drivers/firmware/efi/libstub/random.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Copyright (C) 2016 Linaro Ltd;  <ard.biesheuvel@linaro.org>
   4  */
   5
   6 #include <linux/efi.h>
   7 #include <asm/efi.h>
   8
   9 #include "efistub.h"
  10
  11 typedef union efi_rng_protocol efi_rng_protocol_t;
  12
  13 union efi_rng_protocol {
  14         struct {
  15                 efi_status_t (__efiapi *get_info)(efi_rng_protocol_t *,
  16                                                   unsigned long *,
  17                                                   efi_guid_t *);
  18                 efi_status_t (__efiapi *get_rng)(efi_rng_protocol_t *,
  19                                                  efi_guid_t *, unsigned long,
  20                                                  u8 *out);
  21         };
  22         struct {
  23                 u32 get_info;
  24                 u32 get_rng;
  25         } mixed_mode;
  26 };
  27
  28 /**
  29  * efi_get_random_bytes() - fill a buffer with random bytes
  30  * @size:       size of the buffer
  31  * @out:        caller allocated buffer to receive the random bytes
  32  *
  33  * The call will fail if either the firmware does not implement the
  34  * EFI_RNG_PROTOCOL or there are not enough random bytes available to fill
  35  * the buffer.
  36  *
  37  * Return:      status code
  38  */
  39 efi_status_t efi_get_random_bytes(unsigned long size, u8 *out)
  40 {
  41         efi_guid_t rng_proto = EFI_RNG_PROTOCOL_GUID;
  42         efi_status_t status;
  43         efi_rng_protocol_t *rng = NULL;
  44
  45         status = efi_bs_call(locate_protocol, &rng_proto, NULL, (void **)&rng);
  46         if (status != EFI_SUCCESS)
  47                 return status;
  48
  49         return efi_call_proto(rng, get_rng, NULL, size, out);
  50 }
  51
  52 /**
  53  * efi_random_get_seed() - provide random seed as configuration table
  54  *
  55  * The EFI_RNG_PROTOCOL is used to read random bytes. These random bytes are
  56  * saved as a configuration table which can be used as entropy by the kernel
  57  * for the initialization of its pseudo random number generator.
  58  *
  59  * If the EFI_RNG_PROTOCOL is not available or there are not enough random bytes
  60  * available, the configuration table will not be installed and an error code
  61  * will be returned.
  62  *
  63  * Return:      status code
  64  */
  65 efi_status_t efi_random_get_seed(void)
  66 {
  67         efi_guid_t rng_proto = EFI_RNG_PROTOCOL_GUID;
  68         efi_guid_t rng_algo_raw = EFI_RNG_ALGORITHM_RAW;
  69         efi_guid_t rng_table_guid = LINUX_EFI_RANDOM_SEED_TABLE_GUID;
  70         struct linux_efi_random_seed *prev_seed, *seed = NULL;
  71         int prev_seed_size = 0, seed_size = EFI_RANDOM_SEED_SIZE;
  72         unsigned long nv_seed_size = 0, offset = 0;
  73         efi_rng_protocol_t *rng = NULL;
  74         efi_status_t status;
  75
  76         status = efi_bs_call(locate_protocol, &rng_proto, NULL, (void **)&rng);
  77         if (status != EFI_SUCCESS)
  78                 seed_size = 0;
  79
  80         // Call GetVariable() with a zero length buffer to obtain the size
  81         get_efi_var(L"RandomSeed", &rng_table_guid, NULL, &nv_seed_size, NULL);
  82         if (!seed_size && !nv_seed_size)
  83                 return status;
  84
  85         seed_size += nv_seed_size;
  86
  87         /*
  88          * Check whether a seed was provided by a prior boot stage. In that
  89          * case, instead of overwriting it, let's create a new buffer that can
  90          * hold both, and concatenate the existing and the new seeds.
  91          * Note that we should read the seed size with caution, in case the
  92          * table got corrupted in memory somehow.
  93          */
  94         prev_seed = get_efi_config_table(rng_table_guid);
  95         if (prev_seed && prev_seed->size <= 512U) {
  96                 prev_seed_size = prev_seed->size;
  97                 seed_size += prev_seed_size;
  98         }
  99
 100         /*
 101          * Use EFI_ACPI_RECLAIM_MEMORY here so that it is guaranteed that the
 102          * allocation will survive a kexec reboot (although we refresh the seed
 103          * beforehand)
 104          */
 105         status = efi_bs_call(allocate_pool, EFI_ACPI_RECLAIM_MEMORY,
 106                              struct_size(seed, bits, seed_size),
 107                              (void **)&seed);
 108         if (status != EFI_SUCCESS) {
 109                 efi_warn("Failed to allocate memory for RNG seed.\n");
 110                 goto err_warn;
 111         }
 112
 113         if (rng) {
 114                 status = efi_call_proto(rng, get_rng, &rng_algo_raw,
 115                                         EFI_RANDOM_SEED_SIZE, seed->bits);
 116
 117                 if (status == EFI_UNSUPPORTED)
 118                         /*
 119                          * Use whatever algorithm we have available if the raw algorithm
 120                          * is not implemented.
 121                          */
 122                         status = efi_call_proto(rng, get_rng, NULL,
 123                                                 EFI_RANDOM_SEED_SIZE, seed->bits);
 124
 125                 if (status == EFI_SUCCESS)
 126                         offset = EFI_RANDOM_SEED_SIZE;
 127         }
 128
 129         if (nv_seed_size) {
 130                 status = get_efi_var(L"RandomSeed", &rng_table_guid, NULL,
 131                                      &nv_seed_size, seed->bits + offset);
 132
 133                 if (status == EFI_SUCCESS)
 134                         /*
 135                          * We delete the seed here, and /hope/ that this causes
 136                          * EFI to also zero out its representation on disk.
 137                          * This is somewhat idealistic, but overwriting the
 138                          * variable with zeros is likely just as fraught too.
 139                          * TODO: in the future, maybe we can hash it forward
 140                          * instead, and write a new seed.
 141                          */
 142                         status = set_efi_var(L"RandomSeed", &rng_table_guid, 0,
 143                                              0, NULL);
 144
 145                 if (status == EFI_SUCCESS)
 146                         offset += nv_seed_size;
 147                 else
 148                         memzero_explicit(seed->bits + offset, nv_seed_size);
 149         }
 150
 151         if (!offset)
 152                 goto err_freepool;
 153
 154         if (prev_seed_size) {
 155                 memcpy(seed->bits + offset, prev_seed->bits, prev_seed_size);
 156                 offset += prev_seed_size;
 157         }
 158
 159         seed->size = offset;
 160         status = efi_bs_call(install_configuration_table, &rng_table_guid, seed);
 161         if (status != EFI_SUCCESS)
 162                 goto err_freepool;
 163
 164         if (prev_seed_size) {
 165                 /* wipe and free the old seed if we managed to install the new one */
 166                 memzero_explicit(prev_seed->bits, prev_seed_size);
 167                 efi_bs_call(free_pool, prev_seed);
 168         }
 169         return EFI_SUCCESS;
 170
 171 err_freepool:
 172         memzero_explicit(seed, struct_size(seed, bits, seed_size));
 173         efi_bs_call(free_pool, seed);
 174         efi_warn("Failed to obtain seed from EFI_RNG_PROTOCOL or EFI variable\n");
 175 err_warn:
 176         if (prev_seed)
 177                 efi_warn("Retaining bootloader-supplied seed only");
 178         return status;
 179 }