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
msr_read(struct file * file,char __user * buf,size_t count,loff_t * ppos)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
filter_write(u32 reg)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
msr_write(struct file * file,const char __user * buf,size_t count,loff_t * ppos)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
msr_ioctl(struct file * file,unsigned int ioc,unsigned long arg)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(®s, 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, ®s, 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(®s, 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, ®s, sizeof(regs)))
196 err = -EFAULT;
197 break;
198
199 default:
200 err = -ENOTTY;
201 break;
202 }
203
204 return err;
205 }
206
msr_open(struct inode * inode,struct file * file)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
msr_device_create(unsigned int cpu)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
msr_device_destroy(unsigned int cpu)247 static int msr_device_destroy(unsigned int cpu)
248 {
249 device_destroy(msr_class, MKDEV(MSR_MAJOR, cpu));
250 return 0;
251 }
252
msr_devnode(struct device * dev,umode_t * mode)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
msr_init(void)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
msr_exit(void)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 }
module_exit(msr_exit)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
get_allow_writes(char * buf,const struct kernel_param * kp)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");
334