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 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 } 102 103 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 } 121 122 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.ptr64)) 132 return -EFAULT; 133 #else 134 if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, 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 } 172 173 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 } 208 209 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.ptr64); 221 #else 222 if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64))) 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 */ 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 } 236 237 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 */ 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 ret = rseq_ip_fixup(regs); 286 if (unlikely(ret < 0)) 287 goto error; 288 if (unlikely(rseq_update_cpu_id(t))) 289 goto error; 290 return; 291 292 error: 293 sig = ksig ? ksig->sig : 0; 294 force_sigsegv(sig); 295 } 296 297 #ifdef CONFIG_DEBUG_RSEQ 298 299 /* 300 * Terminate the process if a syscall is issued within a restartable 301 * sequence. 302 */ 303 void rseq_syscall(struct pt_regs *regs) 304 { 305 unsigned long ip = instruction_pointer(regs); 306 struct task_struct *t = current; 307 struct rseq_cs rseq_cs; 308 309 if (!t->rseq) 310 return; 311 if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs)) 312 force_sig(SIGSEGV); 313 } 314 315 #endif 316 317 /* 318 * sys_rseq - setup restartable sequences for caller thread. 319 */ 320 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, 321 int, flags, u32, sig) 322 { 323 int ret; 324 325 if (flags & RSEQ_FLAG_UNREGISTER) { 326 if (flags & ~RSEQ_FLAG_UNREGISTER) 327 return -EINVAL; 328 /* Unregister rseq for current thread. */ 329 if (current->rseq != rseq || !current->rseq) 330 return -EINVAL; 331 if (rseq_len != sizeof(*rseq)) 332 return -EINVAL; 333 if (current->rseq_sig != sig) 334 return -EPERM; 335 ret = rseq_reset_rseq_cpu_id(current); 336 if (ret) 337 return ret; 338 current->rseq = NULL; 339 current->rseq_sig = 0; 340 return 0; 341 } 342 343 if (unlikely(flags)) 344 return -EINVAL; 345 346 if (current->rseq) { 347 /* 348 * If rseq is already registered, check whether 349 * the provided address differs from the prior 350 * one. 351 */ 352 if (current->rseq != rseq || rseq_len != sizeof(*rseq)) 353 return -EINVAL; 354 if (current->rseq_sig != sig) 355 return -EPERM; 356 /* Already registered. */ 357 return -EBUSY; 358 } 359 360 /* 361 * If there was no rseq previously registered, 362 * ensure the provided rseq is properly aligned and valid. 363 */ 364 if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) || 365 rseq_len != sizeof(*rseq)) 366 return -EINVAL; 367 if (!access_ok(rseq, rseq_len)) 368 return -EFAULT; 369 current->rseq = rseq; 370 current->rseq_sig = sig; 371 /* 372 * If rseq was previously inactive, and has just been 373 * registered, ensure the cpu_id_start and cpu_id fields 374 * are updated before returning to user-space. 375 */ 376 rseq_set_notify_resume(current); 377 378 return 0; 379 } 380