offset;
		efi_physical_addr_t target;
		unsigned long pages;

		if (target_slot >= MD_NUM_SLOTS(md)) {
			target_slot -= MD_NUM_SLOTS(md);
			continue;
		}

		target = round_up(md->phys_addr, align) + target_slot * align;
		pages = round_up(size, EFI_PAGE_SIZE) / EFI_PAGE_SIZE;

		status = efi_call_early(allocate_pages, EFI_ALLOCATE_ADDRESS,
					EFI_LOADER_DATA, pages, &target);
		if (status == EFI_SUCCESS)
			*addr = target;
		break;
	}

	efi_call_early(free_pool, memory_map);

	return status;
}