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))
132 		return -EFAULT;
133 #else
134 	if (copy_from_user(&ptr, &t->rseq->rseq_cs, 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);
221 #else
222 	if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
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