drivers/virt/coco/efi_secret/efi_secret.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * efi_secret module
   4  *
   5  * Copyright (C) 2022 IBM Corporation
   6  * Author: Dov Murik <dovmurik@linux.ibm.com>
   7  */
   8
   9 /**
  10  * DOC: efi_secret: Allow reading EFI confidential computing (coco) secret area
  11  * via securityfs interface.
  12  *
  13  * When the module is loaded (and securityfs is mounted, typically under
  14  * /sys/kernel/security), a "secrets/coco" directory is created in securityfs.
  15  * In it, a file is created for each secret entry.  The name of each such file
  16  * is the GUID of the secret entry, and its content is the secret data.
  17  */
  18
  19 #include <linux/platform_device.h>
  20 #include <linux/seq_file.h>
  21 #include <linux/fs.h>
  22 #include <linux/kernel.h>
  23 #include <linux/init.h>
  24 #include <linux/module.h>
  25 #include <linux/io.h>
  26 #include <linux/security.h>
  27 #include <linux/efi.h>
  28 #include <linux/cacheflush.h>
  29
  30 #define EFI_SECRET_NUM_FILES 64
  31
  32 struct efi_secret {
  33         struct dentry *secrets_dir;
  34         struct dentry *fs_dir;
  35         struct dentry *fs_files[EFI_SECRET_NUM_FILES];
  36         void __iomem *secret_data;
  37         u64 secret_data_len;
  38 };
  39
  40 /*
  41  * Structure of the EFI secret area
  42  *
  43  * Offset   Length
  44  * (bytes)  (bytes)  Usage
  45  * -------  -------  -----
  46  *       0       16  Secret table header GUID (must be 1e74f542-71dd-4d66-963e-ef4287ff173b)
  47  *      16        4  Length of bytes of the entire secret area
  48  *
  49  *      20       16  First secret entry's GUID
  50  *      36        4  First secret entry's length in bytes (= 16 + 4 + x)
  51  *      40        x  First secret entry's data
  52  *
  53  *    40+x       16  Second secret entry's GUID
  54  *    56+x        4  Second secret entry's length in bytes (= 16 + 4 + y)
  55  *    60+x        y  Second secret entry's data
  56  *
  57  * (... and so on for additional entries)
  58  *
  59  * The GUID of each secret entry designates the usage of the secret data.
  60  */
  61
  62 /**
  63  * struct secret_header - Header of entire secret area; this should be followed
  64  * by instances of struct secret_entry.
  65  * @guid:       Must be EFI_SECRET_TABLE_HEADER_GUID
  66  * @len:        Length in bytes of entire secret area, including header
  67  */
  68 struct secret_header {
  69         efi_guid_t guid;
  70         u32 len;
  71 } __attribute((packed));
  72
  73 /**
  74  * struct secret_entry - Holds one secret entry
  75  * @guid:       Secret-specific GUID (or NULL_GUID if this secret entry was deleted)
  76  * @len:        Length of secret entry, including its guid and len fields
  77  * @data:       The secret data (full of zeros if this secret entry was deleted)
  78  */
  79 struct secret_entry {
  80         efi_guid_t guid;
  81         u32 len;
  82         u8 data[];
  83 } __attribute((packed));
  84
  85 static size_t secret_entry_data_len(struct secret_entry *e)
  86 {
  87         return e->len - sizeof(*e);
  88 }
  89
  90 static struct efi_secret the_efi_secret;
  91
  92 static inline struct efi_secret *efi_secret_get(void)
  93 {
  94         return &the_efi_secret;
  95 }
  96
  97 static int efi_secret_bin_file_show(struct seq_file *file, void *data)
  98 {
  99         struct secret_entry *e = file->private;
 100
 101         if (e)
 102                 seq_write(file, e->data, secret_entry_data_len(e));
 103
 104         return 0;
 105 }
 106 DEFINE_SHOW_ATTRIBUTE(efi_secret_bin_file);
 107
 108 /*
 109  * Overwrite memory content with zeroes, and ensure that dirty cache lines are
 110  * actually written back to memory, to clear out the secret.
 111  */
 112 static void wipe_memory(void *addr, size_t size)
 113 {
 114         memzero_explicit(addr, size);
 115 #ifdef CONFIG_X86
 116         clflush_cache_range(addr, size);
 117 #endif
 118 }
 119
 120 static int efi_secret_unlink(struct inode *dir, struct dentry *dentry)
 121 {
 122         struct efi_secret *s = efi_secret_get();
 123         struct inode *inode = d_inode(dentry);
 124         struct secret_entry *e = (struct secret_entry *)inode->i_private;
 125         int i;
 126
 127         if (e) {
 128                 /* Zero out the secret data */
 129                 wipe_memory(e->data, secret_entry_data_len(e));
 130                 e->guid = NULL_GUID;
 131         }
 132
 133         inode->i_private = NULL;
 134
 135         for (i = 0; i < EFI_SECRET_NUM_FILES; i++)
 136                 if (s->fs_files[i] == dentry)
 137                         s->fs_files[i] = NULL;
 138
 139         /*
 140          * securityfs_remove tries to lock the directory's inode, but we reach
 141          * the unlink callback when it's already locked
 142          */
 143         inode_unlock(dir);
 144         securityfs_remove(dentry);
 145         inode_lock(dir);
 146
 147         return 0;
 148 }
 149
 150 static const struct inode_operations efi_secret_dir_inode_operations = {
 151         .lookup         = simple_lookup,
 152         .unlink         = efi_secret_unlink,
 153 };
 154
 155 static int efi_secret_map_area(struct platform_device *dev)
 156 {
 157         int ret;
 158         struct efi_secret *s = efi_secret_get();
 159         struct linux_efi_coco_secret_area *secret_area;
 160
 161         if (efi.coco_secret == EFI_INVALID_TABLE_ADDR) {
 162                 dev_err(&dev->dev, "Secret area address is not available\n");
 163                 return -EINVAL;
 164         }
 165
 166         secret_area = memremap(efi.coco_secret, sizeof(*secret_area), MEMREMAP_WB);
 167         if (secret_area == NULL) {
 168                 dev_err(&dev->dev, "Could not map secret area EFI config entry\n");
 169                 return -ENOMEM;
 170         }
 171         if (!secret_area->base_pa || secret_area->size < sizeof(struct secret_header)) {
 172                 dev_err(&dev->dev,
 173                         "Invalid secret area memory location (base_pa=0x%llx size=0x%llx)\n",
 174                         secret_area->base_pa, secret_area->size);
 175                 ret = -EINVAL;
 176                 goto unmap;
 177         }
 178
 179         s->secret_data = ioremap_encrypted(secret_area->base_pa, secret_area->size);
 180         if (s->secret_data == NULL) {
 181                 dev_err(&dev->dev, "Could not map secret area\n");
 182                 ret = -ENOMEM;
 183                 goto unmap;
 184         }
 185
 186         s->secret_data_len = secret_area->size;
 187         ret = 0;
 188
 189 unmap:
 190         memunmap(secret_area);
 191         return ret;
 192 }
 193
 194 static void efi_secret_securityfs_teardown(struct platform_device *dev)
 195 {
 196         struct efi_secret *s = efi_secret_get();
 197         int i;
 198
 199         for (i = (EFI_SECRET_NUM_FILES - 1); i >= 0; i--) {
 200                 securityfs_remove(s->fs_files[i]);
 201                 s->fs_files[i] = NULL;
 202         }
 203
 204         securityfs_remove(s->fs_dir);
 205         s->fs_dir = NULL;
 206
 207         securityfs_remove(s->secrets_dir);
 208         s->secrets_dir = NULL;
 209
 210         dev_dbg(&dev->dev, "Removed securityfs entries\n");
 211 }
 212
 213 static int efi_secret_securityfs_setup(struct platform_device *dev)
 214 {
 215         struct efi_secret *s = efi_secret_get();
 216         int ret = 0, i = 0, bytes_left;
 217         unsigned char *ptr;
 218         struct secret_header *h;
 219         struct secret_entry *e;
 220         struct dentry *dent;
 221         char guid_str[EFI_VARIABLE_GUID_LEN + 1];
 222
 223         ptr = (void __force *)s->secret_data;
 224         h = (struct secret_header *)ptr;
 225         if (efi_guidcmp(h->guid, EFI_SECRET_TABLE_HEADER_GUID)) {
 226                 /*
 227                  * This is not an error: it just means that EFI defines secret
 228                  * area but it was not populated by the Guest Owner.
 229                  */
 230                 dev_dbg(&dev->dev, "EFI secret area does not start with correct GUID\n");
 231                 return -ENODEV;
 232         }
 233         if (h->len < sizeof(*h)) {
 234                 dev_err(&dev->dev, "EFI secret area reported length is too small\n");
 235                 return -EINVAL;
 236         }
 237         if (h->len > s->secret_data_len) {
 238                 dev_err(&dev->dev, "EFI secret area reported length is too big\n");
 239                 return -EINVAL;
 240         }
 241
 242         s->secrets_dir = NULL;
 243         s->fs_dir = NULL;
 244         memset(s->fs_files, 0, sizeof(s->fs_files));
 245
 246         dent = securityfs_create_dir("secrets", NULL);
 247         if (IS_ERR(dent)) {
 248                 dev_err(&dev->dev, "Error creating secrets securityfs directory entry err=%ld\n",
 249                         PTR_ERR(dent));
 250                 return PTR_ERR(dent);
 251         }
 252         s->secrets_dir = dent;
 253
 254         dent = securityfs_create_dir("coco", s->secrets_dir);
 255         if (IS_ERR(dent)) {
 256                 dev_err(&dev->dev, "Error creating coco securityfs directory entry err=%ld\n",
 257                         PTR_ERR(dent));
 258                 return PTR_ERR(dent);
 259         }
 260         d_inode(dent)->i_op = &efi_secret_dir_inode_operations;
 261         s->fs_dir = dent;
 262
 263         bytes_left = h->len - sizeof(*h);
 264         ptr += sizeof(*h);
 265         while (bytes_left >= (int)sizeof(*e) && i < EFI_SECRET_NUM_FILES) {
 266                 e = (struct secret_entry *)ptr;
 267                 if (e->len < sizeof(*e) || e->len > (unsigned int)bytes_left) {
 268                         dev_err(&dev->dev, "EFI secret area is corrupted\n");
 269                         ret = -EINVAL;
 270                         goto err_cleanup;
 271                 }
 272
 273                 /* Skip deleted entries (which will have NULL_GUID) */
 274                 if (efi_guidcmp(e->guid, NULL_GUID)) {
 275                         efi_guid_to_str(&e->guid, guid_str);
 276
 277                         dent = securityfs_create_file(guid_str, 0440, s->fs_dir, (void *)e,
 278                                                       &efi_secret_bin_file_fops);
 279                         if (IS_ERR(dent)) {
 280                                 dev_err(&dev->dev, "Error creating efi_secret securityfs entry\n");
 281                                 ret = PTR_ERR(dent);
 282                                 goto err_cleanup;
 283                         }
 284
 285                         s->fs_files[i++] = dent;
 286                 }
 287                 ptr += e->len;
 288                 bytes_left -= e->len;
 289         }
 290
 291         dev_info(&dev->dev, "Created %d entries in securityfs secrets/coco\n", i);
 292         return 0;
 293
 294 err_cleanup:
 295         efi_secret_securityfs_teardown(dev);
 296         return ret;
 297 }
 298
 299 static void efi_secret_unmap_area(void)
 300 {
 301         struct efi_secret *s = efi_secret_get();
 302
 303         if (s->secret_data) {
 304                 iounmap(s->secret_data);
 305                 s->secret_data = NULL;
 306                 s->secret_data_len = 0;
 307         }
 308 }
 309
 310 static int efi_secret_probe(struct platform_device *dev)
 311 {
 312         int ret;
 313
 314         ret = efi_secret_map_area(dev);
 315         if (ret)
 316                 return ret;
 317
 318         ret = efi_secret_securityfs_setup(dev);
 319         if (ret)
 320                 goto err_unmap;
 321
 322         return ret;
 323
 324 err_unmap:
 325         efi_secret_unmap_area();
 326         return ret;
 327 }
 328
 329 static void efi_secret_remove(struct platform_device *dev)
 330 {
 331         efi_secret_securityfs_teardown(dev);
 332         efi_secret_unmap_area();
 333 }
 334
 335 static struct platform_driver efi_secret_driver = {
 336         .probe = efi_secret_probe,
 337         .remove = efi_secret_remove,
 338         .driver = {
 339                 .name = "efi_secret",
 340         },
 341 };
 342
 343 module_platform_driver(efi_secret_driver);
 344
 345 MODULE_DESCRIPTION("Confidential computing EFI secret area access");
 346 MODULE_AUTHOR("IBM");
 347 MODULE_LICENSE("GPL");
 348 MODULE_ALIAS("platform:efi_secret");