xref: /openbmc/linux/kernel/rseq.c (revision 337cbeb2)
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  
88  	if (put_user(cpu_id, &t->rseq->cpu_id_start))
89  		return -EFAULT;
90  	if (put_user(cpu_id, &t->rseq->cpu_id))
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  	 */
103  	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  	if (put_user(cpu_id, &t->rseq->cpu_id))
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  	u64 ptr;
119  	u32 __user *usig;
120  	u32 sig;
121  	int ret;
122  
123  	if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
124  		return -EFAULT;
125  	if (!ptr) {
126  		memset(rseq_cs, 0, sizeof(*rseq_cs));
127  		return 0;
128  	}
129  	if (ptr >= TASK_SIZE)
130  		return -EINVAL;
131  	urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
132  	if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
133  		return -EFAULT;
134  
135  	if (rseq_cs->start_ip >= TASK_SIZE ||
136  	    rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
137  	    rseq_cs->abort_ip >= TASK_SIZE ||
138  	    rseq_cs->version > 0)
139  		return -EINVAL;
140  	/* Check for overflow. */
141  	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  	if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
145  		return -EINVAL;
146  
147  	usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
148  	ret = get_user(sig, usig);
149  	if (ret)
150  		return ret;
151  
152  	if (current->rseq_sig != sig) {
153  		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  		return -EINVAL;
157  	}
158  	return 0;
159  }
160  
161  static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
162  {
163  	u32 flags, event_mask;
164  	int ret;
165  
166  	/* Get thread flags. */
167  	ret = get_user(flags, &t->rseq->flags);
168  	if (ret)
169  		return ret;
170  
171  	/* Take critical section flags into account. */
172  	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  	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  	preempt_disable();
190  	event_mask = t->rseq_event_mask;
191  	t->rseq_event_mask = 0;
192  	preempt_enable();
193  
194  	return !!(event_mask & ~flags);
195  }
196  
197  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  	if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
208  		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  static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
217  {
218  	return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
219  }
220  
221  static int rseq_ip_fixup(struct pt_regs *regs)
222  {
223  	unsigned long ip = instruction_pointer(regs);
224  	struct task_struct *t = current;
225  	struct rseq_cs rseq_cs;
226  	int ret;
227  
228  	ret = rseq_get_rseq_cs(t, &rseq_cs);
229  	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  	if (!in_rseq_cs(ip, &rseq_cs))
238  		return clear_rseq_cs(t);
239  	ret = rseq_need_restart(t, rseq_cs.flags);
240  	if (ret <= 0)
241  		return ret;
242  	ret = clear_rseq_cs(t);
243  	if (ret)
244  		return ret;
245  	trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
246  			    rseq_cs.abort_ip);
247  	instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
248  	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  void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
263  {
264  	struct task_struct *t = current;
265  	int ret, sig;
266  
267  	if (unlikely(t->flags & PF_EXITING))
268  		return;
269  	if (unlikely(!access_ok(t->rseq, sizeof(*t->rseq))))
270  		goto error;
271  	ret = rseq_ip_fixup(regs);
272  	if (unlikely(ret < 0))
273  		goto error;
274  	if (unlikely(rseq_update_cpu_id(t)))
275  		goto error;
276  	return;
277  
278  error:
279  	sig = ksig ? ksig->sig : 0;
280  	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  SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
308  		int, flags, u32, sig)
309  {
310  	int ret;
311  
312  	if (flags & RSEQ_FLAG_UNREGISTER) {
313  		if (flags & ~RSEQ_FLAG_UNREGISTER)
314  			return -EINVAL;
315  		/* Unregister rseq for current thread. */
316  		if (current->rseq != rseq || !current->rseq)
317  			return -EINVAL;
318  		if (rseq_len != sizeof(*rseq))
319  			return -EINVAL;
320  		if (current->rseq_sig != sig)
321  			return -EPERM;
322  		ret = rseq_reset_rseq_cpu_id(current);
323  		if (ret)
324  			return ret;
325  		current->rseq = NULL;
326  		current->rseq_sig = 0;
327  		return 0;
328  	}
329  
330  	if (unlikely(flags))
331  		return -EINVAL;
332  
333  	if (current->rseq) {
334  		/*
335  		 * If rseq is already registered, check whether
336  		 * the provided address differs from the prior
337  		 * one.
338  		 */
339  		if (current->rseq != rseq || rseq_len != sizeof(*rseq))
340  			return -EINVAL;
341  		if (current->rseq_sig != sig)
342  			return -EPERM;
343  		/* Already registered. */
344  		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  	if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
352  	    rseq_len != sizeof(*rseq))
353  		return -EINVAL;
354  	if (!access_ok(rseq, rseq_len))
355  		return -EFAULT;
356  	current->rseq = rseq;
357  	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  	rseq_set_notify_resume(current);
364  
365  	return 0;
366  }
367