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 /* 12 * Restartable sequences are a lightweight interface that allows 13 * user-level code to be executed atomically relative to scheduler 14 * preemption and signal delivery. Typically used for implementing 15 * per-cpu operations. 16 * 17 * It allows user-space to perform update operations on per-cpu data 18 * without requiring heavy-weight atomic operations. 19 * 20 * Detailed algorithm of rseq user-space assembly sequences: 21 * 22 * init(rseq_cs) 23 * cpu = TLS->rseq::cpu_id_start 24 * [1] TLS->rseq::rseq_cs = rseq_cs 25 * [start_ip] ---------------------------- 26 * [2] if (cpu != TLS->rseq::cpu_id) 27 * goto abort_ip; 28 * [3] <last_instruction_in_cs> 29 * [post_commit_ip] ---------------------------- 30 * 31 * The address of jump target abort_ip must be outside the critical 32 * region, i.e.: 33 * 34 * [abort_ip] < [start_ip] || [abort_ip] >= [post_commit_ip] 35 * 36 * Steps [2]-[3] (inclusive) need to be a sequence of instructions in 37 * userspace that can handle being interrupted between any of those 38 * instructions, and then resumed to the abort_ip. 39 * 40 * 1. Userspace stores the address of the struct rseq_cs assembly 41 * block descriptor into the rseq_cs field of the registered 42 * struct rseq TLS area. This update is performed through a single 43 * store within the inline assembly instruction sequence. 44 * [start_ip] 45 * 46 * 2. Userspace tests to check whether the current cpu_id field match 47 * the cpu number loaded before start_ip, branching to abort_ip 48 * in case of a mismatch. 49 * 50 * If the sequence is preempted or interrupted by a signal 51 * at or after start_ip and before post_commit_ip, then the kernel 52 * clears TLS->__rseq_abi::rseq_cs, and sets the user-space return 53 * ip to abort_ip before returning to user-space, so the preempted 54 * execution resumes at abort_ip. 55 * 56 * 3. Userspace critical section final instruction before 57 * post_commit_ip is the commit. The critical section is 58 * self-terminating. 59 * [post_commit_ip] 60 * 61 * 4. <success> 62 * 63 * On failure at [2], or if interrupted by preempt or signal delivery 64 * between [1] and [3]: 65 * 66 * [abort_ip] 67 * F1. <failure> 68 */ 69 70 /* Required to select the proper per_cpu ops for rseq_stats_inc() */ 71 #define RSEQ_BUILD_SLOW_PATH 72 73 #include <linux/debugfs.h> 74 #include <linux/hrtimer.h> 75 #include <linux/percpu.h> 76 #include <linux/prctl.h> 77 #include <linux/ratelimit.h> 78 #include <linux/rseq_entry.h> 79 #include <linux/sched.h> 80 #include <linux/syscalls.h> 81 #include <linux/uaccess.h> 82 #include <linux/types.h> 83 #include <linux/rseq.h> 84 #include <asm/ptrace.h> 85 86 #define CREATE_TRACE_POINTS 87 #include <trace/events/rseq.h> 88 89 DEFINE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled); 90 91 static inline void rseq_control_debug(bool on) 92 { 93 if (on) 94 static_branch_enable(&rseq_debug_enabled); 95 else 96 static_branch_disable(&rseq_debug_enabled); 97 } 98 99 static int __init rseq_setup_debug(char *str) 100 { 101 bool on; 102 103 if (kstrtobool(str, &on)) 104 return -EINVAL; 105 rseq_control_debug(on); 106 return 1; 107 } 108 __setup("rseq_debug=", rseq_setup_debug); 109 110 #ifdef CONFIG_TRACEPOINTS 111 /* 112 * Out of line, so the actual update functions can be in a header to be 113 * inlined into the exit to user code. 114 */ 115 void __rseq_trace_update(struct task_struct *t) 116 { 117 trace_rseq_update(t); 118 } 119 120 void __rseq_trace_ip_fixup(unsigned long ip, unsigned long start_ip, 121 unsigned long offset, unsigned long abort_ip) 122 { 123 trace_rseq_ip_fixup(ip, start_ip, offset, abort_ip); 124 } 125 #endif /* CONFIG_TRACEPOINTS */ 126 127 #ifdef CONFIG_RSEQ_STATS 128 DEFINE_PER_CPU(struct rseq_stats, rseq_stats); 129 130 static int rseq_stats_show(struct seq_file *m, void *p) 131 { 132 struct rseq_stats stats = { }; 133 unsigned int cpu; 134 135 for_each_possible_cpu(cpu) { 136 stats.exit += data_race(per_cpu(rseq_stats.exit, cpu)); 137 stats.signal += data_race(per_cpu(rseq_stats.signal, cpu)); 138 stats.slowpath += data_race(per_cpu(rseq_stats.slowpath, cpu)); 139 stats.fastpath += data_race(per_cpu(rseq_stats.fastpath, cpu)); 140 stats.ids += data_race(per_cpu(rseq_stats.ids, cpu)); 141 stats.cs += data_race(per_cpu(rseq_stats.cs, cpu)); 142 stats.clear += data_race(per_cpu(rseq_stats.clear, cpu)); 143 stats.fixup += data_race(per_cpu(rseq_stats.fixup, cpu)); 144 if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) { 145 stats.s_granted += data_race(per_cpu(rseq_stats.s_granted, cpu)); 146 stats.s_expired += data_race(per_cpu(rseq_stats.s_expired, cpu)); 147 stats.s_revoked += data_race(per_cpu(rseq_stats.s_revoked, cpu)); 148 stats.s_yielded += data_race(per_cpu(rseq_stats.s_yielded, cpu)); 149 stats.s_aborted += data_race(per_cpu(rseq_stats.s_aborted, cpu)); 150 } 151 } 152 153 seq_printf(m, "exit: %16lu\n", stats.exit); 154 seq_printf(m, "signal: %16lu\n", stats.signal); 155 seq_printf(m, "slowp: %16lu\n", stats.slowpath); 156 seq_printf(m, "fastp: %16lu\n", stats.fastpath); 157 seq_printf(m, "ids: %16lu\n", stats.ids); 158 seq_printf(m, "cs: %16lu\n", stats.cs); 159 seq_printf(m, "clear: %16lu\n", stats.clear); 160 seq_printf(m, "fixup: %16lu\n", stats.fixup); 161 if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) { 162 seq_printf(m, "sgrant: %16lu\n", stats.s_granted); 163 seq_printf(m, "sexpir: %16lu\n", stats.s_expired); 164 seq_printf(m, "srevok: %16lu\n", stats.s_revoked); 165 seq_printf(m, "syield: %16lu\n", stats.s_yielded); 166 seq_printf(m, "sabort: %16lu\n", stats.s_aborted); 167 } 168 return 0; 169 } 170 171 static int rseq_stats_open(struct inode *inode, struct file *file) 172 { 173 return single_open(file, rseq_stats_show, inode->i_private); 174 } 175 176 static const struct file_operations stat_ops = { 177 .open = rseq_stats_open, 178 .read = seq_read, 179 .llseek = seq_lseek, 180 .release = single_release, 181 }; 182 183 static int __init rseq_stats_init(struct dentry *root_dir) 184 { 185 debugfs_create_file("stats", 0444, root_dir, NULL, &stat_ops); 186 return 0; 187 } 188 #else 189 static inline void rseq_stats_init(struct dentry *root_dir) { } 190 #endif /* CONFIG_RSEQ_STATS */ 191 192 static int rseq_debug_show(struct seq_file *m, void *p) 193 { 194 bool on = static_branch_unlikely(&rseq_debug_enabled); 195 196 seq_printf(m, "%d\n", on); 197 return 0; 198 } 199 200 static ssize_t rseq_debug_write(struct file *file, const char __user *ubuf, 201 size_t count, loff_t *ppos) 202 { 203 bool on; 204 205 if (kstrtobool_from_user(ubuf, count, &on)) 206 return -EINVAL; 207 208 rseq_control_debug(on); 209 return count; 210 } 211 212 static int rseq_debug_open(struct inode *inode, struct file *file) 213 { 214 return single_open(file, rseq_debug_show, inode->i_private); 215 } 216 217 static const struct file_operations debug_ops = { 218 .open = rseq_debug_open, 219 .read = seq_read, 220 .write = rseq_debug_write, 221 .llseek = seq_lseek, 222 .release = single_release, 223 }; 224 225 static void rseq_slice_ext_init(struct dentry *root_dir); 226 227 static int __init rseq_debugfs_init(void) 228 { 229 struct dentry *root_dir = debugfs_create_dir("rseq", NULL); 230 231 debugfs_create_file("debug", 0644, root_dir, NULL, &debug_ops); 232 rseq_stats_init(root_dir); 233 if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) 234 rseq_slice_ext_init(root_dir); 235 return 0; 236 } 237 __initcall(rseq_debugfs_init); 238 239 static bool rseq_handle_cs(struct task_struct *t, struct pt_regs *regs) 240 { 241 struct rseq __user *urseq = t->rseq.usrptr; 242 u64 csaddr; 243 244 scoped_user_read_access(urseq, efault) 245 unsafe_get_user(csaddr, &urseq->rseq_cs, efault); 246 if (likely(!csaddr)) 247 return true; 248 return rseq_update_user_cs(t, regs, csaddr); 249 efault: 250 return false; 251 } 252 253 static void rseq_slowpath_update_usr(struct pt_regs *regs) 254 { 255 /* 256 * Preserve has_rseq and user_irq state. The generic entry code clears 257 * user_irq on the way out, the non-generic entry architectures are not 258 * setting user_irq. 259 */ 260 const struct rseq_event evt_mask = { 261 .has_rseq = RSEQ_HAS_RSEQ_VERSION_MASK, 262 .user_irq = true, 263 }; 264 struct task_struct *t = current; 265 struct rseq_ids ids; 266 u32 node_id; 267 bool event; 268 269 if (unlikely(t->flags & PF_EXITING)) 270 return; 271 272 rseq_stat_inc(rseq_stats.slowpath); 273 274 /* 275 * Read and clear the event pending bit first. If the task 276 * was not preempted or migrated or a signal is on the way, 277 * there is no point in doing any of the heavy lifting here 278 * on production kernels. In that case TIF_NOTIFY_RESUME 279 * was raised by some other functionality. 280 * 281 * This is correct because the read/clear operation is 282 * guarded against scheduler preemption, which makes it CPU 283 * local atomic. If the task is preempted right after 284 * re-enabling preemption then TIF_NOTIFY_RESUME is set 285 * again and this function is invoked another time _before_ 286 * the task is able to return to user mode. 287 * 288 * On a debug kernel, invoke the fixup code unconditionally 289 * with the result handed in to allow the detection of 290 * inconsistencies. 291 */ 292 scoped_guard(irq) { 293 event = t->rseq.event.sched_switch; 294 t->rseq.event.all &= evt_mask.all; 295 ids.cpu_id = task_cpu(t); 296 ids.mm_cid = task_mm_cid(t); 297 } 298 299 if (!event) 300 return; 301 302 node_id = cpu_to_node(ids.cpu_id); 303 304 if (unlikely(!rseq_update_usr(t, regs, &ids, node_id))) { 305 /* 306 * Clear the errors just in case this might survive magically, but 307 * leave the rest intact. 308 */ 309 t->rseq.event.error = 0; 310 force_sig(SIGSEGV); 311 } 312 } 313 314 void __rseq_handle_slowpath(struct pt_regs *regs) 315 { 316 /* 317 * If invoked from hypervisors before entering the guest via 318 * resume_user_mode_work(), then @regs is a NULL pointer. 319 * 320 * resume_user_mode_work() clears TIF_NOTIFY_RESUME and re-raises 321 * it before returning from the ioctl() to user space when 322 * rseq_event.sched_switch is set. 323 * 324 * So it's safe to ignore here instead of pointlessly updating it 325 * in the vcpu_run() loop. 326 */ 327 if (!regs) 328 return; 329 330 rseq_slowpath_update_usr(regs); 331 } 332 333 void __rseq_signal_deliver(int sig, struct pt_regs *regs) 334 { 335 rseq_stat_inc(rseq_stats.signal); 336 337 /* 338 * Don't update IDs yet, they are handled on exit to user if 339 * necessary. The important thing is to abort a critical section of 340 * the interrupted context as after this point the instruction 341 * pointer in @regs points to the signal handler. 342 */ 343 if (unlikely(!rseq_handle_cs(current, regs))) { 344 /* 345 * Clear the errors just in case this might survive 346 * magically, but leave the rest intact. 347 */ 348 current->rseq.event.error = 0; 349 force_sigsegv(sig); 350 } 351 352 /* 353 * In legacy mode, force the update of IDs before returning to user 354 * space to stay compatible. 355 */ 356 if (!rseq_v2(current)) 357 rseq_force_update(); 358 } 359 360 /* 361 * Terminate the process if a syscall is issued within a restartable 362 * sequence. 363 */ 364 void __rseq_debug_syscall_return(struct pt_regs *regs) 365 { 366 struct task_struct *t = current; 367 u64 csaddr; 368 369 if (!t->rseq.event.has_rseq) 370 return; 371 if (get_user(csaddr, &t->rseq.usrptr->rseq_cs)) 372 goto fail; 373 if (likely(!csaddr)) 374 return; 375 if (unlikely(csaddr >= TASK_SIZE)) 376 goto fail; 377 if (rseq_debug_update_user_cs(t, regs, csaddr)) 378 return; 379 fail: 380 force_sig(SIGSEGV); 381 } 382 383 #ifdef CONFIG_DEBUG_RSEQ 384 /* Kept around to keep GENERIC_ENTRY=n architectures supported. */ 385 void rseq_syscall(struct pt_regs *regs) 386 { 387 __rseq_debug_syscall_return(regs); 388 } 389 #endif 390 391 static bool rseq_reset_ids(void) 392 { 393 struct rseq __user *rseq = current->rseq.usrptr; 394 395 /* 396 * If this fails, terminate it because this leaves the kernel in 397 * stupid state as exit to user space will try to fixup the ids 398 * again. 399 */ 400 scoped_user_rw_access(rseq, efault) { 401 unsafe_put_user(0, &rseq->cpu_id_start, efault); 402 unsafe_put_user(RSEQ_CPU_ID_UNINITIALIZED, &rseq->cpu_id, efault); 403 unsafe_put_user(0, &rseq->node_id, efault); 404 unsafe_put_user(0, &rseq->mm_cid, efault); 405 } 406 return true; 407 408 efault: 409 force_sig(SIGSEGV); 410 return false; 411 } 412 413 /* The original rseq structure size (including padding) is 32 bytes. */ 414 #define ORIG_RSEQ_SIZE 32 415 416 /* 417 * sys_rseq - setup restartable sequences for caller thread. 418 */ 419 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, int, flags, u32, sig) 420 { 421 u32 rseqfl = 0; 422 u8 version = 1; 423 424 if (flags & RSEQ_FLAG_UNREGISTER) { 425 if (flags & ~RSEQ_FLAG_UNREGISTER) 426 return -EINVAL; 427 /* Unregister rseq for current thread. */ 428 if (current->rseq.usrptr != rseq || !current->rseq.usrptr) 429 return -EINVAL; 430 if (rseq_len != current->rseq.len) 431 return -EINVAL; 432 if (current->rseq.sig != sig) 433 return -EPERM; 434 if (!rseq_reset_ids()) 435 return -EFAULT; 436 rseq_reset(current); 437 return 0; 438 } 439 440 if (unlikely(flags & ~(RSEQ_FLAG_SLICE_EXT_DEFAULT_ON))) 441 return -EINVAL; 442 443 if (current->rseq.usrptr) { 444 /* 445 * If rseq is already registered, check whether 446 * the provided address differs from the prior 447 * one. 448 */ 449 if (current->rseq.usrptr != rseq || rseq_len != current->rseq.len) 450 return -EINVAL; 451 if (current->rseq.sig != sig) 452 return -EPERM; 453 /* Already registered. */ 454 return -EBUSY; 455 } 456 457 /* 458 * If there was no rseq previously registered, ensure the provided rseq 459 * is properly aligned, as communcated to user-space through the ELF 460 * auxiliary vector AT_RSEQ_ALIGN. If rseq_len is the original rseq 461 * size, the required alignment is the original struct rseq alignment. 462 * 463 * The rseq_len is required to be greater or equal to the original rseq 464 * size. In order to be valid, rseq_len is either the original rseq size, 465 * or large enough to contain all supported fields, as communicated to 466 * user-space through the ELF auxiliary vector AT_RSEQ_FEATURE_SIZE. 467 */ 468 if (rseq_len < ORIG_RSEQ_SIZE || 469 (rseq_len == ORIG_RSEQ_SIZE && !IS_ALIGNED((unsigned long)rseq, ORIG_RSEQ_SIZE)) || 470 (rseq_len != ORIG_RSEQ_SIZE && (!IS_ALIGNED((unsigned long)rseq, rseq_alloc_align()) || 471 rseq_len < offsetof(struct rseq, end)))) 472 return -EINVAL; 473 if (!access_ok(rseq, rseq_len)) 474 return -EFAULT; 475 476 /* 477 * The version check effectivly disables time slice extensions until the 478 * RSEQ ABI V2 registration are implemented. 479 */ 480 if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION) && version > 1) { 481 if (rseq_slice_extension_enabled()) { 482 rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE; 483 if (flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON) 484 rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED; 485 } 486 } 487 488 scoped_user_write_access(rseq, efault) { 489 /* 490 * If the rseq_cs pointer is non-NULL on registration, clear it to 491 * avoid a potential segfault on return to user-space. The proper thing 492 * to do would have been to fail the registration but this would break 493 * older libcs that reuse the rseq area for new threads without 494 * clearing the fields. Don't bother reading it, just reset it. 495 */ 496 unsafe_put_user(0UL, &rseq->rseq_cs, efault); 497 unsafe_put_user(rseqfl, &rseq->flags, efault); 498 /* Initialize IDs in user space */ 499 unsafe_put_user(RSEQ_CPU_ID_UNINITIALIZED, &rseq->cpu_id_start, efault); 500 unsafe_put_user(RSEQ_CPU_ID_UNINITIALIZED, &rseq->cpu_id, efault); 501 unsafe_put_user(0U, &rseq->node_id, efault); 502 unsafe_put_user(0U, &rseq->mm_cid, efault); 503 504 /* 505 * All fields past mm_cid are only valid for non-legacy v2 506 * registrations. 507 */ 508 if (version > 1) { 509 if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) 510 unsafe_put_user(0U, &rseq->slice_ctrl.all, efault); 511 } 512 } 513 514 /* 515 * Activate the registration by setting the rseq area address, length 516 * and signature in the task struct. 517 */ 518 current->rseq.usrptr = rseq; 519 current->rseq.len = rseq_len; 520 current->rseq.sig = sig; 521 522 #ifdef CONFIG_RSEQ_SLICE_EXTENSION 523 current->rseq.slice.state.enabled = !!(rseqfl & RSEQ_CS_FLAG_SLICE_EXT_ENABLED); 524 #endif 525 526 /* 527 * If rseq was previously inactive, and has just been 528 * registered, ensure the cpu_id_start and cpu_id fields 529 * are updated before returning to user-space. 530 */ 531 current->rseq.event.has_rseq = true; 532 rseq_force_update(); 533 return 0; 534 535 efault: 536 return -EFAULT; 537 } 538 539 #ifdef CONFIG_RSEQ_SLICE_EXTENSION 540 struct slice_timer { 541 struct hrtimer timer; 542 void *cookie; 543 }; 544 545 static const unsigned int rseq_slice_ext_nsecs_min = 5 * NSEC_PER_USEC; 546 static const unsigned int rseq_slice_ext_nsecs_max = 50 * NSEC_PER_USEC; 547 unsigned int rseq_slice_ext_nsecs __read_mostly = rseq_slice_ext_nsecs_min; 548 static DEFINE_PER_CPU(struct slice_timer, slice_timer); 549 DEFINE_STATIC_KEY_TRUE(rseq_slice_extension_key); 550 551 /* 552 * When the timer expires and the task is still in user space, the return 553 * from interrupt will revoke the grant and schedule. If the task already 554 * entered the kernel via a syscall and the timer fires before the syscall 555 * work was able to cancel it, then depending on the preemption model this 556 * will either reschedule on return from interrupt or in the syscall work 557 * below. 558 */ 559 static enum hrtimer_restart rseq_slice_expired(struct hrtimer *tmr) 560 { 561 struct slice_timer *st = container_of(tmr, struct slice_timer, timer); 562 563 /* 564 * Validate that the task which armed the timer is still on the 565 * CPU. It could have been scheduled out without canceling the 566 * timer. 567 */ 568 if (st->cookie == current && current->rseq.slice.state.granted) { 569 rseq_stat_inc(rseq_stats.s_expired); 570 set_need_resched_current(); 571 } 572 return HRTIMER_NORESTART; 573 } 574 575 bool __rseq_arm_slice_extension_timer(void) 576 { 577 struct slice_timer *st = this_cpu_ptr(&slice_timer); 578 struct task_struct *curr = current; 579 580 lockdep_assert_irqs_disabled(); 581 582 /* 583 * This check prevents a task, which got a time slice extension 584 * granted, from exceeding the maximum scheduling latency when the 585 * grant expired before going out to user space. Don't bother to 586 * clear the grant here, it will be cleaned up automatically before 587 * going out to user space after being scheduled back in. 588 */ 589 if ((unlikely(curr->rseq.slice.expires < ktime_get_mono_fast_ns()))) { 590 set_need_resched_current(); 591 return true; 592 } 593 594 /* 595 * Store the task pointer as a cookie for comparison in the timer 596 * function. This is safe as the timer is CPU local and cannot be 597 * in the expiry function at this point. 598 */ 599 st->cookie = curr; 600 hrtimer_start(&st->timer, curr->rseq.slice.expires, HRTIMER_MODE_ABS_PINNED_HARD); 601 /* Arm the syscall entry work */ 602 set_task_syscall_work(curr, SYSCALL_RSEQ_SLICE); 603 return false; 604 } 605 606 static void rseq_cancel_slice_extension_timer(void) 607 { 608 struct slice_timer *st = this_cpu_ptr(&slice_timer); 609 610 /* 611 * st->cookie can be safely read as preemption is disabled and the 612 * timer is CPU local. 613 * 614 * As this is most probably the first expiring timer, the cancel is 615 * expensive as it has to reprogram the hardware, but that's less 616 * expensive than going through a full hrtimer_interrupt() cycle 617 * for nothing. 618 * 619 * hrtimer_try_to_cancel() is sufficient here as the timer is CPU 620 * local and once the hrtimer code disabled interrupts the timer 621 * callback cannot be running. 622 */ 623 if (st->cookie == current) 624 hrtimer_try_to_cancel(&st->timer); 625 } 626 627 static inline void rseq_slice_set_need_resched(struct task_struct *curr) 628 { 629 /* 630 * The interrupt guard is required to prevent inconsistent state in 631 * this case: 632 * 633 * set_tsk_need_resched() 634 * --> Interrupt 635 * wakeup() 636 * set_tsk_need_resched() 637 * set_preempt_need_resched() 638 * schedule_on_return() 639 * clear_tsk_need_resched() 640 * clear_preempt_need_resched() 641 * set_preempt_need_resched() <- Inconsistent state 642 * 643 * This is safe vs. a remote set of TIF_NEED_RESCHED because that 644 * only sets the already set bit and does not create inconsistent 645 * state. 646 */ 647 scoped_guard(irq) 648 set_need_resched_current(); 649 } 650 651 static void rseq_slice_validate_ctrl(u32 expected) 652 { 653 u32 __user *sctrl = ¤t->rseq.usrptr->slice_ctrl.all; 654 u32 uval; 655 656 if (get_user(uval, sctrl) || uval != expected) 657 force_sig(SIGSEGV); 658 } 659 660 /* 661 * Invoked from syscall entry if a time slice extension was granted and the 662 * kernel did not clear it before user space left the critical section. 663 * 664 * While the recommended way to relinquish the CPU side effect free is 665 * rseq_slice_yield(2), any syscall within a granted slice terminates the 666 * grant and immediately reschedules if required. This supports onion layer 667 * applications, where the code requesting the grant cannot control the 668 * code within the critical section. 669 */ 670 void rseq_syscall_enter_work(long syscall) 671 { 672 struct task_struct *curr = current; 673 struct rseq_slice_ctrl ctrl = { .granted = curr->rseq.slice.state.granted }; 674 675 clear_task_syscall_work(curr, SYSCALL_RSEQ_SLICE); 676 677 if (static_branch_unlikely(&rseq_debug_enabled)) 678 rseq_slice_validate_ctrl(ctrl.all); 679 680 /* 681 * The kernel might have raced, revoked the grant and updated 682 * userspace, but kept the SLICE work set. 683 */ 684 if (!ctrl.granted) 685 return; 686 687 /* 688 * Required to stabilize the per CPU timer pointer and to make 689 * set_tsk_need_resched() correct on PREEMPT[RT] kernels. 690 * 691 * Leaving the scope will reschedule on preemption models FULL, 692 * LAZY and RT if necessary. 693 */ 694 scoped_guard(preempt) { 695 rseq_cancel_slice_extension_timer(); 696 /* 697 * Now that preemption is disabled, quickly check whether 698 * the task was already rescheduled before arriving here. 699 */ 700 if (!curr->rseq.event.sched_switch) { 701 rseq_slice_set_need_resched(curr); 702 703 if (syscall == __NR_rseq_slice_yield) { 704 rseq_stat_inc(rseq_stats.s_yielded); 705 /* Update the yielded state for syscall return */ 706 curr->rseq.slice.yielded = 1; 707 } else { 708 rseq_stat_inc(rseq_stats.s_aborted); 709 } 710 } 711 } 712 /* Reschedule on NONE/VOLUNTARY preemption models */ 713 cond_resched(); 714 715 /* Clear the grant in kernel state and user space */ 716 curr->rseq.slice.state.granted = false; 717 if (put_user(0U, &curr->rseq.usrptr->slice_ctrl.all)) 718 force_sig(SIGSEGV); 719 } 720 721 int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3) 722 { 723 switch (arg2) { 724 case PR_RSEQ_SLICE_EXTENSION_GET: 725 if (arg3) 726 return -EINVAL; 727 return current->rseq.slice.state.enabled ? PR_RSEQ_SLICE_EXT_ENABLE : 0; 728 729 case PR_RSEQ_SLICE_EXTENSION_SET: { 730 u32 rflags, valid = RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE; 731 bool enable = !!(arg3 & PR_RSEQ_SLICE_EXT_ENABLE); 732 733 if (arg3 & ~PR_RSEQ_SLICE_EXT_ENABLE) 734 return -EINVAL; 735 if (!rseq_slice_extension_enabled()) 736 return -ENOTSUPP; 737 if (!current->rseq.usrptr) 738 return -ENXIO; 739 if (!rseq_v2(current)) 740 return -ENOTSUPP; 741 742 /* No change? */ 743 if (enable == !!current->rseq.slice.state.enabled) 744 return 0; 745 746 if (get_user(rflags, ¤t->rseq.usrptr->flags)) 747 goto die; 748 749 if (current->rseq.slice.state.enabled) 750 valid |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED; 751 752 if ((rflags & valid) != valid) 753 goto die; 754 755 rflags &= ~RSEQ_CS_FLAG_SLICE_EXT_ENABLED; 756 rflags |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE; 757 if (enable) 758 rflags |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED; 759 760 if (put_user(rflags, ¤t->rseq.usrptr->flags)) 761 goto die; 762 763 current->rseq.slice.state.enabled = enable; 764 return 0; 765 } 766 default: 767 return -EINVAL; 768 } 769 die: 770 force_sig(SIGSEGV); 771 return -EFAULT; 772 } 773 774 /** 775 * sys_rseq_slice_yield - yield the current processor side effect free if a 776 * task granted with a time slice extension is done with 777 * the critical work before being forced out. 778 * 779 * Return: 1 if the task successfully yielded the CPU within the granted slice. 780 * 0 if the slice extension was either never granted or was revoked by 781 * going over the granted extension, using a syscall other than this one 782 * or being scheduled out earlier due to a subsequent interrupt. 783 * 784 * The syscall does not schedule because the syscall entry work immediately 785 * relinquishes the CPU and schedules if required. 786 */ 787 SYSCALL_DEFINE0(rseq_slice_yield) 788 { 789 int yielded = !!current->rseq.slice.yielded; 790 791 current->rseq.slice.yielded = 0; 792 return yielded; 793 } 794 795 static int rseq_slice_ext_show(struct seq_file *m, void *p) 796 { 797 seq_printf(m, "%d\n", rseq_slice_ext_nsecs); 798 return 0; 799 } 800 801 static ssize_t rseq_slice_ext_write(struct file *file, const char __user *ubuf, 802 size_t count, loff_t *ppos) 803 { 804 unsigned int nsecs; 805 806 if (kstrtouint_from_user(ubuf, count, 10, &nsecs)) 807 return -EINVAL; 808 809 if (nsecs < rseq_slice_ext_nsecs_min) 810 return -ERANGE; 811 812 if (nsecs > rseq_slice_ext_nsecs_max) 813 return -ERANGE; 814 815 rseq_slice_ext_nsecs = nsecs; 816 817 return count; 818 } 819 820 static int rseq_slice_ext_open(struct inode *inode, struct file *file) 821 { 822 return single_open(file, rseq_slice_ext_show, inode->i_private); 823 } 824 825 static const struct file_operations slice_ext_ops = { 826 .open = rseq_slice_ext_open, 827 .read = seq_read, 828 .write = rseq_slice_ext_write, 829 .llseek = seq_lseek, 830 .release = single_release, 831 }; 832 833 static void rseq_slice_ext_init(struct dentry *root_dir) 834 { 835 debugfs_create_file("slice_ext_nsec", 0644, root_dir, NULL, &slice_ext_ops); 836 } 837 838 static int __init rseq_slice_cmdline(char *str) 839 { 840 bool on; 841 842 if (kstrtobool(str, &on)) 843 return 0; 844 845 if (!on) 846 static_branch_disable(&rseq_slice_extension_key); 847 return 1; 848 } 849 __setup("rseq_slice_ext=", rseq_slice_cmdline); 850 851 static int __init rseq_slice_init(void) 852 { 853 unsigned int cpu; 854 855 for_each_possible_cpu(cpu) { 856 hrtimer_setup(per_cpu_ptr(&slice_timer.timer, cpu), rseq_slice_expired, 857 CLOCK_MONOTONIC, HRTIMER_MODE_REL_PINNED_HARD); 858 } 859 return 0; 860 } 861 device_initcall(rseq_slice_init); 862 #else 863 static void rseq_slice_ext_init(struct dentry *root_dir) { } 864 #endif /* CONFIG_RSEQ_SLICE_EXTENSION */ 865