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