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