]>
Commit | Line | Data |
---|---|---|
d7822b1e MD |
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 | ||
84 | static int rseq_update_cpu_id(struct task_struct *t) | |
85 | { | |
86 | u32 cpu_id = raw_smp_processor_id(); | |
87 | ||
8f281770 | 88 | if (put_user(cpu_id, &t->rseq->cpu_id_start)) |
d7822b1e | 89 | return -EFAULT; |
8f281770 | 90 | if (put_user(cpu_id, &t->rseq->cpu_id)) |
d7822b1e MD |
91 | return -EFAULT; |
92 | trace_rseq_update(t); | |
93 | return 0; | |
94 | } | |
95 | ||
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 | */ | |
8f281770 | 103 | if (put_user(cpu_id_start, &t->rseq->cpu_id_start)) |
d7822b1e MD |
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 | */ | |
8f281770 | 110 | if (put_user(cpu_id, &t->rseq->cpu_id)) |
d7822b1e MD |
111 | return -EFAULT; |
112 | return 0; | |
113 | } | |
114 | ||
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 | unsigned long ptr; | |
119 | u32 __user *usig; | |
120 | u32 sig; | |
121 | int ret; | |
122 | ||
8f281770 | 123 | ret = get_user(ptr, &t->rseq->rseq_cs); |
d7822b1e MD |
124 | if (ret) |
125 | return ret; | |
126 | if (!ptr) { | |
127 | memset(rseq_cs, 0, sizeof(*rseq_cs)); | |
128 | return 0; | |
129 | } | |
130 | urseq_cs = (struct rseq_cs __user *)ptr; | |
131 | if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs))) | |
132 | return -EFAULT; | |
d7822b1e | 133 | |
e96d7135 MD |
134 | if (rseq_cs->start_ip >= TASK_SIZE || |
135 | rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE || | |
136 | rseq_cs->abort_ip >= TASK_SIZE || | |
137 | rseq_cs->version > 0) | |
138 | return -EINVAL; | |
139 | /* Check for overflow. */ | |
140 | if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip) | |
141 | return -EINVAL; | |
d7822b1e MD |
142 | /* Ensure that abort_ip is not in the critical section. */ |
143 | if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset) | |
144 | return -EINVAL; | |
145 | ||
e96d7135 | 146 | usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32)); |
d7822b1e MD |
147 | ret = get_user(sig, usig); |
148 | if (ret) | |
149 | return ret; | |
150 | ||
151 | if (current->rseq_sig != sig) { | |
152 | printk_ratelimited(KERN_WARNING | |
153 | "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n", | |
154 | sig, current->rseq_sig, current->pid, usig); | |
e96d7135 | 155 | return -EINVAL; |
d7822b1e MD |
156 | } |
157 | return 0; | |
158 | } | |
159 | ||
160 | static int rseq_need_restart(struct task_struct *t, u32 cs_flags) | |
161 | { | |
162 | u32 flags, event_mask; | |
163 | int ret; | |
164 | ||
165 | /* Get thread flags. */ | |
8f281770 | 166 | ret = get_user(flags, &t->rseq->flags); |
d7822b1e MD |
167 | if (ret) |
168 | return ret; | |
169 | ||
170 | /* Take critical section flags into account. */ | |
171 | flags |= cs_flags; | |
172 | ||
173 | /* | |
174 | * Restart on signal can only be inhibited when restart on | |
175 | * preempt and restart on migrate are inhibited too. Otherwise, | |
176 | * a preempted signal handler could fail to restart the prior | |
177 | * execution context on sigreturn. | |
178 | */ | |
179 | if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) && | |
180 | (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) != | |
181 | RSEQ_CS_PREEMPT_MIGRATE_FLAGS)) | |
182 | return -EINVAL; | |
183 | ||
184 | /* | |
185 | * Load and clear event mask atomically with respect to | |
186 | * scheduler preemption. | |
187 | */ | |
188 | preempt_disable(); | |
189 | event_mask = t->rseq_event_mask; | |
190 | t->rseq_event_mask = 0; | |
191 | preempt_enable(); | |
192 | ||
193 | return !!(event_mask & ~flags); | |
194 | } | |
195 | ||
196 | static int clear_rseq_cs(struct task_struct *t) | |
197 | { | |
198 | /* | |
199 | * The rseq_cs field is set to NULL on preemption or signal | |
200 | * delivery on top of rseq assembly block, as well as on top | |
201 | * of code outside of the rseq assembly block. This performs | |
202 | * a lazy clear of the rseq_cs field. | |
203 | * | |
204 | * Set rseq_cs to NULL with single-copy atomicity. | |
205 | */ | |
8f281770 | 206 | return put_user(0UL, &t->rseq->rseq_cs); |
d7822b1e MD |
207 | } |
208 | ||
209 | /* | |
210 | * Unsigned comparison will be true when ip >= start_ip, and when | |
211 | * ip < start_ip + post_commit_offset. | |
212 | */ | |
213 | static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs) | |
214 | { | |
215 | return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset; | |
216 | } | |
217 | ||
218 | static int rseq_ip_fixup(struct pt_regs *regs) | |
219 | { | |
220 | unsigned long ip = instruction_pointer(regs); | |
221 | struct task_struct *t = current; | |
222 | struct rseq_cs rseq_cs; | |
223 | int ret; | |
224 | ||
225 | ret = rseq_get_rseq_cs(t, &rseq_cs); | |
226 | if (ret) | |
227 | return ret; | |
228 | ||
229 | /* | |
230 | * Handle potentially not being within a critical section. | |
231 | * If not nested over a rseq critical section, restart is useless. | |
232 | * Clear the rseq_cs pointer and return. | |
233 | */ | |
234 | if (!in_rseq_cs(ip, &rseq_cs)) | |
235 | return clear_rseq_cs(t); | |
236 | ret = rseq_need_restart(t, rseq_cs.flags); | |
237 | if (ret <= 0) | |
238 | return ret; | |
239 | ret = clear_rseq_cs(t); | |
240 | if (ret) | |
241 | return ret; | |
242 | trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset, | |
243 | rseq_cs.abort_ip); | |
244 | instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip); | |
245 | return 0; | |
246 | } | |
247 | ||
248 | /* | |
249 | * This resume handler must always be executed between any of: | |
250 | * - preemption, | |
251 | * - signal delivery, | |
252 | * and return to user-space. | |
253 | * | |
254 | * This is how we can ensure that the entire rseq critical section, | |
255 | * consisting of both the C part and the assembly instruction sequence, | |
256 | * will issue the commit instruction only if executed atomically with | |
257 | * respect to other threads scheduled on the same CPU, and with respect | |
258 | * to signal handlers. | |
259 | */ | |
784e0300 | 260 | void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs) |
d7822b1e MD |
261 | { |
262 | struct task_struct *t = current; | |
784e0300 | 263 | int ret, sig; |
d7822b1e MD |
264 | |
265 | if (unlikely(t->flags & PF_EXITING)) | |
266 | return; | |
267 | if (unlikely(!access_ok(VERIFY_WRITE, t->rseq, sizeof(*t->rseq)))) | |
268 | goto error; | |
269 | ret = rseq_ip_fixup(regs); | |
270 | if (unlikely(ret < 0)) | |
271 | goto error; | |
272 | if (unlikely(rseq_update_cpu_id(t))) | |
273 | goto error; | |
274 | return; | |
275 | ||
276 | error: | |
784e0300 WD |
277 | sig = ksig ? ksig->sig : 0; |
278 | force_sigsegv(sig, t); | |
d7822b1e MD |
279 | } |
280 | ||
281 | #ifdef CONFIG_DEBUG_RSEQ | |
282 | ||
283 | /* | |
284 | * Terminate the process if a syscall is issued within a restartable | |
285 | * sequence. | |
286 | */ | |
287 | void rseq_syscall(struct pt_regs *regs) | |
288 | { | |
289 | unsigned long ip = instruction_pointer(regs); | |
290 | struct task_struct *t = current; | |
291 | struct rseq_cs rseq_cs; | |
292 | ||
293 | if (!t->rseq) | |
294 | return; | |
295 | if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) || | |
296 | rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs)) | |
297 | force_sig(SIGSEGV, t); | |
298 | } | |
299 | ||
300 | #endif | |
301 | ||
302 | /* | |
303 | * sys_rseq - setup restartable sequences for caller thread. | |
304 | */ | |
305 | SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, | |
306 | int, flags, u32, sig) | |
307 | { | |
308 | int ret; | |
309 | ||
310 | if (flags & RSEQ_FLAG_UNREGISTER) { | |
311 | /* Unregister rseq for current thread. */ | |
312 | if (current->rseq != rseq || !current->rseq) | |
313 | return -EINVAL; | |
314 | if (current->rseq_len != rseq_len) | |
315 | return -EINVAL; | |
316 | if (current->rseq_sig != sig) | |
317 | return -EPERM; | |
318 | ret = rseq_reset_rseq_cpu_id(current); | |
319 | if (ret) | |
320 | return ret; | |
321 | current->rseq = NULL; | |
322 | current->rseq_len = 0; | |
323 | current->rseq_sig = 0; | |
324 | return 0; | |
325 | } | |
326 | ||
327 | if (unlikely(flags)) | |
328 | return -EINVAL; | |
329 | ||
330 | if (current->rseq) { | |
331 | /* | |
332 | * If rseq is already registered, check whether | |
333 | * the provided address differs from the prior | |
334 | * one. | |
335 | */ | |
336 | if (current->rseq != rseq || current->rseq_len != rseq_len) | |
337 | return -EINVAL; | |
338 | if (current->rseq_sig != sig) | |
339 | return -EPERM; | |
340 | /* Already registered. */ | |
341 | return -EBUSY; | |
342 | } | |
343 | ||
344 | /* | |
345 | * If there was no rseq previously registered, | |
346 | * ensure the provided rseq is properly aligned and valid. | |
347 | */ | |
348 | if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) || | |
349 | rseq_len != sizeof(*rseq)) | |
350 | return -EINVAL; | |
351 | if (!access_ok(VERIFY_WRITE, rseq, rseq_len)) | |
352 | return -EFAULT; | |
353 | current->rseq = rseq; | |
354 | current->rseq_len = rseq_len; | |
355 | current->rseq_sig = sig; | |
356 | /* | |
357 | * If rseq was previously inactive, and has just been | |
358 | * registered, ensure the cpu_id_start and cpu_id fields | |
359 | * are updated before returning to user-space. | |
360 | */ | |
361 | rseq_set_notify_resume(current); | |
362 | ||
363 | return 0; | |
364 | } |