security/loadpin/loadpin.c

   1 /*
   2  * Module and Firmware Pinning Security Module
   3  *
   4  * Copyright 2011-2016 Google Inc.
   5  *
   6  * Author: Kees Cook <keescook@chromium.org>
   7  *
   8  * This software is licensed under the terms of the GNU General Public
   9  * License version 2, as published by the Free Software Foundation, and
  10  * may be copied, distributed, and modified under those terms.
  11  *
  12  * This program is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15  * GNU General Public License for more details.
  16  */
  17
  18 #define pr_fmt(fmt) "LoadPin: " fmt
  19
  20 #include <linux/module.h>
  21 #include <linux/fs.h>
  22 #include <linux/fs_struct.h>
  23 #include <linux/lsm_hooks.h>
  24 #include <linux/mount.h>
  25 #include <linux/path.h>
  26 #include <linux/sched.h>        /* current */
  27 #include <linux/string_helpers.h>
  28
  29 static void report_load(const char *origin, struct file *file, char *operation)
  30 {
  31         char *cmdline, *pathname;
  32
  33         pathname = kstrdup_quotable_file(file, GFP_KERNEL);
  34         cmdline = kstrdup_quotable_cmdline(current, GFP_KERNEL);
  35
  36         pr_notice("%s %s obj=%s%s%s pid=%d cmdline=%s%s%s\n",
  37                   origin, operation,
  38                   (pathname && pathname[0] != '<') ? "\"" : "",
  39                   pathname,
  40                   (pathname && pathname[0] != '<') ? "\"" : "",
  41                   task_pid_nr(current),
  42                   cmdline ? "\"" : "", cmdline, cmdline ? "\"" : "");
  43
  44         kfree(cmdline);
  45         kfree(pathname);
  46 }
  47
  48 static int enabled = IS_ENABLED(CONFIG_SECURITY_LOADPIN_ENABLED);
  49 static struct super_block *pinned_root;
  50 static DEFINE_SPINLOCK(pinned_root_spinlock);
  51
  52 #ifdef CONFIG_SYSCTL
  53 static int zero;
  54 static int one = 1;
  55
  56 static struct ctl_path loadpin_sysctl_path[] = {
  57         { .procname = "kernel", },
  58         { .procname = "loadpin", },
  59         { }
  60 };
  61
  62 static struct ctl_table loadpin_sysctl_table[] = {
  63         {
  64                 .procname       = "enabled",
  65                 .data           = &enabled,
  66                 .maxlen         = sizeof(int),
  67                 .mode           = 0644,
  68                 .proc_handler   = proc_dointvec_minmax,
  69                 .extra1         = &zero,
  70                 .extra2         = &one,
  71         },
  72         { }
  73 };
  74
  75 /*
  76  * This must be called after early kernel init, since then the rootdev
  77  * is available.
  78  */
  79 static void check_pinning_enforcement(struct super_block *mnt_sb)
  80 {
  81         bool ro = false;
  82
  83         /*
  84          * If load pinning is not enforced via a read-only block
  85          * device, allow sysctl to change modes for testing.
  86          */
  87         if (mnt_sb->s_bdev) {
  88                 ro = bdev_read_only(mnt_sb->s_bdev);
  89                 pr_info("dev(%u,%u): %s\n",
  90                         MAJOR(mnt_sb->s_bdev->bd_dev),
  91                         MINOR(mnt_sb->s_bdev->bd_dev),
  92                         ro ? "read-only" : "writable");
  93         } else
  94                 pr_info("mnt_sb lacks block device, treating as: writable\n");
  95
  96         if (!ro) {
  97                 if (!register_sysctl_paths(loadpin_sysctl_path,
  98                                            loadpin_sysctl_table))
  99                         pr_notice("sysctl registration failed!\n");
 100                 else
 101                         pr_info("load pinning can be disabled.\n");
 102         } else
 103                 pr_info("load pinning engaged.\n");
 104 }
 105 #else
 106 static void check_pinning_enforcement(struct super_block *mnt_sb)
 107 {
 108         pr_info("load pinning engaged.\n");
 109 }
 110 #endif
 111
 112 static void loadpin_sb_free_security(struct super_block *mnt_sb)
 113 {
 114         /*
 115          * When unmounting the filesystem we were using for load
 116          * pinning, we acknowledge the superblock release, but make sure
 117          * no other modules or firmware can be loaded.
 118          */
 119         if (!IS_ERR_OR_NULL(pinned_root) && mnt_sb == pinned_root) {
 120                 pinned_root = ERR_PTR(-EIO);
 121                 pr_info("umount pinned fs: refusing further loads\n");
 122         }
 123 }
 124
 125 static int loadpin_read_file(struct file *file, enum kernel_read_file_id id)
 126 {
 127         struct super_block *load_root;
 128         const char *origin = kernel_read_file_id_str(id);
 129
 130         /* This handles the older init_module API that has a NULL file. */
 131         if (!file) {
 132                 if (!enabled) {
 133                         report_load(origin, NULL, "old-api-pinning-ignored");
 134                         return 0;
 135                 }
 136
 137                 report_load(origin, NULL, "old-api-denied");
 138                 return -EPERM;
 139         }
 140
 141         load_root = file->f_path.mnt->mnt_sb;
 142
 143         /* First loaded module/firmware defines the root for all others. */
 144         spin_lock(&pinned_root_spinlock);
 145         /*
 146          * pinned_root is only NULL at startup. Otherwise, it is either
 147          * a valid reference, or an ERR_PTR.
 148          */
 149         if (!pinned_root) {
 150                 pinned_root = load_root;
 151                 /*
 152                  * Unlock now since it's only pinned_root we care about.
 153                  * In the worst case, we will (correctly) report pinning
 154                  * failures before we have announced that pinning is
 155                  * enabled. This would be purely cosmetic.
 156                  */
 157                 spin_unlock(&pinned_root_spinlock);
 158                 check_pinning_enforcement(pinned_root);
 159                 report_load(origin, file, "pinned");
 160         } else {
 161                 spin_unlock(&pinned_root_spinlock);
 162         }
 163
 164         if (IS_ERR_OR_NULL(pinned_root) || load_root != pinned_root) {
 165                 if (unlikely(!enabled)) {
 166                         report_load(origin, file, "pinning-ignored");
 167                         return 0;
 168                 }
 169
 170                 report_load(origin, file, "denied");
 171                 return -EPERM;
 172         }
 173
 174         return 0;
 175 }
 176
 177 static struct security_hook_list loadpin_hooks[] = {
 178         LSM_HOOK_INIT(sb_free_security, loadpin_sb_free_security),
 179         LSM_HOOK_INIT(kernel_read_file, loadpin_read_file),
 180 };
 181
 182 void __init loadpin_add_hooks(void)
 183 {
 184         pr_info("ready to pin (currently %sabled)", enabled ? "en" : "dis");
 185         security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
 186 }
 187
 188 /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
 189 module_param(enabled, int, 0);
 190 MODULE_PARM_DESC(enabled, "Pin module/firmware loading (default: true)");