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 88 if (put_user(cpu_id, &t->rseq->cpu_id_start)) 89 return -EFAULT; 90 if (put_user(cpu_id, &t->rseq->cpu_id)) 91 return -EFAULT; 92 trace_rseq_update(t); 93 return 0; 94 } 95 96 static int rseq_reset_rseq_cpu_id(struct task_struct *t) 97 { 98 u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED; 99 100 /* 101 * Reset cpu_id_start to its initial state (0). 102 */ 103 if (put_user(cpu_id_start, &t->rseq->cpu_id_start)) 104 return -EFAULT; 105 /* 106 * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming 107 * in after unregistration can figure out that rseq needs to be 108 * registered again. 109 */ 110 if (put_user(cpu_id, &t->rseq->cpu_id)) 111 return -EFAULT; 112 return 0; 113 } 114 115 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs) 116 { 117 struct rseq_cs __user *urseq_cs; 118 u64 ptr; 119 u32 __user *usig; 120 u32 sig; 121 int ret; 122 123 if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr))) 124 return -EFAULT; 125 if (!ptr) { 126 memset(rseq_cs, 0, sizeof(*rseq_cs)); 127 return 0; 128 } 129 if (ptr >= TASK_SIZE) 130 return -EINVAL; 131 urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr; 132 if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs))) 133 return -EFAULT; 134 135 if (rseq_cs->start_ip >= TASK_SIZE || 136 rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE || 137 rseq_cs->abort_ip >= TASK_SIZE || 138 rseq_cs->version > 0) 139 return -EINVAL; 140 /* Check for overflow. */ 141 if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip) 142 return -EINVAL; 143 /* Ensure that abort_ip is not in the critical section. */ 144 if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset) 145 return -EINVAL; 146 147 usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32)); 148 ret = get_user(sig, usig); 149 if (ret) 150 return ret; 151 152 if (current->rseq_sig != sig) { 153 printk_ratelimited(KERN_WARNING 154 "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n", 155 sig, current->rseq_sig, current->pid, usig); 156 return -EINVAL; 157 } 158 return 0; 159 } 160 161 static int rseq_need_restart(struct task_struct *t, u32 cs_flags) 162 { 163 u32 flags, event_mask; 164 int ret; 165 166 /* Get thread flags. */ 167 ret = get_user(flags, &t->rseq->flags); 168 if (ret) 169 return ret; 170 171 /* Take critical section flags into account. */ 172 flags |= cs_flags; 173 174 /* 175 * Restart on signal can only be inhibited when restart on 176 * preempt and restart on migrate are inhibited too. Otherwise, 177 * a preempted signal handler could fail to restart the prior 178 * execution context on sigreturn. 179 */ 180 if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) && 181 (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) != 182 RSEQ_CS_PREEMPT_MIGRATE_FLAGS)) 183 return -EINVAL; 184 185 /* 186 * Load and clear event mask atomically with respect to 187 * scheduler preemption. 188 */ 189 preempt_disable(); 190 event_mask = t->rseq_event_mask; 191 t->rseq_event_mask = 0; 192 preempt_enable(); 193 194 return !!(event_mask & ~flags); 195 } 196 197 static int clear_rseq_cs(struct task_struct *t) 198 { 199 /* 200 * The rseq_cs field is set to NULL on preemption or signal 201 * delivery on top of rseq assembly block, as well as on top 202 * of code outside of the rseq assembly block. This performs 203 * a lazy clear of the rseq_cs field. 204 * 205 * Set rseq_cs to NULL. 206 */ 207 if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64))) 208 return -EFAULT; 209 return 0; 210 } 211 212 /* 213 * Unsigned comparison will be true when ip >= start_ip, and when 214 * ip < start_ip + post_commit_offset. 215 */ 216 static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs) 217 { 218 return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset; 219 } 220 221 static int rseq_ip_fixup(struct pt_regs *regs) 222 { 223 unsigned long ip = instruction_pointer(regs); 224 struct task_struct *t = current; 225 struct rseq_cs rseq_cs; 226 int ret; 227 228 ret = rseq_get_rseq_cs(t, &rseq_cs); 229 if (ret) 230 return ret; 231 232 /* 233 * Handle potentially not being within a critical section. 234 * If not nested over a rseq critical section, restart is useless. 235 * Clear the rseq_cs pointer and return. 236 */ 237 if (!in_rseq_cs(ip, &rseq_cs)) 238 return clear_rseq_cs(t); 239 ret = rseq_need_restart(t, rseq_cs.flags); 240 if (ret <= 0) 241 return ret; 242 ret = clear_rseq_cs(t); 243 if (ret) 244 return ret; 245 trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset, 246 rseq_cs.abort_ip); 247 instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip); 248 return 0; 249 } 250 251 /* 252 * This resume handler must always be executed between any of: 253 * - preemption, 254 * - signal delivery, 255 * and return to user-space. 256 * 257 * This is how we can ensure that the entire rseq critical section, 258 * consisting of both the C part and the assembly instruction sequence, 259 * will issue the commit instruction only if executed atomically with 260 * respect to other threads scheduled on the same CPU, and with respect 261 * to signal handlers. 262 */ 263 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs) 264 { 265 struct task_struct *t = current; 266 int ret, sig; 267 268 if (unlikely(t->flags & PF_EXITING)) 269 return; 270 if (unlikely(!access_ok(VERIFY_WRITE, t->rseq, sizeof(*t->rseq)))) 271 goto error; 272 ret = rseq_ip_fixup(regs); 273 if (unlikely(ret < 0)) 274 goto error; 275 if (unlikely(rseq_update_cpu_id(t))) 276 goto error; 277 return; 278 279 error: 280 sig = ksig ? ksig->sig : 0; 281 force_sigsegv(sig, t); 282 } 283 284 #ifdef CONFIG_DEBUG_RSEQ 285 286 /* 287 * Terminate the process if a syscall is issued within a restartable 288 * sequence. 289 */ 290 void rseq_syscall(struct pt_regs *regs) 291 { 292 unsigned long ip = instruction_pointer(regs); 293 struct task_struct *t = current; 294 struct rseq_cs rseq_cs; 295 296 if (!t->rseq) 297 return; 298 if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) || 299 rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs)) 300 force_sig(SIGSEGV, t); 301 } 302 303 #endif 304 305 /* 306 * sys_rseq - setup restartable sequences for caller thread. 307 */ 308 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, 309 int, flags, u32, sig) 310 { 311 int ret; 312 313 if (flags & RSEQ_FLAG_UNREGISTER) { 314 /* Unregister rseq for current thread. */ 315 if (current->rseq != rseq || !current->rseq) 316 return -EINVAL; 317 if (current->rseq_len != rseq_len) 318 return -EINVAL; 319 if (current->rseq_sig != sig) 320 return -EPERM; 321 ret = rseq_reset_rseq_cpu_id(current); 322 if (ret) 323 return ret; 324 current->rseq = NULL; 325 current->rseq_len = 0; 326 current->rseq_sig = 0; 327 return 0; 328 } 329 330 if (unlikely(flags)) 331 return -EINVAL; 332 333 if (current->rseq) { 334 /* 335 * If rseq is already registered, check whether 336 * the provided address differs from the prior 337 * one. 338 */ 339 if (current->rseq != rseq || current->rseq_len != rseq_len) 340 return -EINVAL; 341 if (current->rseq_sig != sig) 342 return -EPERM; 343 /* Already registered. */ 344 return -EBUSY; 345 } 346 347 /* 348 * If there was no rseq previously registered, 349 * ensure the provided rseq is properly aligned and valid. 350 */ 351 if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) || 352 rseq_len != sizeof(*rseq)) 353 return -EINVAL; 354 if (!access_ok(VERIFY_WRITE, rseq, rseq_len)) 355 return -EFAULT; 356 current->rseq = rseq; 357 current->rseq_len = rseq_len; 358 current->rseq_sig = sig; 359 /* 360 * If rseq was previously inactive, and has just been 361 * registered, ensure the cpu_id_start and cpu_id fields 362 * are updated before returning to user-space. 363 */ 364 rseq_set_notify_resume(current); 365 366 return 0; 367 } 368