1 // SPDX-License-Identifier: GPL-2.0+ 2 /* 3 * Restartable sequences system call 4 * 5 * Copyright (C) 2015, Google, Inc., 6 * Paul Turner <[email protected]> and Andrew Hunter <[email protected]> 7 * Copyright (C) 2015-2018, EfficiOS Inc., 8 * Mathieu Desnoyers <[email protected]> 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 if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr))) 131 return -EFAULT; 132 if (!ptr) { 133 memset(rseq_cs, 0, sizeof(*rseq_cs)); 134 return 0; 135 } 136 if (ptr >= TASK_SIZE) 137 return -EINVAL; 138 urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr; 139 if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs))) 140 return -EFAULT; 141 142 if (rseq_cs->start_ip >= TASK_SIZE || 143 rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE || 144 rseq_cs->abort_ip >= TASK_SIZE || 145 rseq_cs->version > 0) 146 return -EINVAL; 147 /* Check for overflow. */ 148 if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip) 149 return -EINVAL; 150 /* Ensure that abort_ip is not in the critical section. */ 151 if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset) 152 return -EINVAL; 153 154 usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32)); 155 ret = get_user(sig, usig); 156 if (ret) 157 return ret; 158 159 if (current->rseq_sig != sig) { 160 printk_ratelimited(KERN_WARNING 161 "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n", 162 sig, current->rseq_sig, current->pid, usig); 163 return -EINVAL; 164 } 165 return 0; 166 } 167 168 static int rseq_need_restart(struct task_struct *t, u32 cs_flags) 169 { 170 u32 flags, event_mask; 171 int ret; 172 173 /* Get thread flags. */ 174 ret = get_user(flags, &t->rseq->flags); 175 if (ret) 176 return ret; 177 178 /* Take critical section flags into account. */ 179 flags |= cs_flags; 180 181 /* 182 * Restart on signal can only be inhibited when restart on 183 * preempt and restart on migrate are inhibited too. Otherwise, 184 * a preempted signal handler could fail to restart the prior 185 * execution context on sigreturn. 186 */ 187 if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) && 188 (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) != 189 RSEQ_CS_PREEMPT_MIGRATE_FLAGS)) 190 return -EINVAL; 191 192 /* 193 * Load and clear event mask atomically with respect to 194 * scheduler preemption. 195 */ 196 preempt_disable(); 197 event_mask = t->rseq_event_mask; 198 t->rseq_event_mask = 0; 199 preempt_enable(); 200 201 return !!(event_mask & ~flags); 202 } 203 204 static int clear_rseq_cs(struct task_struct *t) 205 { 206 /* 207 * The rseq_cs field is set to NULL on preemption or signal 208 * delivery on top of rseq assembly block, as well as on top 209 * of code outside of the rseq assembly block. This performs 210 * a lazy clear of the rseq_cs field. 211 * 212 * Set rseq_cs to NULL. 213 */ 214 if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64))) 215 return -EFAULT; 216 return 0; 217 } 218 219 /* 220 * Unsigned comparison will be true when ip >= start_ip, and when 221 * ip < start_ip + post_commit_offset. 222 */ 223 static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs) 224 { 225 return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset; 226 } 227 228 static int rseq_ip_fixup(struct pt_regs *regs) 229 { 230 unsigned long ip = instruction_pointer(regs); 231 struct task_struct *t = current; 232 struct rseq_cs rseq_cs; 233 int ret; 234 235 ret = rseq_get_rseq_cs(t, &rseq_cs); 236 if (ret) 237 return ret; 238 239 /* 240 * Handle potentially not being within a critical section. 241 * If not nested over a rseq critical section, restart is useless. 242 * Clear the rseq_cs pointer and return. 243 */ 244 if (!in_rseq_cs(ip, &rseq_cs)) 245 return clear_rseq_cs(t); 246 ret = rseq_need_restart(t, rseq_cs.flags); 247 if (ret <= 0) 248 return ret; 249 ret = clear_rseq_cs(t); 250 if (ret) 251 return ret; 252 trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset, 253 rseq_cs.abort_ip); 254 instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip); 255 return 0; 256 } 257 258 /* 259 * This resume handler must always be executed between any of: 260 * - preemption, 261 * - signal delivery, 262 * and return to user-space. 263 * 264 * This is how we can ensure that the entire rseq critical section 265 * will issue the commit instruction only if executed atomically with 266 * respect to other threads scheduled on the same CPU, and with respect 267 * to signal handlers. 268 */ 269 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs) 270 { 271 struct task_struct *t = current; 272 int ret, sig; 273 274 if (unlikely(t->flags & PF_EXITING)) 275 return; 276 if (unlikely(!access_ok(t->rseq, sizeof(*t->rseq)))) 277 goto error; 278 ret = rseq_ip_fixup(regs); 279 if (unlikely(ret < 0)) 280 goto error; 281 if (unlikely(rseq_update_cpu_id(t))) 282 goto error; 283 return; 284 285 error: 286 sig = ksig ? ksig->sig : 0; 287 force_sigsegv(sig); 288 } 289 290 #ifdef CONFIG_DEBUG_RSEQ 291 292 /* 293 * Terminate the process if a syscall is issued within a restartable 294 * sequence. 295 */ 296 void rseq_syscall(struct pt_regs *regs) 297 { 298 unsigned long ip = instruction_pointer(regs); 299 struct task_struct *t = current; 300 struct rseq_cs rseq_cs; 301 302 if (!t->rseq) 303 return; 304 if (!access_ok(t->rseq, sizeof(*t->rseq)) || 305 rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs)) 306 force_sig(SIGSEGV); 307 } 308 309 #endif 310 311 /* 312 * sys_rseq - setup restartable sequences for caller thread. 313 */ 314 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, 315 int, flags, u32, sig) 316 { 317 int ret; 318 319 if (flags & RSEQ_FLAG_UNREGISTER) { 320 if (flags & ~RSEQ_FLAG_UNREGISTER) 321 return -EINVAL; 322 /* Unregister rseq for current thread. */ 323 if (current->rseq != rseq || !current->rseq) 324 return -EINVAL; 325 if (rseq_len != sizeof(*rseq)) 326 return -EINVAL; 327 if (current->rseq_sig != sig) 328 return -EPERM; 329 ret = rseq_reset_rseq_cpu_id(current); 330 if (ret) 331 return ret; 332 current->rseq = NULL; 333 current->rseq_sig = 0; 334 return 0; 335 } 336 337 if (unlikely(flags)) 338 return -EINVAL; 339 340 if (current->rseq) { 341 /* 342 * If rseq is already registered, check whether 343 * the provided address differs from the prior 344 * one. 345 */ 346 if (current->rseq != rseq || rseq_len != sizeof(*rseq)) 347 return -EINVAL; 348 if (current->rseq_sig != sig) 349 return -EPERM; 350 /* Already registered. */ 351 return -EBUSY; 352 } 353 354 /* 355 * If there was no rseq previously registered, 356 * ensure the provided rseq is properly aligned and valid. 357 */ 358 if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) || 359 rseq_len != sizeof(*rseq)) 360 return -EINVAL; 361 if (!access_ok(rseq, rseq_len)) 362 return -EFAULT; 363 current->rseq = rseq; 364 current->rseq_sig = sig; 365 /* 366 * If rseq was previously inactive, and has just been 367 * registered, ensure the cpu_id_start and cpu_id fields 368 * are updated before returning to user-space. 369 */ 370 rseq_set_notify_resume(current); 371 372 return 0; 373 } 374