projects/linux·blob v5.15-rc2 · refs

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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 <[email protected]> and Andrew Hunter <[email protected]>
7 * Copyright (C) 2015-2018, EfficiOS Inc.,
8 * Mathieu Desnoyers <[email protected]>
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
84static 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
97efault_end:
98 user_write_access_end();
99efault:
100 return -EFAULT;
101}
102
103static 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
122static 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
173static 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
209static 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 */
232static 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
237static 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 */
278void __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 ret = rseq_ip_fixup(regs);
286 if (unlikely(ret < 0))
287 goto error;
288 if (unlikely(rseq_update_cpu_id(t)))
289 goto error;
290 return;
291
292error:
293 sig = ksig ? ksig->sig : 0;
294 force_sigsegv(sig);
295}
296
297#ifdef CONFIG_DEBUG_RSEQ
298
299/*
300 * Terminate the process if a syscall is issued within a restartable
301 * sequence.
302 */
303void rseq_syscall(struct pt_regs *regs)
304{
305 unsigned long ip = instruction_pointer(regs);
306 struct task_struct *t = current;
307 struct rseq_cs rseq_cs;
308
309 if (!t->rseq)
310 return;
311 if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
312 force_sig(SIGSEGV);
313}
314
315#endif
316
317/*
318 * sys_rseq - setup restartable sequences for caller thread.
319 */
320SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
321 int, flags, u32, sig)
322{
323 int ret;
324
325 if (flags & RSEQ_FLAG_UNREGISTER) {
326 if (flags & ~RSEQ_FLAG_UNREGISTER)
327 return -EINVAL;
328 /* Unregister rseq for current thread. */
329 if (current->rseq != rseq || !current->rseq)
330 return -EINVAL;
331 if (rseq_len != sizeof(*rseq))
332 return -EINVAL;
333 if (current->rseq_sig != sig)
334 return -EPERM;
335 ret = rseq_reset_rseq_cpu_id(current);
336 if (ret)
337 return ret;
338 current->rseq = NULL;
339 current->rseq_sig = 0;
340 return 0;
341 }
342
343 if (unlikely(flags))
344 return -EINVAL;
345
346 if (current->rseq) {
347 /*
348 * If rseq is already registered, check whether
349 * the provided address differs from the prior
350 * one.
351 */
352 if (current->rseq != rseq || rseq_len != sizeof(*rseq))
353 return -EINVAL;
354 if (current->rseq_sig != sig)
355 return -EPERM;
356 /* Already registered. */
357 return -EBUSY;
358 }
359
360 /*
361 * If there was no rseq previously registered,
362 * ensure the provided rseq is properly aligned and valid.
363 */
364 if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
365 rseq_len != sizeof(*rseq))
366 return -EINVAL;
367 if (!access_ok(rseq, rseq_len))
368 return -EFAULT;
369 current->rseq = rseq;
370 current->rseq_sig = sig;
371 /*
372 * If rseq was previously inactive, and has just been
373 * registered, ensure the cpu_id_start and cpu_id fields
374 * are updated before returning to user-space.
375 */
376 rseq_set_notify_resume(current);
377
378 return 0;
379}