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TOMOYO Linux Cross Reference
Linux/kernel/rseq.c

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  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  * will issue the commit instruction only if executed atomically with
259  * respect to other threads scheduled on the same CPU, and with respect
260  * to signal handlers.
261  */
262 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
263 {
264         struct task_struct *t = current;
265         int ret, sig;
266 
267         if (unlikely(t->flags & PF_EXITING))
268                 return;
269         if (unlikely(!access_ok(t->rseq, sizeof(*t->rseq))))
270                 goto error;
271         ret = rseq_ip_fixup(regs);
272         if (unlikely(ret < 0))
273                 goto error;
274         if (unlikely(rseq_update_cpu_id(t)))
275                 goto error;
276         return;
277 
278 error:
279         sig = ksig ? ksig->sig : 0;
280         force_sigsegv(sig);
281 }
282 
283 #ifdef CONFIG_DEBUG_RSEQ
284 
285 /*
286  * Terminate the process if a syscall is issued within a restartable
287  * sequence.
288  */
289 void rseq_syscall(struct pt_regs *regs)
290 {
291         unsigned long ip = instruction_pointer(regs);
292         struct task_struct *t = current;
293         struct rseq_cs rseq_cs;
294 
295         if (!t->rseq)
296                 return;
297         if (!access_ok(t->rseq, sizeof(*t->rseq)) ||
298             rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
299                 force_sig(SIGSEGV);
300 }
301 
302 #endif
303 
304 /*
305  * sys_rseq - setup restartable sequences for caller thread.
306  */
307 SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
308                 int, flags, u32, sig)
309 {
310         int ret;
311 
312         if (flags & RSEQ_FLAG_UNREGISTER) {
313                 /* Unregister rseq for current thread. */
314                 if (current->rseq != rseq || !current->rseq)
315                         return -EINVAL;
316                 if (rseq_len != sizeof(*rseq))
317                         return -EINVAL;
318                 if (current->rseq_sig != sig)
319                         return -EPERM;
320                 ret = rseq_reset_rseq_cpu_id(current);
321                 if (ret)
322                         return ret;
323                 current->rseq = NULL;
324                 current->rseq_sig = 0;
325                 return 0;
326         }
327 
328         if (unlikely(flags))
329                 return -EINVAL;
330 
331         if (current->rseq) {
332                 /*
333                  * If rseq is already registered, check whether
334                  * the provided address differs from the prior
335                  * one.
336                  */
337                 if (current->rseq != rseq || rseq_len != sizeof(*rseq))
338                         return -EINVAL;
339                 if (current->rseq_sig != sig)
340                         return -EPERM;
341                 /* Already registered. */
342                 return -EBUSY;
343         }
344 
345         /*
346          * If there was no rseq previously registered,
347          * ensure the provided rseq is properly aligned and valid.
348          */
349         if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
350             rseq_len != sizeof(*rseq))
351                 return -EINVAL;
352         if (!access_ok(rseq, rseq_len))
353                 return -EFAULT;
354         current->rseq = rseq;
355         current->rseq_sig = sig;
356         /*
357          * If rseq was previously inactive, and has just been
358          * registered, ensure the cpu_id_start and cpu_id fields
359          * are updated before returning to user-space.
360          */
361         rseq_set_notify_resume(current);
362 
363         return 0;
364 }
365 

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