1 // SPDX-License-Identifier: GPL-2.0+
2 /*
3 * Restartable sequences system call
4 *
5 * Copyright (C) 2015, Google, Inc.,
6 * Paul Turner <pjt@google.com> and Andrew Hunter <ahh@google.com>
7 * Copyright (C) 2015-2018, EfficiOS Inc.,
8 * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
9 */
10
11 #include <linux/sched.h>
12 #include <linux/uaccess.h>
13 #include <linux/syscalls.h>
14 #include <linux/rseq.h>
15 #include <linux/types.h>
16 #include <asm/ptrace.h>
17
18 #define CREATE_TRACE_POINTS
19 #include <trace/events/rseq.h>
20
21 /* The original rseq structure size (including padding) is 32 bytes. */
22 #define ORIG_RSEQ_SIZE 32
23
24 #define RSEQ_CS_NO_RESTART_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT | \
25 RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
26 RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)
27
28 /*
29 *
30 * Restartable sequences are a lightweight interface that allows
31 * user-level code to be executed atomically relative to scheduler
32 * preemption and signal delivery. Typically used for implementing
33 * per-cpu operations.
34 *
35 * It allows user-space to perform update operations on per-cpu data
36 * without requiring heavy-weight atomic operations.
37 *
38 * Detailed algorithm of rseq user-space assembly sequences:
39 *
40 * init(rseq_cs)
41 * cpu = TLS->rseq::cpu_id_start
42 * [1] TLS->rseq::rseq_cs = rseq_cs
43 * [start_ip] ----------------------------
44 * [2] if (cpu != TLS->rseq::cpu_id)
45 * goto abort_ip;
46 * [3] <last_instruction_in_cs>
47 * [post_commit_ip] ----------------------------
48 *
49 * The address of jump target abort_ip must be outside the critical
50 * region, i.e.:
51 *
52 * [abort_ip] < [start_ip] || [abort_ip] >= [post_commit_ip]
53 *
54 * Steps [2]-[3] (inclusive) need to be a sequence of instructions in
55 * userspace that can handle being interrupted between any of those
56 * instructions, and then resumed to the abort_ip.
57 *
58 * 1. Userspace stores the address of the struct rseq_cs assembly
59 * block descriptor into the rseq_cs field of the registered
60 * struct rseq TLS area. This update is performed through a single
61 * store within the inline assembly instruction sequence.
62 * [start_ip]
63 *
64 * 2. Userspace tests to check whether the current cpu_id field match
65 * the cpu number loaded before start_ip, branching to abort_ip
66 * in case of a mismatch.
67 *
68 * If the sequence is preempted or interrupted by a signal
69 * at or after start_ip and before post_commit_ip, then the kernel
70 * clears TLS->__rseq_abi::rseq_cs, and sets the user-space return
71 * ip to abort_ip before returning to user-space, so the preempted
72 * execution resumes at abort_ip.
73 *
74 * 3. Userspace critical section final instruction before
75 * post_commit_ip is the commit. The critical section is
76 * self-terminating.
77 * [post_commit_ip]
78 *
79 * 4. <success>
80 *
81 * On failure at [2], or if interrupted by preempt or signal delivery
82 * between [1] and [3]:
83 *
84 * [abort_ip]
85 * F1. <failure>
86 */
87
rseq_update_cpu_node_id(struct task_struct * t)88 static int rseq_update_cpu_node_id(struct task_struct *t)
89 {
90 struct rseq __user *rseq = t->rseq;
91 u32 cpu_id = raw_smp_processor_id();
92 u32 node_id = cpu_to_node(cpu_id);
93 u32 mm_cid = task_mm_cid(t);
94
95 WARN_ON_ONCE((int) mm_cid < 0);
96 if (!user_write_access_begin(rseq, t->rseq_len))
97 goto efault;
98 unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
99 unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
100 unsafe_put_user(node_id, &rseq->node_id, efault_end);
101 unsafe_put_user(mm_cid, &rseq->mm_cid, efault_end);
102 /*
103 * Additional feature fields added after ORIG_RSEQ_SIZE
104 * need to be conditionally updated only if
105 * t->rseq_len != ORIG_RSEQ_SIZE.
106 */
107 user_write_access_end();
108 trace_rseq_update(t);
109 return 0;
110
111 efault_end:
112 user_write_access_end();
113 efault:
114 return -EFAULT;
115 }
116
rseq_reset_rseq_cpu_node_id(struct task_struct * t)117 static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
118 {
119 u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED, node_id = 0,
120 mm_cid = 0;
121
122 /*
123 * Reset cpu_id_start to its initial state (0).
124 */
125 if (put_user(cpu_id_start, &t->rseq->cpu_id_start))
126 return -EFAULT;
127 /*
128 * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
129 * in after unregistration can figure out that rseq needs to be
130 * registered again.
131 */
132 if (put_user(cpu_id, &t->rseq->cpu_id))
133 return -EFAULT;
134 /*
135 * Reset node_id to its initial state (0).
136 */
137 if (put_user(node_id, &t->rseq->node_id))
138 return -EFAULT;
139 /*
140 * Reset mm_cid to its initial state (0).
141 */
142 if (put_user(mm_cid, &t->rseq->mm_cid))
143 return -EFAULT;
144 /*
145 * Additional feature fields added after ORIG_RSEQ_SIZE
146 * need to be conditionally reset only if
147 * t->rseq_len != ORIG_RSEQ_SIZE.
148 */
149 return 0;
150 }
151
rseq_get_rseq_cs(struct task_struct * t,struct rseq_cs * rseq_cs)152 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
153 {
154 struct rseq_cs __user *urseq_cs;
155 u64 ptr;
156 u32 __user *usig;
157 u32 sig;
158 int ret;
159
160 #ifdef CONFIG_64BIT
161 if (get_user(ptr, &t->rseq->rseq_cs))
162 return -EFAULT;
163 #else
164 if (copy_from_user(&ptr, &t->rseq->rseq_cs, sizeof(ptr)))
165 return -EFAULT;
166 #endif
167 if (!ptr) {
168 memset(rseq_cs, 0, sizeof(*rseq_cs));
169 return 0;
170 }
171 if (ptr >= TASK_SIZE)
172 return -EINVAL;
173 urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
174 if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
175 return -EFAULT;
176
177 if (rseq_cs->start_ip >= TASK_SIZE ||
178 rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
179 rseq_cs->abort_ip >= TASK_SIZE ||
180 rseq_cs->version > 0)
181 return -EINVAL;
182 /* Check for overflow. */
183 if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
184 return -EINVAL;
185 /* Ensure that abort_ip is not in the critical section. */
186 if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
187 return -EINVAL;
188
189 usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
190 ret = get_user(sig, usig);
191 if (ret)
192 return ret;
193
194 if (current->rseq_sig != sig) {
195 printk_ratelimited(KERN_WARNING
196 "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
197 sig, current->rseq_sig, current->pid, usig);
198 return -EINVAL;
199 }
200 return 0;
201 }
202
rseq_warn_flags(const char * str,u32 flags)203 static bool rseq_warn_flags(const char *str, u32 flags)
204 {
205 u32 test_flags;
206
207 if (!flags)
208 return false;
209 test_flags = flags & RSEQ_CS_NO_RESTART_FLAGS;
210 if (test_flags)
211 pr_warn_once("Deprecated flags (%u) in %s ABI structure", test_flags, str);
212 test_flags = flags & ~RSEQ_CS_NO_RESTART_FLAGS;
213 if (test_flags)
214 pr_warn_once("Unknown flags (%u) in %s ABI structure", test_flags, str);
215 return true;
216 }
217
rseq_need_restart(struct task_struct * t,u32 cs_flags)218 static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
219 {
220 u32 flags, event_mask;
221 int ret;
222
223 if (rseq_warn_flags("rseq_cs", cs_flags))
224 return -EINVAL;
225
226 /* Get thread flags. */
227 ret = get_user(flags, &t->rseq->flags);
228 if (ret)
229 return ret;
230
231 if (rseq_warn_flags("rseq", flags))
232 return -EINVAL;
233
234 /*
235 * Load and clear event mask atomically with respect to
236 * scheduler preemption.
237 */
238 preempt_disable();
239 event_mask = t->rseq_event_mask;
240 t->rseq_event_mask = 0;
241 preempt_enable();
242
243 return !!event_mask;
244 }
245
clear_rseq_cs(struct task_struct * t)246 static int clear_rseq_cs(struct task_struct *t)
247 {
248 /*
249 * The rseq_cs field is set to NULL on preemption or signal
250 * delivery on top of rseq assembly block, as well as on top
251 * of code outside of the rseq assembly block. This performs
252 * a lazy clear of the rseq_cs field.
253 *
254 * Set rseq_cs to NULL.
255 */
256 #ifdef CONFIG_64BIT
257 return put_user(0UL, &t->rseq->rseq_cs);
258 #else
259 if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
260 return -EFAULT;
261 return 0;
262 #endif
263 }
264
265 /*
266 * Unsigned comparison will be true when ip >= start_ip, and when
267 * ip < start_ip + post_commit_offset.
268 */
in_rseq_cs(unsigned long ip,struct rseq_cs * rseq_cs)269 static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
270 {
271 return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
272 }
273
rseq_ip_fixup(struct pt_regs * regs)274 static int rseq_ip_fixup(struct pt_regs *regs)
275 {
276 unsigned long ip = instruction_pointer(regs);
277 struct task_struct *t = current;
278 struct rseq_cs rseq_cs;
279 int ret;
280
281 ret = rseq_get_rseq_cs(t, &rseq_cs);
282 if (ret)
283 return ret;
284
285 /*
286 * Handle potentially not being within a critical section.
287 * If not nested over a rseq critical section, restart is useless.
288 * Clear the rseq_cs pointer and return.
289 */
290 if (!in_rseq_cs(ip, &rseq_cs))
291 return clear_rseq_cs(t);
292 ret = rseq_need_restart(t, rseq_cs.flags);
293 if (ret <= 0)
294 return ret;
295 ret = clear_rseq_cs(t);
296 if (ret)
297 return ret;
298 trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
299 rseq_cs.abort_ip);
300 instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
301 return 0;
302 }
303
304 /*
305 * This resume handler must always be executed between any of:
306 * - preemption,
307 * - signal delivery,
308 * and return to user-space.
309 *
310 * This is how we can ensure that the entire rseq critical section
311 * will issue the commit instruction only if executed atomically with
312 * respect to other threads scheduled on the same CPU, and with respect
313 * to signal handlers.
314 */
__rseq_handle_notify_resume(struct ksignal * ksig,struct pt_regs * regs)315 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
316 {
317 struct task_struct *t = current;
318 int ret, sig;
319
320 if (unlikely(t->flags & PF_EXITING))
321 return;
322
323 /*
324 * regs is NULL if and only if the caller is in a syscall path. Skip
325 * fixup and leave rseq_cs as is so that rseq_sycall() will detect and
326 * kill a misbehaving userspace on debug kernels.
327 */
328 if (regs) {
329 ret = rseq_ip_fixup(regs);
330 if (unlikely(ret < 0))
331 goto error;
332 }
333 if (unlikely(rseq_update_cpu_node_id(t)))
334 goto error;
335 return;
336
337 error:
338 sig = ksig ? ksig->sig : 0;
339 force_sigsegv(sig);
340 }
341
342 #ifdef CONFIG_DEBUG_RSEQ
343
344 /*
345 * Terminate the process if a syscall is issued within a restartable
346 * sequence.
347 */
rseq_syscall(struct pt_regs * regs)348 void rseq_syscall(struct pt_regs *regs)
349 {
350 unsigned long ip = instruction_pointer(regs);
351 struct task_struct *t = current;
352 struct rseq_cs rseq_cs;
353
354 if (!t->rseq)
355 return;
356 if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
357 force_sig(SIGSEGV);
358 }
359
360 #endif
361
362 /*
363 * sys_rseq - setup restartable sequences for caller thread.
364 */
SYSCALL_DEFINE4(rseq,struct rseq __user *,rseq,u32,rseq_len,int,flags,u32,sig)365 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
366 int, flags, u32, sig)
367 {
368 int ret;
369
370 if (flags & RSEQ_FLAG_UNREGISTER) {
371 if (flags & ~RSEQ_FLAG_UNREGISTER)
372 return -EINVAL;
373 /* Unregister rseq for current thread. */
374 if (current->rseq != rseq || !current->rseq)
375 return -EINVAL;
376 if (rseq_len != current->rseq_len)
377 return -EINVAL;
378 if (current->rseq_sig != sig)
379 return -EPERM;
380 ret = rseq_reset_rseq_cpu_node_id(current);
381 if (ret)
382 return ret;
383 current->rseq = NULL;
384 current->rseq_sig = 0;
385 current->rseq_len = 0;
386 return 0;
387 }
388
389 if (unlikely(flags))
390 return -EINVAL;
391
392 if (current->rseq) {
393 /*
394 * If rseq is already registered, check whether
395 * the provided address differs from the prior
396 * one.
397 */
398 if (current->rseq != rseq || rseq_len != current->rseq_len)
399 return -EINVAL;
400 if (current->rseq_sig != sig)
401 return -EPERM;
402 /* Already registered. */
403 return -EBUSY;
404 }
405
406 /*
407 * If there was no rseq previously registered, ensure the provided rseq
408 * is properly aligned, as communcated to user-space through the ELF
409 * auxiliary vector AT_RSEQ_ALIGN. If rseq_len is the original rseq
410 * size, the required alignment is the original struct rseq alignment.
411 *
412 * In order to be valid, rseq_len is either the original rseq size, or
413 * large enough to contain all supported fields, as communicated to
414 * user-space through the ELF auxiliary vector AT_RSEQ_FEATURE_SIZE.
415 */
416 if (rseq_len < ORIG_RSEQ_SIZE ||
417 (rseq_len == ORIG_RSEQ_SIZE && !IS_ALIGNED((unsigned long)rseq, ORIG_RSEQ_SIZE)) ||
418 (rseq_len != ORIG_RSEQ_SIZE && (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
419 rseq_len < offsetof(struct rseq, end))))
420 return -EINVAL;
421 if (!access_ok(rseq, rseq_len))
422 return -EFAULT;
423 current->rseq = rseq;
424 current->rseq_len = rseq_len;
425 current->rseq_sig = sig;
426 /*
427 * If rseq was previously inactive, and has just been
428 * registered, ensure the cpu_id_start and cpu_id fields
429 * are updated before returning to user-space.
430 */
431 rseq_set_notify_resume(current);
432
433 return 0;
434 }
435