arch/x86/kernel/msr.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /* ----------------------------------------------------------------------- *
   3  *
   4  *   Copyright 2000-2008 H. Peter Anvin - All Rights Reserved
   5  *   Copyright 2009 Intel Corporation; author: H. Peter Anvin
   6  *
   7  * ----------------------------------------------------------------------- */
   8
   9 /*
  10  * x86 MSR access device
  11  *
  12  * This device is accessed by lseek() to the appropriate register number
  13  * and then read/write in chunks of 8 bytes.  A larger size means multiple
  14  * reads or writes of the same register.
  15  *
  16  * This driver uses /dev/cpu/%d/msr where %d is the minor number, and on
  17  * an SMP box will direct the access to CPU %d.
  18  */
  19
  20 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  21
  22 #include <linux/module.h>
  23
  24 #include <linux/types.h>
  25 #include <linux/errno.h>
  26 #include <linux/fcntl.h>
  27 #include <linux/init.h>
  28 #include <linux/poll.h>
  29 #include <linux/smp.h>
  30 #include <linux/major.h>
  31 #include <linux/fs.h>
  32 #include <linux/device.h>
  33 #include <linux/cpu.h>
  34 #include <linux/notifier.h>
  35 #include <linux/uaccess.h>
  36 #include <linux/gfp.h>
  37 #include <linux/security.h>
  38
  39 #include <asm/cpufeature.h>
  40 #include <asm/msr.h>
  41
  42 static enum cpuhp_state cpuhp_msr_state;
  43
  44 enum allow_write_msrs {
  45         MSR_WRITES_ON,
  46         MSR_WRITES_OFF,
  47         MSR_WRITES_DEFAULT,
  48 };
  49
  50 static enum allow_write_msrs allow_writes = MSR_WRITES_DEFAULT;
  51
  52 static ssize_t msr_read(struct file *file, char __user *buf,
  53                         size_t count, loff_t *ppos)
  54 {
  55         u32 __user *tmp = (u32 __user *) buf;
  56         u32 data[2];
  57         u32 reg = *ppos;
  58         int cpu = iminor(file_inode(file));
  59         int err = 0;
  60         ssize_t bytes = 0;
  61
  62         if (count % 8)
  63                 return -EINVAL; /* Invalid chunk size */
  64
  65         for (; count; count -= 8) {
  66                 err = rdmsr_safe_on_cpu(cpu, reg, &data[0], &data[1]);
  67                 if (err)
  68                         break;
  69                 if (copy_to_user(tmp, &data, 8)) {
  70                         err = -EFAULT;
  71                         break;
  72                 }
  73                 tmp += 2;
  74                 bytes += 8;
  75         }
  76
  77         return bytes ? bytes : err;
  78 }
  79
  80 static int filter_write(u32 reg)
  81 {
  82         /*
  83          * MSRs writes usually happen all at once, and can easily saturate kmsg.
  84          * Only allow one message every 30 seconds.
  85          *
  86          * It's possible to be smarter here and do it (for example) per-MSR, but
  87          * it would certainly be more complex, and this is enough at least to
  88          * avoid saturating the ring buffer.
  89          */
  90         static DEFINE_RATELIMIT_STATE(fw_rs, 30 * HZ, 1);
  91
  92         switch (allow_writes) {
  93         case MSR_WRITES_ON:  return 0;
  94         case MSR_WRITES_OFF: return -EPERM;
  95         default: break;
  96         }
  97
  98         if (!__ratelimit(&fw_rs))
  99                 return 0;
 100
 101         pr_warn("Write to unrecognized MSR 0x%x by %s (pid: %d).\n",
 102                 reg, current->comm, current->pid);
 103         pr_warn("See https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/about for details.\n");
 104
 105         return 0;
 106 }
 107
 108 static ssize_t msr_write(struct file *file, const char __user *buf,
 109                          size_t count, loff_t *ppos)
 110 {
 111         const u32 __user *tmp = (const u32 __user *)buf;
 112         u32 data[2];
 113         u32 reg = *ppos;
 114         int cpu = iminor(file_inode(file));
 115         int err = 0;
 116         ssize_t bytes = 0;
 117
 118         err = security_locked_down(LOCKDOWN_MSR);
 119         if (err)
 120                 return err;
 121
 122         err = filter_write(reg);
 123         if (err)
 124                 return err;
 125
 126         if (count % 8)
 127                 return -EINVAL; /* Invalid chunk size */
 128
 129         for (; count; count -= 8) {
 130                 if (copy_from_user(&data, tmp, 8)) {
 131                         err = -EFAULT;
 132                         break;
 133                 }
 134
 135                 add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
 136
 137                 err = wrmsr_safe_on_cpu(cpu, reg, data[0], data[1]);
 138                 if (err)
 139                         break;
 140
 141                 tmp += 2;
 142                 bytes += 8;
 143         }
 144
 145         return bytes ? bytes : err;
 146 }
 147
 148 static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
 149 {
 150         u32 __user *uregs = (u32 __user *)arg;
 151         u32 regs[8];
 152         int cpu = iminor(file_inode(file));
 153         int err;
 154
 155         switch (ioc) {
 156         case X86_IOC_RDMSR_REGS:
 157                 if (!(file->f_mode & FMODE_READ)) {
 158                         err = -EBADF;
 159                         break;
 160                 }
 161                 if (copy_from_user(&regs, uregs, sizeof(regs))) {
 162                         err = -EFAULT;
 163                         break;
 164                 }
 165                 err = rdmsr_safe_regs_on_cpu(cpu, regs);
 166                 if (err)
 167                         break;
 168                 if (copy_to_user(uregs, &regs, sizeof(regs)))
 169                         err = -EFAULT;
 170                 break;
 171
 172         case X86_IOC_WRMSR_REGS:
 173                 if (!(file->f_mode & FMODE_WRITE)) {
 174                         err = -EBADF;
 175                         break;
 176                 }
 177                 if (copy_from_user(&regs, uregs, sizeof(regs))) {
 178                         err = -EFAULT;
 179                         break;
 180                 }
 181                 err = security_locked_down(LOCKDOWN_MSR);
 182                 if (err)
 183                         break;
 184
 185                 err = filter_write(regs[1]);
 186                 if (err)
 187                         return err;
 188
 189                 add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
 190
 191                 err = wrmsr_safe_regs_on_cpu(cpu, regs);
 192                 if (err)
 193                         break;
 194                 if (copy_to_user(uregs, &regs, sizeof(regs)))
 195                         err = -EFAULT;
 196                 break;
 197
 198         default:
 199                 err = -ENOTTY;
 200                 break;
 201         }
 202
 203         return err;
 204 }
 205
 206 static int msr_open(struct inode *inode, struct file *file)
 207 {
 208         unsigned int cpu = iminor(file_inode(file));
 209         struct cpuinfo_x86 *c;
 210
 211         if (!capable(CAP_SYS_RAWIO))
 212                 return -EPERM;
 213
 214         if (cpu >= nr_cpu_ids || !cpu_online(cpu))
 215                 return -ENXIO;  /* No such CPU */
 216
 217         c = &cpu_data(cpu);
 218         if (!cpu_has(c, X86_FEATURE_MSR))
 219                 return -EIO;    /* MSR not supported */
 220
 221         return 0;
 222 }
 223
 224 /*
 225  * File operations we support
 226  */
 227 static const struct file_operations msr_fops = {
 228         .owner = THIS_MODULE,
 229         .llseek = no_seek_end_llseek,
 230         .read = msr_read,
 231         .write = msr_write,
 232         .open = msr_open,
 233         .unlocked_ioctl = msr_ioctl,
 234         .compat_ioctl = msr_ioctl,
 235 };
 236
 237 static char *msr_devnode(const struct device *dev, umode_t *mode)
 238 {
 239         return kasprintf(GFP_KERNEL, "cpu/%u/msr", MINOR(dev->devt));
 240 }
 241
 242 static const struct class msr_class = {
 243         .name           = "msr",
 244         .devnode        = msr_devnode,
 245 };
 246
 247 static int msr_device_create(unsigned int cpu)
 248 {
 249         struct device *dev;
 250
 251         dev = device_create(&msr_class, NULL, MKDEV(MSR_MAJOR, cpu), NULL,
 252                             "msr%d", cpu);
 253         return PTR_ERR_OR_ZERO(dev);
 254 }
 255
 256 static int msr_device_destroy(unsigned int cpu)
 257 {
 258         device_destroy(&msr_class, MKDEV(MSR_MAJOR, cpu));
 259         return 0;
 260 }
 261
 262 static int __init msr_init(void)
 263 {
 264         int err;
 265
 266         if (__register_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr", &msr_fops)) {
 267                 pr_err("unable to get major %d for msr\n", MSR_MAJOR);
 268                 return -EBUSY;
 269         }
 270         err = class_register(&msr_class);
 271         if (err)
 272                 goto out_chrdev;
 273
 274         err  = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "x86/msr:online",
 275                                  msr_device_create, msr_device_destroy);
 276         if (err < 0)
 277                 goto out_class;
 278         cpuhp_msr_state = err;
 279         return 0;
 280
 281 out_class:
 282         class_unregister(&msr_class);
 283 out_chrdev:
 284         __unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
 285         return err;
 286 }
 287 module_init(msr_init);
 288
 289 static void __exit msr_exit(void)
 290 {
 291         cpuhp_remove_state(cpuhp_msr_state);
 292         class_unregister(&msr_class);
 293         __unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
 294 }
 295 module_exit(msr_exit)
 296
 297 static int set_allow_writes(const char *val, const struct kernel_param *cp)
 298 {
 299         /* val is NUL-terminated, see kernfs_fop_write() */
 300         char *s = strstrip((char *)val);
 301
 302         if (!strcmp(s, "on"))
 303                 allow_writes = MSR_WRITES_ON;
 304         else if (!strcmp(s, "off"))
 305                 allow_writes = MSR_WRITES_OFF;
 306         else
 307                 allow_writes = MSR_WRITES_DEFAULT;
 308
 309         return 0;
 310 }
 311
 312 static int get_allow_writes(char *buf, const struct kernel_param *kp)
 313 {
 314         const char *res;
 315
 316         switch (allow_writes) {
 317         case MSR_WRITES_ON:  res = "on"; break;
 318         case MSR_WRITES_OFF: res = "off"; break;
 319         default: res = "default"; break;
 320         }
 321
 322         return sprintf(buf, "%s\n", res);
 323 }
 324
 325 static const struct kernel_param_ops allow_writes_ops = {
 326         .set = set_allow_writes,
 327         .get = get_allow_writes
 328 };
 329
 330 module_param_cb(allow_writes, &allow_writes_ops, NULL, 0600);
 331
 332 MODULE_AUTHOR("H. Peter Anvin <hpa@zytor.com>");
 333 MODULE_DESCRIPTION("x86 generic MSR driver");
 334 MODULE_LICENSE("GPL");