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 int rseq_need_restart(struct task_struct *t, u32 cs_flags) 175 { 176 u32 flags, event_mask; 177 int ret; 178 179 if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS) || cs_flags) 180 return -EINVAL; 181 182 /* Get thread flags. */ 183 ret = get_user(flags, &t->rseq->flags); 184 if (ret) 185 return ret; 186 187 if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS) || flags) 188 return -EINVAL; 189 190 /* 191 * Load and clear event mask atomically with respect to 192 * scheduler preemption. 193 */ 194 preempt_disable(); 195 event_mask = t->rseq_event_mask; 196 t->rseq_event_mask = 0; 197 preempt_enable(); 198 199 return !!event_mask; 200 } 201 202 static int clear_rseq_cs(struct task_struct *t) 203 { 204 /* 205 * The rseq_cs field is set to NULL on preemption or signal 206 * delivery on top of rseq assembly block, as well as on top 207 * of code outside of the rseq assembly block. This performs 208 * a lazy clear of the rseq_cs field. 209 * 210 * Set rseq_cs to NULL. 211 */ 212 #ifdef CONFIG_64BIT 213 return put_user(0UL, &t->rseq->rseq_cs); 214 #else 215 if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs))) 216 return -EFAULT; 217 return 0; 218 #endif 219 } 220 221 /* 222 * Unsigned comparison will be true when ip >= start_ip, and when 223 * ip < start_ip + post_commit_offset. 224 */ 225 static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs) 226 { 227 return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset; 228 } 229 230 static int rseq_ip_fixup(struct pt_regs *regs) 231 { 232 unsigned long ip = instruction_pointer(regs); 233 struct task_struct *t = current; 234 struct rseq_cs rseq_cs; 235 int ret; 236 237 ret = rseq_get_rseq_cs(t, &rseq_cs); 238 if (ret) 239 return ret; 240 241 /* 242 * Handle potentially not being within a critical section. 243 * If not nested over a rseq critical section, restart is useless. 244 * Clear the rseq_cs pointer and return. 245 */ 246 if (!in_rseq_cs(ip, &rseq_cs)) 247 return clear_rseq_cs(t); 248 ret = rseq_need_restart(t, rseq_cs.flags); 249 if (ret <= 0) 250 return ret; 251 ret = clear_rseq_cs(t); 252 if (ret) 253 return ret; 254 trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset, 255 rseq_cs.abort_ip); 256 instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip); 257 return 0; 258 } 259 260 /* 261 * This resume handler must always be executed between any of: 262 * - preemption, 263 * - signal delivery, 264 * and return to user-space. 265 * 266 * This is how we can ensure that the entire rseq critical section 267 * will issue the commit instruction only if executed atomically with 268 * respect to other threads scheduled on the same CPU, and with respect 269 * to signal handlers. 270 */ 271 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs) 272 { 273 struct task_struct *t = current; 274 int ret, sig; 275 276 if (unlikely(t->flags & PF_EXITING)) 277 return; 278 279 /* 280 * regs is NULL if and only if the caller is in a syscall path. Skip 281 * fixup and leave rseq_cs as is so that rseq_sycall() will detect and 282 * kill a misbehaving userspace on debug kernels. 283 */ 284 if (regs) { 285 ret = rseq_ip_fixup(regs); 286 if (unlikely(ret < 0)) 287 goto error; 288 } 289 if (unlikely(rseq_update_cpu_id(t))) 290 goto error; 291 return; 292 293 error: 294 sig = ksig ? ksig->sig : 0; 295 force_sigsegv(sig); 296 } 297 298 #ifdef CONFIG_DEBUG_RSEQ 299 300 /* 301 * Terminate the process if a syscall is issued within a restartable 302 * sequence. 303 */ 304 void rseq_syscall(struct pt_regs *regs) 305 { 306 unsigned long ip = instruction_pointer(regs); 307 struct task_struct *t = current; 308 struct rseq_cs rseq_cs; 309 310 if (!t->rseq) 311 return; 312 if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs)) 313 force_sig(SIGSEGV); 314 } 315 316 #endif 317 318 /* 319 * sys_rseq - setup restartable sequences for caller thread. 320 */ 321 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, 322 int, flags, u32, sig) 323 { 324 int ret; 325 326 if (flags & RSEQ_FLAG_UNREGISTER) { 327 if (flags & ~RSEQ_FLAG_UNREGISTER) 328 return -EINVAL; 329 /* Unregister rseq for current thread. */ 330 if (current->rseq != rseq || !current->rseq) 331 return -EINVAL; 332 if (rseq_len != sizeof(*rseq)) 333 return -EINVAL; 334 if (current->rseq_sig != sig) 335 return -EPERM; 336 ret = rseq_reset_rseq_cpu_id(current); 337 if (ret) 338 return ret; 339 current->rseq = NULL; 340 current->rseq_sig = 0; 341 return 0; 342 } 343 344 if (unlikely(flags)) 345 return -EINVAL; 346 347 if (current->rseq) { 348 /* 349 * If rseq is already registered, check whether 350 * the provided address differs from the prior 351 * one. 352 */ 353 if (current->rseq != rseq || rseq_len != sizeof(*rseq)) 354 return -EINVAL; 355 if (current->rseq_sig != sig) 356 return -EPERM; 357 /* Already registered. */ 358 return -EBUSY; 359 } 360 361 /* 362 * If there was no rseq previously registered, 363 * ensure the provided rseq is properly aligned and valid. 364 */ 365 if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) || 366 rseq_len != sizeof(*rseq)) 367 return -EINVAL; 368 if (!access_ok(rseq, rseq_len)) 369 return -EFAULT; 370 current->rseq = rseq; 371 current->rseq_sig = sig; 372 /* 373 * If rseq was previously inactive, and has just been 374 * registered, ensure the cpu_id_start and cpu_id fields 375 * are updated before returning to user-space. 376 */ 377 rseq_set_notify_resume(current); 378 379 return 0; 380 } 381