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