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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 : 0 : static int rseq_update_cpu_id(struct task_struct *t)
85 : : {
86 : 0 : u32 cpu_id = raw_smp_processor_id();
87 : :
88 [ # # ]: 0 : if (put_user(cpu_id, &t->rseq->cpu_id_start))
89 : : return -EFAULT;
90 [ # # ]: 0 : if (put_user(cpu_id, &t->rseq->cpu_id))
91 : : return -EFAULT;
92 : 0 : trace_rseq_update(t);
93 : 0 : return 0;
94 : : }
95 : :
96 : 0 : static int rseq_reset_rseq_cpu_id(struct task_struct *t)
97 : : {
98 : 0 : 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 [ # # ]: 0 : 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 [ # # ]: 0 : if (put_user(cpu_id, &t->rseq->cpu_id))
111 : : return -EFAULT;
112 : : return 0;
113 : : }
114 : :
115 : 0 : static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
116 : : {
117 : 0 : struct rseq_cs __user *urseq_cs;
118 : 0 : u64 ptr;
119 : 0 : u32 __user *usig;
120 : 0 : u32 sig;
121 : 0 : int ret;
122 : :
123 [ # # ]: 0 : if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
124 : : return -EFAULT;
125 [ # # ]: 0 : if (!ptr) {
126 : 0 : memset(rseq_cs, 0, sizeof(*rseq_cs));
127 : 0 : return 0;
128 : : }
129 [ # # # # : 0 : if (ptr >= TASK_SIZE)
# # ]
130 : : return -EINVAL;
131 : 0 : urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
132 [ # # # # ]: 0 : if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
133 : 0 : return -EFAULT;
134 : :
135 [ # # # # : 0 : if (rseq_cs->start_ip >= TASK_SIZE ||
# # # # ]
136 [ # # # # : 0 : rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
# # ]
137 [ # # # # ]: 0 : rseq_cs->abort_ip >= TASK_SIZE ||
138 [ # # ]: 0 : rseq_cs->version > 0)
139 : 0 : return -EINVAL;
140 : : /* Check for overflow. */
141 [ # # ]: 0 : 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 [ # # ]: 0 : if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
145 : : return -EINVAL;
146 : :
147 : 0 : usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
148 : 0 : ret = get_user(sig, usig);
149 [ # # ]: 0 : if (ret)
150 : : return ret;
151 : :
152 [ # # ]: 0 : if (current->rseq_sig != sig) {
153 [ # # ]: 0 : 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 : 0 : return -EINVAL;
157 : : }
158 : : return 0;
159 : : }
160 : :
161 : 0 : static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
162 : : {
163 : 0 : u32 flags, event_mask;
164 : 0 : int ret;
165 : :
166 : : /* Get thread flags. */
167 : 0 : ret = get_user(flags, &t->rseq->flags);
168 [ # # ]: 0 : if (ret)
169 : : return ret;
170 : :
171 : : /* Take critical section flags into account. */
172 : 0 : 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 [ # # # # ]: 0 : 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 : 0 : preempt_disable();
190 : 0 : event_mask = t->rseq_event_mask;
191 : 0 : t->rseq_event_mask = 0;
192 : 0 : preempt_enable();
193 : :
194 : 0 : return !!(event_mask & ~flags);
195 : : }
196 : :
197 : 0 : 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 [ # # # # ]: 0 : if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
208 : 0 : 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 : : */
216 : 0 : static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
217 : : {
218 : 0 : return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
219 : : }
220 : :
221 : 0 : static int rseq_ip_fixup(struct pt_regs *regs)
222 : : {
223 : 0 : unsigned long ip = instruction_pointer(regs);
224 : 0 : struct task_struct *t = current;
225 : 0 : struct rseq_cs rseq_cs;
226 : 0 : int ret;
227 : :
228 : 0 : ret = rseq_get_rseq_cs(t, &rseq_cs);
229 [ # # ]: 0 : 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 [ # # ]: 0 : if (!in_rseq_cs(ip, &rseq_cs))
238 : 0 : return clear_rseq_cs(t);
239 : 0 : ret = rseq_need_restart(t, rseq_cs.flags);
240 [ # # ]: 0 : if (ret <= 0)
241 : 0 : return ret;
242 : 0 : ret = clear_rseq_cs(t);
243 : 0 : if (ret)
244 : : return ret;
245 : 0 : trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
246 : 0 : rseq_cs.abort_ip);
247 : 0 : instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
248 : 0 : 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 : : */
262 : 0 : void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
263 : : {
264 [ # # ]: 0 : struct task_struct *t = current;
265 : 0 : int ret, sig;
266 : :
267 [ # # ]: 0 : if (unlikely(t->flags & PF_EXITING))
268 : : return;
269 [ # # ]: 0 : if (unlikely(!access_ok(t->rseq, sizeof(*t->rseq))))
270 : 0 : goto error;
271 : 0 : ret = rseq_ip_fixup(regs);
272 [ # # ]: 0 : if (unlikely(ret < 0))
273 : 0 : goto error;
274 [ # # ]: 0 : if (unlikely(rseq_update_cpu_id(t)))
275 : 0 : goto error;
276 : : return;
277 : :
278 : 0 : error:
279 [ # # ]: 0 : sig = ksig ? ksig->sig : 0;
280 : 0 : 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 : : */
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 : : */
307 : 0 : SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
308 : : int, flags, u32, sig)
309 : : {
310 : 0 : int ret;
311 : :
312 [ # # ]: 0 : if (flags & RSEQ_FLAG_UNREGISTER) {
313 [ # # ]: 0 : if (flags & ~RSEQ_FLAG_UNREGISTER)
314 : : return -EINVAL;
315 : : /* Unregister rseq for current thread. */
316 [ # # # # ]: 0 : if (current->rseq != rseq || !current->rseq)
317 : : return -EINVAL;
318 [ # # ]: 0 : if (rseq_len != sizeof(*rseq))
319 : : return -EINVAL;
320 [ # # ]: 0 : if (current->rseq_sig != sig)
321 : : return -EPERM;
322 : 0 : ret = rseq_reset_rseq_cpu_id(current);
323 : 0 : if (ret)
324 : : return ret;
325 : 0 : current->rseq = NULL;
326 : 0 : current->rseq_sig = 0;
327 : 0 : return 0;
328 : : }
329 : :
330 [ # # ]: 0 : if (unlikely(flags))
331 : : return -EINVAL;
332 : :
333 [ # # ]: 0 : if (current->rseq) {
334 : : /*
335 : : * If rseq is already registered, check whether
336 : : * the provided address differs from the prior
337 : : * one.
338 : : */
339 [ # # # # ]: 0 : if (current->rseq != rseq || rseq_len != sizeof(*rseq))
340 : : return -EINVAL;
341 [ # # ]: 0 : if (current->rseq_sig != sig)
342 : : return -EPERM;
343 : : /* Already registered. */
344 : 0 : 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 [ # # # # ]: 0 : if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
352 : : rseq_len != sizeof(*rseq))
353 : : return -EINVAL;
354 [ # # ]: 0 : if (!access_ok(rseq, rseq_len))
355 : : return -EFAULT;
356 [ # # ]: 0 : current->rseq = rseq;
357 : 0 : 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 [ # # ]: 0 : rseq_set_notify_resume(current);
364 : :
365 : : return 0;
366 : : }
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