security/loadpin/loadpin.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Module and Firmware Pinning Security Module
   4  *
   5  * Copyright 2011-2016 Google Inc.
   6  *
   7  * Author: Kees Cook <keescook@chromium.org>
   8  */
   9
  10 #define pr_fmt(fmt) "LoadPin: " fmt
  11
  12 #include <linux/module.h>
  13 #include <linux/fs.h>
  14 #include <linux/kernel_read_file.h>
  15 #include <linux/lsm_hooks.h>
  16 #include <linux/mount.h>
  17 #include <linux/blkdev.h>
  18 #include <linux/path.h>
  19 #include <linux/sched.h>        /* current */
  20 #include <linux/string_helpers.h>
  21
  22 static void report_load(const char *origin, struct file *file, char *operation)
  23 {
  24         char *cmdline, *pathname;
  25
  26         pathname = kstrdup_quotable_file(file, GFP_KERNEL);
  27         cmdline = kstrdup_quotable_cmdline(current, GFP_KERNEL);
  28
  29         pr_notice("%s %s obj=%s%s%s pid=%d cmdline=%s%s%s\n",
  30                   origin, operation,
  31                   (pathname && pathname[0] != '<') ? "\"" : "",
  32                   pathname,
  33                   (pathname && pathname[0] != '<') ? "\"" : "",
  34                   task_pid_nr(current),
  35                   cmdline ? "\"" : "", cmdline, cmdline ? "\"" : "");
  36
  37         kfree(cmdline);
  38         kfree(pathname);
  39 }
  40
  41 static int enforce = IS_ENABLED(CONFIG_SECURITY_LOADPIN_ENFORCE);
  42 static char *exclude_read_files[READING_MAX_ID];
  43 static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
  44 static struct super_block *pinned_root;
  45 static DEFINE_SPINLOCK(pinned_root_spinlock);
  46
  47 #ifdef CONFIG_SYSCTL
  48
  49 static struct ctl_path loadpin_sysctl_path[] = {
  50         { .procname = "kernel", },
  51         { .procname = "loadpin", },
  52         { }
  53 };
  54
  55 static struct ctl_table loadpin_sysctl_table[] = {
  56         {
  57                 .procname       = "enforce",
  58                 .data           = &enforce,
  59                 .maxlen         = sizeof(int),
  60                 .mode           = 0644,
  61                 .proc_handler   = proc_dointvec_minmax,
  62                 .extra1         = SYSCTL_ZERO,
  63                 .extra2         = SYSCTL_ONE,
  64         },
  65         { }
  66 };
  67
  68 /*
  69  * This must be called after early kernel init, since then the rootdev
  70  * is available.
  71  */
  72 static void check_pinning_enforcement(struct super_block *mnt_sb)
  73 {
  74         bool ro = false;
  75
  76         /*
  77          * If load pinning is not enforced via a read-only block
  78          * device, allow sysctl to change modes for testing.
  79          */
  80         if (mnt_sb->s_bdev) {
  81                 char bdev[BDEVNAME_SIZE];
  82
  83                 ro = bdev_read_only(mnt_sb->s_bdev);
  84                 bdevname(mnt_sb->s_bdev, bdev);
  85                 pr_info("%s (%u:%u): %s\n", bdev,
  86                         MAJOR(mnt_sb->s_bdev->bd_dev),
  87                         MINOR(mnt_sb->s_bdev->bd_dev),
  88                         ro ? "read-only" : "writable");
  89         } else
  90                 pr_info("mnt_sb lacks block device, treating as: writable\n");
  91
  92         if (!ro) {
  93                 if (!register_sysctl_paths(loadpin_sysctl_path,
  94                                            loadpin_sysctl_table))
  95                         pr_notice("sysctl registration failed!\n");
  96                 else
  97                         pr_info("enforcement can be disabled.\n");
  98         } else
  99                 pr_info("load pinning engaged.\n");
 100 }
 101 #else
 102 static void check_pinning_enforcement(struct super_block *mnt_sb)
 103 {
 104         pr_info("load pinning engaged.\n");
 105 }
 106 #endif
 107
 108 static void loadpin_sb_free_security(struct super_block *mnt_sb)
 109 {
 110         /*
 111          * When unmounting the filesystem we were using for load
 112          * pinning, we acknowledge the superblock release, but make sure
 113          * no other modules or firmware can be loaded.
 114          */
 115         if (!IS_ERR_OR_NULL(pinned_root) && mnt_sb == pinned_root) {
 116                 pinned_root = ERR_PTR(-EIO);
 117                 pr_info("umount pinned fs: refusing further loads\n");
 118         }
 119 }
 120
 121 static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
 122                              bool contents)
 123 {
 124         struct super_block *load_root;
 125         const char *origin = kernel_read_file_id_str(id);
 126
 127         /*
 128          * If we will not know that we'll be seeing the full contents
 129          * then we cannot trust a load will be complete and unchanged
 130          * off disk. Treat all contents=false hooks as if there were
 131          * no associated file struct.
 132          */
 133         if (!contents)
 134                 file = NULL;
 135
 136         /* If the file id is excluded, ignore the pinning. */
 137         if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
 138             ignore_read_file_id[id]) {
 139                 report_load(origin, file, "pinning-excluded");
 140                 return 0;
 141         }
 142
 143         /* This handles the older init_module API that has a NULL file. */
 144         if (!file) {
 145                 if (!enforce) {
 146                         report_load(origin, NULL, "old-api-pinning-ignored");
 147                         return 0;
 148                 }
 149
 150                 report_load(origin, NULL, "old-api-denied");
 151                 return -EPERM;
 152         }
 153
 154         load_root = file->f_path.mnt->mnt_sb;
 155
 156         /* First loaded module/firmware defines the root for all others. */
 157         spin_lock(&pinned_root_spinlock);
 158         /*
 159          * pinned_root is only NULL at startup. Otherwise, it is either
 160          * a valid reference, or an ERR_PTR.
 161          */
 162         if (!pinned_root) {
 163                 pinned_root = load_root;
 164                 /*
 165                  * Unlock now since it's only pinned_root we care about.
 166                  * In the worst case, we will (correctly) report pinning
 167                  * failures before we have announced that pinning is
 168                  * enforcing. This would be purely cosmetic.
 169                  */
 170                 spin_unlock(&pinned_root_spinlock);
 171                 check_pinning_enforcement(pinned_root);
 172                 report_load(origin, file, "pinned");
 173         } else {
 174                 spin_unlock(&pinned_root_spinlock);
 175         }
 176
 177         if (IS_ERR_OR_NULL(pinned_root) || load_root != pinned_root) {
 178                 if (unlikely(!enforce)) {
 179                         report_load(origin, file, "pinning-ignored");
 180                         return 0;
 181                 }
 182
 183                 report_load(origin, file, "denied");
 184                 return -EPERM;
 185         }
 186
 187         return 0;
 188 }
 189
 190 static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
 191 {
 192         return loadpin_read_file(NULL, (enum kernel_read_file_id) id, contents);
 193 }
 194
 195 static struct security_hook_list loadpin_hooks[] __lsm_ro_after_init = {
 196         LSM_HOOK_INIT(sb_free_security, loadpin_sb_free_security),
 197         LSM_HOOK_INIT(kernel_read_file, loadpin_read_file),
 198         LSM_HOOK_INIT(kernel_load_data, loadpin_load_data),
 199 };
 200
 201 static void __init parse_exclude(void)
 202 {
 203         int i, j;
 204         char *cur;
 205
 206         /*
 207          * Make sure all the arrays stay within expected sizes. This
 208          * is slightly weird because kernel_read_file_str[] includes
 209          * READING_MAX_ID, which isn't actually meaningful here.
 210          */
 211         BUILD_BUG_ON(ARRAY_SIZE(exclude_read_files) !=
 212                      ARRAY_SIZE(ignore_read_file_id));
 213         BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) <
 214                      ARRAY_SIZE(ignore_read_file_id));
 215
 216         for (i = 0; i < ARRAY_SIZE(exclude_read_files); i++) {
 217                 cur = exclude_read_files[i];
 218                 if (!cur)
 219                         break;
 220                 if (*cur == '\0')
 221                         continue;
 222
 223                 for (j = 0; j < ARRAY_SIZE(ignore_read_file_id); j++) {
 224                         if (strcmp(cur, kernel_read_file_str[j]) == 0) {
 225                                 pr_info("excluding: %s\n",
 226                                         kernel_read_file_str[j]);
 227                                 ignore_read_file_id[j] = 1;
 228                                 /*
 229                                  * Can not break, because one read_file_str
 230                                  * may map to more than on read_file_id.
 231                                  */
 232                         }
 233                 }
 234         }
 235 }
 236
 237 static int __init loadpin_init(void)
 238 {
 239         pr_info("ready to pin (currently %senforcing)\n",
 240                 enforce ? "" : "not ");
 241         parse_exclude();
 242         security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
 243         return 0;
 244 }
 245
 246 DEFINE_LSM(loadpin) = {
 247         .name = "loadpin",
 248         .init = loadpin_init,
 249 };
 250
 251 /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
 252 module_param(enforce, int, 0);
 253 MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
 254 module_param_array_named(exclude, exclude_read_files, charp, NULL, 0);
 255 MODULE_PARM_DESC(exclude, "Exclude pinning specific read file types");