xref: /openbmc/linux/kernel/rseq.c (revision f7b01bb0)
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 /* The original rseq structure size (including padding) is 32 bytes. */
22 #define ORIG_RSEQ_SIZE		32
23 
24 #define RSEQ_CS_NO_RESTART_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT | \
25 				  RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
26 				  RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)
27 
28 /*
29  *
30  * Restartable sequences are a lightweight interface that allows
31  * user-level code to be executed atomically relative to scheduler
32  * preemption and signal delivery. Typically used for implementing
33  * per-cpu operations.
34  *
35  * It allows user-space to perform update operations on per-cpu data
36  * without requiring heavy-weight atomic operations.
37  *
38  * Detailed algorithm of rseq user-space assembly sequences:
39  *
40  *                     init(rseq_cs)
41  *                     cpu = TLS->rseq::cpu_id_start
42  *   [1]               TLS->rseq::rseq_cs = rseq_cs
43  *   [start_ip]        ----------------------------
44  *   [2]               if (cpu != TLS->rseq::cpu_id)
45  *                             goto abort_ip;
46  *   [3]               <last_instruction_in_cs>
47  *   [post_commit_ip]  ----------------------------
48  *
49  *   The address of jump target abort_ip must be outside the critical
50  *   region, i.e.:
51  *
52  *     [abort_ip] < [start_ip]  || [abort_ip] >= [post_commit_ip]
53  *
54  *   Steps [2]-[3] (inclusive) need to be a sequence of instructions in
55  *   userspace that can handle being interrupted between any of those
56  *   instructions, and then resumed to the abort_ip.
57  *
58  *   1.  Userspace stores the address of the struct rseq_cs assembly
59  *       block descriptor into the rseq_cs field of the registered
60  *       struct rseq TLS area. This update is performed through a single
61  *       store within the inline assembly instruction sequence.
62  *       [start_ip]
63  *
64  *   2.  Userspace tests to check whether the current cpu_id field match
65  *       the cpu number loaded before start_ip, branching to abort_ip
66  *       in case of a mismatch.
67  *
68  *       If the sequence is preempted or interrupted by a signal
69  *       at or after start_ip and before post_commit_ip, then the kernel
70  *       clears TLS->__rseq_abi::rseq_cs, and sets the user-space return
71  *       ip to abort_ip before returning to user-space, so the preempted
72  *       execution resumes at abort_ip.
73  *
74  *   3.  Userspace critical section final instruction before
75  *       post_commit_ip is the commit. The critical section is
76  *       self-terminating.
77  *       [post_commit_ip]
78  *
79  *   4.  <success>
80  *
81  *   On failure at [2], or if interrupted by preempt or signal delivery
82  *   between [1] and [3]:
83  *
84  *       [abort_ip]
85  *   F1. <failure>
86  */
87 
rseq_update_cpu_node_id(struct task_struct * t)88 static int rseq_update_cpu_node_id(struct task_struct *t)
89 {
90 	struct rseq __user *rseq = t->rseq;
91 	u32 cpu_id = raw_smp_processor_id();
92 	u32 node_id = cpu_to_node(cpu_id);
93 	u32 mm_cid = task_mm_cid(t);
94 
95 	WARN_ON_ONCE((int) mm_cid < 0);
96 	if (!user_write_access_begin(rseq, t->rseq_len))
97 		goto efault;
98 	unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
99 	unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
100 	unsafe_put_user(node_id, &rseq->node_id, efault_end);
101 	unsafe_put_user(mm_cid, &rseq->mm_cid, efault_end);
102 	/*
103 	 * Additional feature fields added after ORIG_RSEQ_SIZE
104 	 * need to be conditionally updated only if
105 	 * t->rseq_len != ORIG_RSEQ_SIZE.
106 	 */
107 	user_write_access_end();
108 	trace_rseq_update(t);
109 	return 0;
110 
111 efault_end:
112 	user_write_access_end();
113 efault:
114 	return -EFAULT;
115 }
116 
rseq_reset_rseq_cpu_node_id(struct task_struct * t)117 static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
118 {
119 	u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED, node_id = 0,
120 	    mm_cid = 0;
121 
122 	/*
123 	 * Reset cpu_id_start to its initial state (0).
124 	 */
125 	if (put_user(cpu_id_start, &t->rseq->cpu_id_start))
126 		return -EFAULT;
127 	/*
128 	 * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
129 	 * in after unregistration can figure out that rseq needs to be
130 	 * registered again.
131 	 */
132 	if (put_user(cpu_id, &t->rseq->cpu_id))
133 		return -EFAULT;
134 	/*
135 	 * Reset node_id to its initial state (0).
136 	 */
137 	if (put_user(node_id, &t->rseq->node_id))
138 		return -EFAULT;
139 	/*
140 	 * Reset mm_cid to its initial state (0).
141 	 */
142 	if (put_user(mm_cid, &t->rseq->mm_cid))
143 		return -EFAULT;
144 	/*
145 	 * Additional feature fields added after ORIG_RSEQ_SIZE
146 	 * need to be conditionally reset only if
147 	 * t->rseq_len != ORIG_RSEQ_SIZE.
148 	 */
149 	return 0;
150 }
151 
rseq_get_rseq_cs(struct task_struct * t,struct rseq_cs * rseq_cs)152 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
153 {
154 	struct rseq_cs __user *urseq_cs;
155 	u64 ptr;
156 	u32 __user *usig;
157 	u32 sig;
158 	int ret;
159 
160 #ifdef CONFIG_64BIT
161 	if (get_user(ptr, &t->rseq->rseq_cs))
162 		return -EFAULT;
163 #else
164 	if (copy_from_user(&ptr, &t->rseq->rseq_cs, sizeof(ptr)))
165 		return -EFAULT;
166 #endif
167 	if (!ptr) {
168 		memset(rseq_cs, 0, sizeof(*rseq_cs));
169 		return 0;
170 	}
171 	if (ptr >= TASK_SIZE)
172 		return -EINVAL;
173 	urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
174 	if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
175 		return -EFAULT;
176 
177 	if (rseq_cs->start_ip >= TASK_SIZE ||
178 	    rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
179 	    rseq_cs->abort_ip >= TASK_SIZE ||
180 	    rseq_cs->version > 0)
181 		return -EINVAL;
182 	/* Check for overflow. */
183 	if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
184 		return -EINVAL;
185 	/* Ensure that abort_ip is not in the critical section. */
186 	if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
187 		return -EINVAL;
188 
189 	usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
190 	ret = get_user(sig, usig);
191 	if (ret)
192 		return ret;
193 
194 	if (current->rseq_sig != sig) {
195 		printk_ratelimited(KERN_WARNING
196 			"Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
197 			sig, current->rseq_sig, current->pid, usig);
198 		return -EINVAL;
199 	}
200 	return 0;
201 }
202 
rseq_warn_flags(const char * str,u32 flags)203 static bool rseq_warn_flags(const char *str, u32 flags)
204 {
205 	u32 test_flags;
206 
207 	if (!flags)
208 		return false;
209 	test_flags = flags & RSEQ_CS_NO_RESTART_FLAGS;
210 	if (test_flags)
211 		pr_warn_once("Deprecated flags (%u) in %s ABI structure", test_flags, str);
212 	test_flags = flags & ~RSEQ_CS_NO_RESTART_FLAGS;
213 	if (test_flags)
214 		pr_warn_once("Unknown flags (%u) in %s ABI structure", test_flags, str);
215 	return true;
216 }
217 
rseq_need_restart(struct task_struct * t,u32 cs_flags)218 static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
219 {
220 	u32 flags, event_mask;
221 	int ret;
222 
223 	if (rseq_warn_flags("rseq_cs", cs_flags))
224 		return -EINVAL;
225 
226 	/* Get thread flags. */
227 	ret = get_user(flags, &t->rseq->flags);
228 	if (ret)
229 		return ret;
230 
231 	if (rseq_warn_flags("rseq", flags))
232 		return -EINVAL;
233 
234 	/*
235 	 * Load and clear event mask atomically with respect to
236 	 * scheduler preemption.
237 	 */
238 	preempt_disable();
239 	event_mask = t->rseq_event_mask;
240 	t->rseq_event_mask = 0;
241 	preempt_enable();
242 
243 	return !!event_mask;
244 }
245 
clear_rseq_cs(struct task_struct * t)246 static int clear_rseq_cs(struct task_struct *t)
247 {
248 	/*
249 	 * The rseq_cs field is set to NULL on preemption or signal
250 	 * delivery on top of rseq assembly block, as well as on top
251 	 * of code outside of the rseq assembly block. This performs
252 	 * a lazy clear of the rseq_cs field.
253 	 *
254 	 * Set rseq_cs to NULL.
255 	 */
256 #ifdef CONFIG_64BIT
257 	return put_user(0UL, &t->rseq->rseq_cs);
258 #else
259 	if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
260 		return -EFAULT;
261 	return 0;
262 #endif
263 }
264 
265 /*
266  * Unsigned comparison will be true when ip >= start_ip, and when
267  * ip < start_ip + post_commit_offset.
268  */
in_rseq_cs(unsigned long ip,struct rseq_cs * rseq_cs)269 static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
270 {
271 	return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
272 }
273 
rseq_ip_fixup(struct pt_regs * regs)274 static int rseq_ip_fixup(struct pt_regs *regs)
275 {
276 	unsigned long ip = instruction_pointer(regs);
277 	struct task_struct *t = current;
278 	struct rseq_cs rseq_cs;
279 	int ret;
280 
281 	ret = rseq_get_rseq_cs(t, &rseq_cs);
282 	if (ret)
283 		return ret;
284 
285 	/*
286 	 * Handle potentially not being within a critical section.
287 	 * If not nested over a rseq critical section, restart is useless.
288 	 * Clear the rseq_cs pointer and return.
289 	 */
290 	if (!in_rseq_cs(ip, &rseq_cs))
291 		return clear_rseq_cs(t);
292 	ret = rseq_need_restart(t, rseq_cs.flags);
293 	if (ret <= 0)
294 		return ret;
295 	ret = clear_rseq_cs(t);
296 	if (ret)
297 		return ret;
298 	trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
299 			    rseq_cs.abort_ip);
300 	instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
301 	return 0;
302 }
303 
304 /*
305  * This resume handler must always be executed between any of:
306  * - preemption,
307  * - signal delivery,
308  * and return to user-space.
309  *
310  * This is how we can ensure that the entire rseq critical section
311  * will issue the commit instruction only if executed atomically with
312  * respect to other threads scheduled on the same CPU, and with respect
313  * to signal handlers.
314  */
__rseq_handle_notify_resume(struct ksignal * ksig,struct pt_regs * regs)315 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
316 {
317 	struct task_struct *t = current;
318 	int ret, sig;
319 
320 	if (unlikely(t->flags & PF_EXITING))
321 		return;
322 
323 	/*
324 	 * regs is NULL if and only if the caller is in a syscall path.  Skip
325 	 * fixup and leave rseq_cs as is so that rseq_sycall() will detect and
326 	 * kill a misbehaving userspace on debug kernels.
327 	 */
328 	if (regs) {
329 		ret = rseq_ip_fixup(regs);
330 		if (unlikely(ret < 0))
331 			goto error;
332 	}
333 	if (unlikely(rseq_update_cpu_node_id(t)))
334 		goto error;
335 	return;
336 
337 error:
338 	sig = ksig ? ksig->sig : 0;
339 	force_sigsegv(sig);
340 }
341 
342 #ifdef CONFIG_DEBUG_RSEQ
343 
344 /*
345  * Terminate the process if a syscall is issued within a restartable
346  * sequence.
347  */
rseq_syscall(struct pt_regs * regs)348 void rseq_syscall(struct pt_regs *regs)
349 {
350 	unsigned long ip = instruction_pointer(regs);
351 	struct task_struct *t = current;
352 	struct rseq_cs rseq_cs;
353 
354 	if (!t->rseq)
355 		return;
356 	if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
357 		force_sig(SIGSEGV);
358 }
359 
360 #endif
361 
362 /*
363  * sys_rseq - setup restartable sequences for caller thread.
364  */
SYSCALL_DEFINE4(rseq,struct rseq __user *,rseq,u32,rseq_len,int,flags,u32,sig)365 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
366 		int, flags, u32, sig)
367 {
368 	int ret;
369 
370 	if (flags & RSEQ_FLAG_UNREGISTER) {
371 		if (flags & ~RSEQ_FLAG_UNREGISTER)
372 			return -EINVAL;
373 		/* Unregister rseq for current thread. */
374 		if (current->rseq != rseq || !current->rseq)
375 			return -EINVAL;
376 		if (rseq_len != current->rseq_len)
377 			return -EINVAL;
378 		if (current->rseq_sig != sig)
379 			return -EPERM;
380 		ret = rseq_reset_rseq_cpu_node_id(current);
381 		if (ret)
382 			return ret;
383 		current->rseq = NULL;
384 		current->rseq_sig = 0;
385 		current->rseq_len = 0;
386 		return 0;
387 	}
388 
389 	if (unlikely(flags))
390 		return -EINVAL;
391 
392 	if (current->rseq) {
393 		/*
394 		 * If rseq is already registered, check whether
395 		 * the provided address differs from the prior
396 		 * one.
397 		 */
398 		if (current->rseq != rseq || rseq_len != current->rseq_len)
399 			return -EINVAL;
400 		if (current->rseq_sig != sig)
401 			return -EPERM;
402 		/* Already registered. */
403 		return -EBUSY;
404 	}
405 
406 	/*
407 	 * If there was no rseq previously registered, ensure the provided rseq
408 	 * is properly aligned, as communcated to user-space through the ELF
409 	 * auxiliary vector AT_RSEQ_ALIGN. If rseq_len is the original rseq
410 	 * size, the required alignment is the original struct rseq alignment.
411 	 *
412 	 * In order to be valid, rseq_len is either the original rseq size, or
413 	 * large enough to contain all supported fields, as communicated to
414 	 * user-space through the ELF auxiliary vector AT_RSEQ_FEATURE_SIZE.
415 	 */
416 	if (rseq_len < ORIG_RSEQ_SIZE ||
417 	    (rseq_len == ORIG_RSEQ_SIZE && !IS_ALIGNED((unsigned long)rseq, ORIG_RSEQ_SIZE)) ||
418 	    (rseq_len != ORIG_RSEQ_SIZE && (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
419 					    rseq_len < offsetof(struct rseq, end))))
420 		return -EINVAL;
421 	if (!access_ok(rseq, rseq_len))
422 		return -EFAULT;
423 	current->rseq = rseq;
424 	current->rseq_len = rseq_len;
425 	current->rseq_sig = sig;
426 	/*
427 	 * If rseq was previously inactive, and has just been
428 	 * registered, ensure the cpu_id_start and cpu_id fields
429 	 * are updated before returning to user-space.
430 	 */
431 	rseq_set_notify_resume(current);
432 
433 	return 0;
434 }
435