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 struct rseq __user *rseq = t->rseq;
88
89 if (!user_write_access_begin(rseq, sizeof(*rseq)))
90 goto efault;
91 unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
92 unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
93 user_write_access_end();
94 trace_rseq_update(t);
95 return 0;
96
97 efault_end:
98 user_write_access_end();
99 efault:
100 return -EFAULT;
101 }
102
rseq_reset_rseq_cpu_id(struct task_struct * t)103 static int rseq_reset_rseq_cpu_id(struct task_struct *t)
104 {
105 u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED;
106
107 /*
108 * Reset cpu_id_start to its initial state (0).
109 */
110 if (put_user(cpu_id_start, &t->rseq->cpu_id_start))
111 return -EFAULT;
112 /*
113 * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
114 * in after unregistration can figure out that rseq needs to be
115 * registered again.
116 */
117 if (put_user(cpu_id, &t->rseq->cpu_id))
118 return -EFAULT;
119 return 0;
120 }
121
rseq_get_rseq_cs(struct task_struct * t,struct rseq_cs * rseq_cs)122 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
123 {
124 struct rseq_cs __user *urseq_cs;
125 u64 ptr;
126 u32 __user *usig;
127 u32 sig;
128 int ret;
129
130 #ifdef CONFIG_64BIT
131 if (get_user(ptr, &t->rseq->rseq_cs.ptr64))
132 return -EFAULT;
133 #else
134 if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
135 return -EFAULT;
136 #endif
137 if (!ptr) {
138 memset(rseq_cs, 0, sizeof(*rseq_cs));
139 return 0;
140 }
141 if (ptr >= TASK_SIZE)
142 return -EINVAL;
143 urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
144 if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
145 return -EFAULT;
146
147 if (rseq_cs->start_ip >= TASK_SIZE ||
148 rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
149 rseq_cs->abort_ip >= TASK_SIZE ||
150 rseq_cs->version > 0)
151 return -EINVAL;
152 /* Check for overflow. */
153 if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
154 return -EINVAL;
155 /* Ensure that abort_ip is not in the critical section. */
156 if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
157 return -EINVAL;
158
159 usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
160 ret = get_user(sig, usig);
161 if (ret)
162 return ret;
163
164 if (current->rseq_sig != sig) {
165 printk_ratelimited(KERN_WARNING
166 "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
167 sig, current->rseq_sig, current->pid, usig);
168 return -EINVAL;
169 }
170 return 0;
171 }
172
rseq_need_restart(struct task_struct * t,u32 cs_flags)173 static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
174 {
175 u32 flags, event_mask;
176 int ret;
177
178 /* Get thread flags. */
179 ret = get_user(flags, &t->rseq->flags);
180 if (ret)
181 return ret;
182
183 /* Take critical section flags into account. */
184 flags |= cs_flags;
185
186 /*
187 * Restart on signal can only be inhibited when restart on
188 * preempt and restart on migrate are inhibited too. Otherwise,
189 * a preempted signal handler could fail to restart the prior
190 * execution context on sigreturn.
191 */
192 if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) &&
193 (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) !=
194 RSEQ_CS_PREEMPT_MIGRATE_FLAGS))
195 return -EINVAL;
196
197 /*
198 * Load and clear event mask atomically with respect to
199 * scheduler preemption.
200 */
201 preempt_disable();
202 event_mask = t->rseq_event_mask;
203 t->rseq_event_mask = 0;
204 preempt_enable();
205
206 return !!(event_mask & ~flags);
207 }
208
clear_rseq_cs(struct task_struct * t)209 static int clear_rseq_cs(struct task_struct *t)
210 {
211 /*
212 * The rseq_cs field is set to NULL on preemption or signal
213 * delivery on top of rseq assembly block, as well as on top
214 * of code outside of the rseq assembly block. This performs
215 * a lazy clear of the rseq_cs field.
216 *
217 * Set rseq_cs to NULL.
218 */
219 #ifdef CONFIG_64BIT
220 return put_user(0UL, &t->rseq->rseq_cs.ptr64);
221 #else
222 if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
223 return -EFAULT;
224 return 0;
225 #endif
226 }
227
228 /*
229 * Unsigned comparison will be true when ip >= start_ip, and when
230 * ip < start_ip + post_commit_offset.
231 */
in_rseq_cs(unsigned long ip,struct rseq_cs * rseq_cs)232 static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
233 {
234 return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
235 }
236
rseq_ip_fixup(struct pt_regs * regs)237 static int rseq_ip_fixup(struct pt_regs *regs)
238 {
239 unsigned long ip = instruction_pointer(regs);
240 struct task_struct *t = current;
241 struct rseq_cs rseq_cs;
242 int ret;
243
244 ret = rseq_get_rseq_cs(t, &rseq_cs);
245 if (ret)
246 return ret;
247
248 /*
249 * Handle potentially not being within a critical section.
250 * If not nested over a rseq critical section, restart is useless.
251 * Clear the rseq_cs pointer and return.
252 */
253 if (!in_rseq_cs(ip, &rseq_cs))
254 return clear_rseq_cs(t);
255 ret = rseq_need_restart(t, rseq_cs.flags);
256 if (ret <= 0)
257 return ret;
258 ret = clear_rseq_cs(t);
259 if (ret)
260 return ret;
261 trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
262 rseq_cs.abort_ip);
263 instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
264 return 0;
265 }
266
267 /*
268 * This resume handler must always be executed between any of:
269 * - preemption,
270 * - signal delivery,
271 * and return to user-space.
272 *
273 * This is how we can ensure that the entire rseq critical section
274 * will issue the commit instruction only if executed atomically with
275 * respect to other threads scheduled on the same CPU, and with respect
276 * to signal handlers.
277 */
__rseq_handle_notify_resume(struct ksignal * ksig,struct pt_regs * regs)278 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
279 {
280 struct task_struct *t = current;
281 int ret, sig;
282
283 if (unlikely(t->flags & PF_EXITING))
284 return;
285
286 /*
287 * regs is NULL if and only if the caller is in a syscall path. Skip
288 * fixup and leave rseq_cs as is so that rseq_sycall() will detect and
289 * kill a misbehaving userspace on debug kernels.
290 */
291 if (regs) {
292 ret = rseq_ip_fixup(regs);
293 if (unlikely(ret < 0))
294 goto error;
295 }
296 if (unlikely(rseq_update_cpu_id(t)))
297 goto error;
298 return;
299
300 error:
301 sig = ksig ? ksig->sig : 0;
302 force_sigsegv(sig);
303 }
304
305 #ifdef CONFIG_DEBUG_RSEQ
306
307 /*
308 * Terminate the process if a syscall is issued within a restartable
309 * sequence.
310 */
rseq_syscall(struct pt_regs * regs)311 void rseq_syscall(struct pt_regs *regs)
312 {
313 unsigned long ip = instruction_pointer(regs);
314 struct task_struct *t = current;
315 struct rseq_cs rseq_cs;
316
317 if (!t->rseq)
318 return;
319 if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
320 force_sig(SIGSEGV);
321 }
322
323 #endif
324
325 /*
326 * sys_rseq - setup restartable sequences for caller thread.
327 */
SYSCALL_DEFINE4(rseq,struct rseq __user *,rseq,u32,rseq_len,int,flags,u32,sig)328 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
329 int, flags, u32, sig)
330 {
331 int ret;
332
333 if (flags & RSEQ_FLAG_UNREGISTER) {
334 if (flags & ~RSEQ_FLAG_UNREGISTER)
335 return -EINVAL;
336 /* Unregister rseq for current thread. */
337 if (current->rseq != rseq || !current->rseq)
338 return -EINVAL;
339 if (rseq_len != sizeof(*rseq))
340 return -EINVAL;
341 if (current->rseq_sig != sig)
342 return -EPERM;
343 ret = rseq_reset_rseq_cpu_id(current);
344 if (ret)
345 return ret;
346 current->rseq = NULL;
347 current->rseq_sig = 0;
348 return 0;
349 }
350
351 if (unlikely(flags))
352 return -EINVAL;
353
354 if (current->rseq) {
355 /*
356 * If rseq is already registered, check whether
357 * the provided address differs from the prior
358 * one.
359 */
360 if (current->rseq != rseq || rseq_len != sizeof(*rseq))
361 return -EINVAL;
362 if (current->rseq_sig != sig)
363 return -EPERM;
364 /* Already registered. */
365 return -EBUSY;
366 }
367
368 /*
369 * If there was no rseq previously registered,
370 * ensure the provided rseq is properly aligned and valid.
371 */
372 if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
373 rseq_len != sizeof(*rseq))
374 return -EINVAL;
375 if (!access_ok(rseq, rseq_len))
376 return -EFAULT;
377 current->rseq = rseq;
378 current->rseq_sig = sig;
379 /*
380 * If rseq was previously inactive, and has just been
381 * registered, ensure the cpu_id_start and cpu_id fields
382 * are updated before returning to user-space.
383 */
384 rseq_set_notify_resume(current);
385
386 return 0;
387 }
388
