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 #define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE | \
22 RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT)
23
24 /*
25 *
26 * Restartable sequences are a lightweight interface that allows
27 * user-level code to be executed atomically relative to scheduler
28 * preemption and signal delivery. Typically used for implementing
29 * per-cpu operations.
30 *
31 * It allows user-space to perform update operations on per-cpu data
32 * without requiring heavy-weight atomic operations.
33 *
34 * Detailed algorithm of rseq user-space assembly sequences:
35 *
36 * init(rseq_cs)
37 * cpu = TLS->rseq::cpu_id_start
38 * [1] TLS->rseq::rseq_cs = rseq_cs
39 * [start_ip] ----------------------------
40 * [2] if (cpu != TLS->rseq::cpu_id)
41 * goto abort_ip;
42 * [3] <last_instruction_in_cs>
43 * [post_commit_ip] ----------------------------
44 *
45 * The address of jump target abort_ip must be outside the critical
46 * region, i.e.:
47 *
48 * [abort_ip] < [start_ip] || [abort_ip] >= [post_commit_ip]
49 *
50 * Steps [2]-[3] (inclusive) need to be a sequence of instructions in
51 * userspace that can handle being interrupted between any of those
52 * instructions, and then resumed to the abort_ip.
53 *
54 * 1. Userspace stores the address of the struct rseq_cs assembly
55 * block descriptor into the rseq_cs field of the registered
56 * struct rseq TLS area. This update is performed through a single
57 * store within the inline assembly instruction sequence.
58 * [start_ip]
59 *
60 * 2. Userspace tests to check whether the current cpu_id field match
61 * the cpu number loaded before start_ip, branching to abort_ip
62 * in case of a mismatch.
63 *
64 * If the sequence is preempted or interrupted by a signal
65 * at or after start_ip and before post_commit_ip, then the kernel
66 * clears TLS->__rseq_abi::rseq_cs, and sets the user-space return
67 * ip to abort_ip before returning to user-space, so the preempted
68 * execution resumes at abort_ip.
69 *
70 * 3. Userspace critical section final instruction before
71 * post_commit_ip is the commit. The critical section is
72 * self-terminating.
73 * [post_commit_ip]
74 *
75 * 4. <success>
76 *
77 * On failure at [2], or if interrupted by preempt or signal delivery
78 * between [1] and [3]:
79 *
80 * [abort_ip]
81 * F1. <failure>
82 */
83
rseq_update_cpu_id(struct task_struct * t)84 static int rseq_update_cpu_id(struct task_struct *t)
85 {
86 u32 cpu_id = raw_smp_processor_id();
87
88 if (put_user(cpu_id, &t->rseq->cpu_id_start))
89 return -EFAULT;
90 if (put_user(cpu_id, &t->rseq->cpu_id))
91 return -EFAULT;
92 trace_rseq_update(t);
93 return 0;
94 }
95
rseq_reset_rseq_cpu_id(struct task_struct * t)96 static int rseq_reset_rseq_cpu_id(struct task_struct *t)
97 {
98 u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED;
99
100 /*
101 * Reset cpu_id_start to its initial state (0).
102 */
103 if (put_user(cpu_id_start, &t->rseq->cpu_id_start))
104 return -EFAULT;
105 /*
106 * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
107 * in after unregistration can figure out that rseq needs to be
108 * registered again.
109 */
110 if (put_user(cpu_id, &t->rseq->cpu_id))
111 return -EFAULT;
112 return 0;
113 }
114
rseq_get_rseq_cs(struct task_struct * t,struct rseq_cs * rseq_cs)115 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
116 {
117 struct rseq_cs __user *urseq_cs;
118 u64 ptr;
119 u32 __user *usig;
120 u32 sig;
121 int ret;
122
123 if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
124 return -EFAULT;
125 if (!ptr) {
126 memset(rseq_cs, 0, sizeof(*rseq_cs));
127 return 0;
128 }
129 if (ptr >= TASK_SIZE)
130 return -EINVAL;
131 urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
132 if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
133 return -EFAULT;
134
135 if (rseq_cs->start_ip >= TASK_SIZE ||
136 rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
137 rseq_cs->abort_ip >= TASK_SIZE ||
138 rseq_cs->version > 0)
139 return -EINVAL;
140 /* Check for overflow. */
141 if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
142 return -EINVAL;
143 /* Ensure that abort_ip is not in the critical section. */
144 if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
145 return -EINVAL;
146
147 usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
148 ret = get_user(sig, usig);
149 if (ret)
150 return ret;
151
152 if (current->rseq_sig != sig) {
153 printk_ratelimited(KERN_WARNING
154 "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
155 sig, current->rseq_sig, current->pid, usig);
156 return -EINVAL;
157 }
158 return 0;
159 }
160
rseq_need_restart(struct task_struct * t,u32 cs_flags)161 static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
162 {
163 u32 flags, event_mask;
164 int ret;
165
166 /* Get thread flags. */
167 ret = get_user(flags, &t->rseq->flags);
168 if (ret)
169 return ret;
170
171 /* Take critical section flags into account. */
172 flags |= cs_flags;
173
174 /*
175 * Restart on signal can only be inhibited when restart on
176 * preempt and restart on migrate are inhibited too. Otherwise,
177 * a preempted signal handler could fail to restart the prior
178 * execution context on sigreturn.
179 */
180 if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) &&
181 (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) !=
182 RSEQ_CS_PREEMPT_MIGRATE_FLAGS))
183 return -EINVAL;
184
185 /*
186 * Load and clear event mask atomically with respect to
187 * scheduler preemption.
188 */
189 preempt_disable();
190 event_mask = t->rseq_event_mask;
191 t->rseq_event_mask = 0;
192 preempt_enable();
193
194 return !!(event_mask & ~flags);
195 }
196
clear_rseq_cs(struct task_struct * t)197 static int clear_rseq_cs(struct task_struct *t)
198 {
199 /*
200 * The rseq_cs field is set to NULL on preemption or signal
201 * delivery on top of rseq assembly block, as well as on top
202 * of code outside of the rseq assembly block. This performs
203 * a lazy clear of the rseq_cs field.
204 *
205 * Set rseq_cs to NULL.
206 */
207 if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
208 return -EFAULT;
209 return 0;
210 }
211
212 /*
213 * Unsigned comparison will be true when ip >= start_ip, and when
214 * ip < start_ip + post_commit_offset.
215 */
in_rseq_cs(unsigned long ip,struct rseq_cs * rseq_cs)216 static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
217 {
218 return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
219 }
220
rseq_ip_fixup(struct pt_regs * regs)221 static int rseq_ip_fixup(struct pt_regs *regs)
222 {
223 unsigned long ip = instruction_pointer(regs);
224 struct task_struct *t = current;
225 struct rseq_cs rseq_cs;
226 int ret;
227
228 ret = rseq_get_rseq_cs(t, &rseq_cs);
229 if (ret)
230 return ret;
231
232 /*
233 * Handle potentially not being within a critical section.
234 * If not nested over a rseq critical section, restart is useless.
235 * Clear the rseq_cs pointer and return.
236 */
237 if (!in_rseq_cs(ip, &rseq_cs))
238 return clear_rseq_cs(t);
239 ret = rseq_need_restart(t, rseq_cs.flags);
240 if (ret <= 0)
241 return ret;
242 ret = clear_rseq_cs(t);
243 if (ret)
244 return ret;
245 trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
246 rseq_cs.abort_ip);
247 instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
248 return 0;
249 }
250
251 /*
252 * This resume handler must always be executed between any of:
253 * - preemption,
254 * - signal delivery,
255 * and return to user-space.
256 *
257 * This is how we can ensure that the entire rseq critical section
258 * will issue the commit instruction only if executed atomically with
259 * respect to other threads scheduled on the same CPU, and with respect
260 * to signal handlers.
261 */
__rseq_handle_notify_resume(struct ksignal * ksig,struct pt_regs * regs)262 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
263 {
264 struct task_struct *t = current;
265 int ret, sig;
266
267 if (unlikely(t->flags & PF_EXITING))
268 return;
269 if (unlikely(!access_ok(t->rseq, sizeof(*t->rseq))))
270 goto error;
271 ret = rseq_ip_fixup(regs);
272 if (unlikely(ret < 0))
273 goto error;
274 if (unlikely(rseq_update_cpu_id(t)))
275 goto error;
276 return;
277
278 error:
279 sig = ksig ? ksig->sig : 0;
280 force_sigsegv(sig);
281 }
282
283 #ifdef CONFIG_DEBUG_RSEQ
284
285 /*
286 * Terminate the process if a syscall is issued within a restartable
287 * sequence.
288 */
rseq_syscall(struct pt_regs * regs)289 void rseq_syscall(struct pt_regs *regs)
290 {
291 unsigned long ip = instruction_pointer(regs);
292 struct task_struct *t = current;
293 struct rseq_cs rseq_cs;
294
295 if (!t->rseq)
296 return;
297 if (!access_ok(t->rseq, sizeof(*t->rseq)) ||
298 rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
299 force_sig(SIGSEGV);
300 }
301
302 #endif
303
304 /*
305 * sys_rseq - setup restartable sequences for caller thread.
306 */
SYSCALL_DEFINE4(rseq,struct rseq __user *,rseq,u32,rseq_len,int,flags,u32,sig)307 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
308 int, flags, u32, sig)
309 {
310 int ret;
311
312 if (flags & RSEQ_FLAG_UNREGISTER) {
313 if (flags & ~RSEQ_FLAG_UNREGISTER)
314 return -EINVAL;
315 /* Unregister rseq for current thread. */
316 if (current->rseq != rseq || !current->rseq)
317 return -EINVAL;
318 if (rseq_len != sizeof(*rseq))
319 return -EINVAL;
320 if (current->rseq_sig != sig)
321 return -EPERM;
322 ret = rseq_reset_rseq_cpu_id(current);
323 if (ret)
324 return ret;
325 current->rseq = NULL;
326 current->rseq_sig = 0;
327 return 0;
328 }
329
330 if (unlikely(flags))
331 return -EINVAL;
332
333 if (current->rseq) {
334 /*
335 * If rseq is already registered, check whether
336 * the provided address differs from the prior
337 * one.
338 */
339 if (current->rseq != rseq || rseq_len != sizeof(*rseq))
340 return -EINVAL;
341 if (current->rseq_sig != sig)
342 return -EPERM;
343 /* Already registered. */
344 return -EBUSY;
345 }
346
347 /*
348 * If there was no rseq previously registered,
349 * ensure the provided rseq is properly aligned and valid.
350 */
351 if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
352 rseq_len != sizeof(*rseq))
353 return -EINVAL;
354 if (!access_ok(rseq, rseq_len))
355 return -EFAULT;
356 current->rseq = rseq;
357 current->rseq_sig = sig;
358 /*
359 * If rseq was previously inactive, and has just been
360 * registered, ensure the cpu_id_start and cpu_id fields
361 * are updated before returning to user-space.
362 */
363 rseq_set_notify_resume(current);
364
365 return 0;
366 }
367
