kernel/rseq.c

   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 /* The original rseq structure size (including padding) is 32 bytes. */
  22 #define ORIG_RSEQ_SIZE          32
  23
  24 #define RSEQ_CS_NO_RESTART_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT | \
  25                                   RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
  26                                   RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)
  27
  28 /*
  29  *
  30  * Restartable sequences are a lightweight interface that allows
  31  * user-level code to be executed atomically relative to scheduler
  32  * preemption and signal delivery. Typically used for implementing
  33  * per-cpu operations.
  34  *
  35  * It allows user-space to perform update operations on per-cpu data
  36  * without requiring heavy-weight atomic operations.
  37  *
  38  * Detailed algorithm of rseq user-space assembly sequences:
  39  *
  40  *                     init(rseq_cs)
  41  *                     cpu = TLS->rseq::cpu_id_start
  42  *   [1]               TLS->rseq::rseq_cs = rseq_cs
  43  *   [start_ip]        ----------------------------
  44  *   [2]               if (cpu != TLS->rseq::cpu_id)
  45  *                             goto abort_ip;
  46  *   [3]               <last_instruction_in_cs>
  47  *   [post_commit_ip]  ----------------------------
  48  *
  49  *   The address of jump target abort_ip must be outside the critical
  50  *   region, i.e.:
  51  *
  52  *     [abort_ip] < [start_ip]  || [abort_ip] >= [post_commit_ip]
  53  *
  54  *   Steps [2]-[3] (inclusive) need to be a sequence of instructions in
  55  *   userspace that can handle being interrupted between any of those
  56  *   instructions, and then resumed to the abort_ip.
  57  *
  58  *   1.  Userspace stores the address of the struct rseq_cs assembly
  59  *       block descriptor into the rseq_cs field of the registered
  60  *       struct rseq TLS area. This update is performed through a single
  61  *       store within the inline assembly instruction sequence.
  62  *       [start_ip]
  63  *
  64  *   2.  Userspace tests to check whether the current cpu_id field match
  65  *       the cpu number loaded before start_ip, branching to abort_ip
  66  *       in case of a mismatch.
  67  *
  68  *       If the sequence is preempted or interrupted by a signal
  69  *       at or after start_ip and before post_commit_ip, then the kernel
  70  *       clears TLS->__rseq_abi::rseq_cs, and sets the user-space return
  71  *       ip to abort_ip before returning to user-space, so the preempted
  72  *       execution resumes at abort_ip.
  73  *
  74  *   3.  Userspace critical section final instruction before
  75  *       post_commit_ip is the commit. The critical section is
  76  *       self-terminating.
  77  *       [post_commit_ip]
  78  *
  79  *   4.  <success>
  80  *
  81  *   On failure at [2], or if interrupted by preempt or signal delivery
  82  *   between [1] and [3]:
  83  *
  84  *       [abort_ip]
  85  *   F1. <failure>
  86  */
  87
  88 static int rseq_update_cpu_node_id(struct task_struct *t)
  89 {
  90         struct rseq __user *rseq = t->rseq;
  91         u32 cpu_id = raw_smp_processor_id();
  92         u32 node_id = cpu_to_node(cpu_id);
  93         u32 mm_cid = task_mm_cid(t);
  94
  95         WARN_ON_ONCE((int) mm_cid < 0);
  96         if (!user_write_access_begin(rseq, t->rseq_len))
  97                 goto efault;
  98         unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
  99         unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
 100         unsafe_put_user(node_id, &rseq->node_id, efault_end);
 101         unsafe_put_user(mm_cid, &rseq->mm_cid, efault_end);
 102         /*
 103          * Additional feature fields added after ORIG_RSEQ_SIZE
 104          * need to be conditionally updated only if
 105          * t->rseq_len != ORIG_RSEQ_SIZE.
 106          */
 107         user_write_access_end();
 108         trace_rseq_update(t);
 109         return 0;
 110
 111 efault_end:
 112         user_write_access_end();
 113 efault:
 114         return -EFAULT;
 115 }
 116
 117 static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
 118 {
 119         u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED, node_id = 0,
 120             mm_cid = 0;
 121
 122         /*
 123          * Reset cpu_id_start to its initial state (0).
 124          */
 125         if (put_user(cpu_id_start, &t->rseq->cpu_id_start))
 126                 return -EFAULT;
 127         /*
 128          * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
 129          * in after unregistration can figure out that rseq needs to be
 130          * registered again.
 131          */
 132         if (put_user(cpu_id, &t->rseq->cpu_id))
 133                 return -EFAULT;
 134         /*
 135          * Reset node_id to its initial state (0).
 136          */
 137         if (put_user(node_id, &t->rseq->node_id))
 138                 return -EFAULT;
 139         /*
 140          * Reset mm_cid to its initial state (0).
 141          */
 142         if (put_user(mm_cid, &t->rseq->mm_cid))
 143                 return -EFAULT;
 144         /*
 145          * Additional feature fields added after ORIG_RSEQ_SIZE
 146          * need to be conditionally reset only if
 147          * t->rseq_len != ORIG_RSEQ_SIZE.
 148          */
 149         return 0;
 150 }
 151
 152 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
 153 {
 154         struct rseq_cs __user *urseq_cs;
 155         u64 ptr;
 156         u32 __user *usig;
 157         u32 sig;
 158         int ret;
 159
 160 #ifdef CONFIG_64BIT
 161         if (get_user(ptr, &t->rseq->rseq_cs))
 162                 return -EFAULT;
 163 #else
 164         if (copy_from_user(&ptr, &t->rseq->rseq_cs, sizeof(ptr)))
 165                 return -EFAULT;
 166 #endif
 167         if (!ptr) {
 168                 memset(rseq_cs, 0, sizeof(*rseq_cs));
 169                 return 0;
 170         }
 171         if (ptr >= TASK_SIZE)
 172                 return -EINVAL;
 173         urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
 174         if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
 175                 return -EFAULT;
 176
 177         if (rseq_cs->start_ip >= TASK_SIZE ||
 178             rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
 179             rseq_cs->abort_ip >= TASK_SIZE ||
 180             rseq_cs->version > 0)
 181                 return -EINVAL;
 182         /* Check for overflow. */
 183         if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
 184                 return -EINVAL;
 185         /* Ensure that abort_ip is not in the critical section. */
 186         if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
 187                 return -EINVAL;
 188
 189         usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
 190         ret = get_user(sig, usig);
 191         if (ret)
 192                 return ret;
 193
 194         if (current->rseq_sig != sig) {
 195                 printk_ratelimited(KERN_WARNING
 196                         "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
 197                         sig, current->rseq_sig, current->pid, usig);
 198                 return -EINVAL;
 199         }
 200         return 0;
 201 }
 202
 203 static bool rseq_warn_flags(const char *str, u32 flags)
 204 {
 205         u32 test_flags;
 206
 207         if (!flags)
 208                 return false;
 209         test_flags = flags & RSEQ_CS_NO_RESTART_FLAGS;
 210         if (test_flags)
 211                 pr_warn_once("Deprecated flags (%u) in %s ABI structure", test_flags, str);
 212         test_flags = flags & ~RSEQ_CS_NO_RESTART_FLAGS;
 213         if (test_flags)
 214                 pr_warn_once("Unknown flags (%u) in %s ABI structure", test_flags, str);
 215         return true;
 216 }
 217
 218 static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
 219 {
 220         u32 flags, event_mask;
 221         int ret;
 222
 223         if (rseq_warn_flags("rseq_cs", cs_flags))
 224                 return -EINVAL;
 225
 226         /* Get thread flags. */
 227         ret = get_user(flags, &t->rseq->flags);
 228         if (ret)
 229                 return ret;
 230
 231         if (rseq_warn_flags("rseq", flags))
 232                 return -EINVAL;
 233
 234         /*
 235          * Load and clear event mask atomically with respect to
 236          * scheduler preemption.
 237          */
 238         preempt_disable();
 239         event_mask = t->rseq_event_mask;
 240         t->rseq_event_mask = 0;
 241         preempt_enable();
 242
 243         return !!event_mask;
 244 }
 245
 246 static int clear_rseq_cs(struct task_struct *t)
 247 {
 248         /*
 249          * The rseq_cs field is set to NULL on preemption or signal
 250          * delivery on top of rseq assembly block, as well as on top
 251          * of code outside of the rseq assembly block. This performs
 252          * a lazy clear of the rseq_cs field.
 253          *
 254          * Set rseq_cs to NULL.
 255          */
 256 #ifdef CONFIG_64BIT
 257         return put_user(0UL, &t->rseq->rseq_cs);
 258 #else
 259         if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
 260                 return -EFAULT;
 261         return 0;
 262 #endif
 263 }
 264
 265 /*
 266  * Unsigned comparison will be true when ip >= start_ip, and when
 267  * ip < start_ip + post_commit_offset.
 268  */
 269 static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
 270 {
 271         return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
 272 }
 273
 274 static int rseq_ip_fixup(struct pt_regs *regs)
 275 {
 276         unsigned long ip = instruction_pointer(regs);
 277         struct task_struct *t = current;
 278         struct rseq_cs rseq_cs;
 279         int ret;
 280
 281         ret = rseq_get_rseq_cs(t, &rseq_cs);
 282         if (ret)
 283                 return ret;
 284
 285         /*
 286          * Handle potentially not being within a critical section.
 287          * If not nested over a rseq critical section, restart is useless.
 288          * Clear the rseq_cs pointer and return.
 289          */
 290         if (!in_rseq_cs(ip, &rseq_cs))
 291                 return clear_rseq_cs(t);
 292         ret = rseq_need_restart(t, rseq_cs.flags);
 293         if (ret <= 0)
 294                 return ret;
 295         ret = clear_rseq_cs(t);
 296         if (ret)
 297                 return ret;
 298         trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
 299                             rseq_cs.abort_ip);
 300         instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
 301         return 0;
 302 }
 303
 304 /*
 305  * This resume handler must always be executed between any of:
 306  * - preemption,
 307  * - signal delivery,
 308  * and return to user-space.
 309  *
 310  * This is how we can ensure that the entire rseq critical section
 311  * will issue the commit instruction only if executed atomically with
 312  * respect to other threads scheduled on the same CPU, and with respect
 313  * to signal handlers.
 314  */
 315 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 316 {
 317         struct task_struct *t = current;
 318         int ret, sig;
 319
 320         if (unlikely(t->flags & PF_EXITING))
 321                 return;
 322
 323         /*
 324          * regs is NULL if and only if the caller is in a syscall path.  Skip
 325          * fixup and leave rseq_cs as is so that rseq_sycall() will detect and
 326          * kill a misbehaving userspace on debug kernels.
 327          */
 328         if (regs) {
 329                 ret = rseq_ip_fixup(regs);
 330                 if (unlikely(ret < 0))
 331                         goto error;
 332         }
 333         if (unlikely(rseq_update_cpu_node_id(t)))
 334                 goto error;
 335         return;
 336
 337 error:
 338         sig = ksig ? ksig->sig : 0;
 339         force_sigsegv(sig);
 340 }
 341
 342 #ifdef CONFIG_DEBUG_RSEQ
 343
 344 /*
 345  * Terminate the process if a syscall is issued within a restartable
 346  * sequence.
 347  */
 348 void rseq_syscall(struct pt_regs *regs)
 349 {
 350         unsigned long ip = instruction_pointer(regs);
 351         struct task_struct *t = current;
 352         struct rseq_cs rseq_cs;
 353
 354         if (!t->rseq)
 355                 return;
 356         if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
 357                 force_sig(SIGSEGV);
 358 }
 359
 360 #endif
 361
 362 /*
 363  * sys_rseq - setup restartable sequences for caller thread.
 364  */
 365 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 366                 int, flags, u32, sig)
 367 {
 368         int ret;
 369
 370         if (flags & RSEQ_FLAG_UNREGISTER) {
 371                 if (flags & ~RSEQ_FLAG_UNREGISTER)
 372                         return -EINVAL;
 373                 /* Unregister rseq for current thread. */
 374                 if (current->rseq != rseq || !current->rseq)
 375                         return -EINVAL;
 376                 if (rseq_len != current->rseq_len)
 377                         return -EINVAL;
 378                 if (current->rseq_sig != sig)
 379                         return -EPERM;
 380                 ret = rseq_reset_rseq_cpu_node_id(current);
 381                 if (ret)
 382                         return ret;
 383                 current->rseq = NULL;
 384                 current->rseq_sig = 0;
 385                 current->rseq_len = 0;
 386                 return 0;
 387         }
 388
 389         if (unlikely(flags))
 390                 return -EINVAL;
 391
 392         if (current->rseq) {
 393                 /*
 394                  * If rseq is already registered, check whether
 395                  * the provided address differs from the prior
 396                  * one.
 397                  */
 398                 if (current->rseq != rseq || rseq_len != current->rseq_len)
 399                         return -EINVAL;
 400                 if (current->rseq_sig != sig)
 401                         return -EPERM;
 402                 /* Already registered. */
 403                 return -EBUSY;
 404         }
 405
 406         /*
 407          * If there was no rseq previously registered, ensure the provided rseq
 408          * is properly aligned, as communcated to user-space through the ELF
 409          * auxiliary vector AT_RSEQ_ALIGN. If rseq_len is the original rseq
 410          * size, the required alignment is the original struct rseq alignment.
 411          *
 412          * In order to be valid, rseq_len is either the original rseq size, or
 413          * large enough to contain all supported fields, as communicated to
 414          * user-space through the ELF auxiliary vector AT_RSEQ_FEATURE_SIZE.
 415          */
 416         if (rseq_len < ORIG_RSEQ_SIZE ||
 417             (rseq_len == ORIG_RSEQ_SIZE && !IS_ALIGNED((unsigned long)rseq, ORIG_RSEQ_SIZE)) ||
 418             (rseq_len != ORIG_RSEQ_SIZE && (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
 419                                             rseq_len < offsetof(struct rseq, end))))
 420                 return -EINVAL;
 421         if (!access_ok(rseq, rseq_len))
 422                 return -EFAULT;
 423         current->rseq = rseq;
 424         current->rseq_len = rseq_len;
 425         current->rseq_sig = sig;
 426         /*
 427          * If rseq was previously inactive, and has just been
 428          * registered, ensure the cpu_id_start and cpu_id fields
 429          * are updated before returning to user-space.
 430          */
 431         rseq_set_notify_resume(current);
 432
 433         return 0;
 434 }