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