// SPDX-License-Identifier: GPL-2.0+
  /*
   * Restartable sequences system call
   *
   * Copyright (C) 2015, Google, Inc.,
   * Paul Turner <pjt@google.com> and Andrew Hunter <ahh@google.com>
   * Copyright (C) 2015-2018, EfficiOS Inc.,
   * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   */
  
  #include <linux/sched.h>
  #include <linux/uaccess.h>
  #include <linux/syscalls.h>
  #include <linux/rseq.h>
  #include <linux/types.h>
  #include <asm/ptrace.h>
  
  #define CREATE_TRACE_POINTS
  #include <trace/events/rseq.h>
  
  #define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE | \
  				       RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT)
  
  /*
   *
   * Restartable sequences are a lightweight interface that allows
   * user-level code to be executed atomically relative to scheduler
   * preemption and signal delivery. Typically used for implementing
   * per-cpu operations.
   *
   * It allows user-space to perform update operations on per-cpu data
   * without requiring heavy-weight atomic operations.
   *
   * Detailed algorithm of rseq user-space assembly sequences:
   *
   *                     init(rseq_cs)
   *                     cpu = TLS->rseq::cpu_id_start
   *   [1]               TLS->rseq::rseq_cs = rseq_cs
   *   [start_ip]        ----------------------------
   *   [2]               if (cpu != TLS->rseq::cpu_id)
   *                             goto abort_ip;
   *   [3]               <last_instruction_in_cs>
   *   [post_commit_ip]  ----------------------------
   *
   *   The address of jump target abort_ip must be outside the critical
   *   region, i.e.:
   *
   *     [abort_ip] < [start_ip]  || [abort_ip] >= [post_commit_ip]
   *
   *   Steps [2]-[3] (inclusive) need to be a sequence of instructions in
   *   userspace that can handle being interrupted between any of those
   *   instructions, and then resumed to the abort_ip.
   *
   *   1.  Userspace stores the address of the struct rseq_cs assembly
   *       block descriptor into the rseq_cs field of the registered
   *       struct rseq TLS area. This update is performed through a single
   *       store within the inline assembly instruction sequence.
   *       [start_ip]
   *
   *   2.  Userspace tests to check whether the current cpu_id field match
   *       the cpu number loaded before start_ip, branching to abort_ip
   *       in case of a mismatch.
   *
   *       If the sequence is preempted or interrupted by a signal
   *       at or after start_ip and before post_commit_ip, then the kernel
   *       clears TLS->__rseq_abi::rseq_cs, and sets the user-space return
   *       ip to abort_ip before returning to user-space, so the preempted
   *       execution resumes at abort_ip.
   *
   *   3.  Userspace critical section final instruction before
   *       post_commit_ip is the commit. The critical section is
   *       self-terminating.
   *       [post_commit_ip]
   *
   *   4.  <success>
   *
   *   On failure at [2], or if interrupted by preempt or signal delivery
   *   between [1] and [3]:
   *
   *       [abort_ip]
   *   F1. <failure>
   */
  
  static int rseq_update_cpu_id(struct task_struct *t)
  {
  	u32 cpu_id = raw_smp_processor_id();

  	if (put_user(cpu_id, &t->rseq->cpu_id_start))

  		return -EFAULT;

  	if (put_user(cpu_id, &t->rseq->cpu_id))

  		return -EFAULT;
  	trace_rseq_update(t);
  	return 0;
  }
  
  static int rseq_reset_rseq_cpu_id(struct task_struct *t)
  {
  	u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED;
  
  	/*
  	 * Reset cpu_id_start to its initial state (0).
  	 */

  	if (put_user(cpu_id_start, &t->rseq->cpu_id_start))

  		return -EFAULT;
  	/*
  	 * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
  	 * in after unregistration can figure out that rseq needs to be
  	 * registered again.
  	 */

  	if (put_user(cpu_id, &t->rseq->cpu_id))

  		return -EFAULT;
  	return 0;
  }
  
  static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
  {
  	struct rseq_cs __user *urseq_cs;

  	u64 ptr;

  	u32 __user *usig;
  	u32 sig;
  	int ret;

  	if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
  		return -EFAULT;

  	if (!ptr) {
  		memset(rseq_cs, 0, sizeof(*rseq_cs));
  		return 0;
  	}

  	if (ptr >= TASK_SIZE)
  		return -EINVAL;
  	urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;

  	if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
  		return -EFAULT;

  	if (rseq_cs->start_ip >= TASK_SIZE ||
  	    rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
  	    rseq_cs->abort_ip >= TASK_SIZE ||
  	    rseq_cs->version > 0)
  		return -EINVAL;
  	/* Check for overflow. */
  	if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
  		return -EINVAL;

  	/* Ensure that abort_ip is not in the critical section. */
  	if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
  		return -EINVAL;

  	usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));

  	ret = get_user(sig, usig);
  	if (ret)
  		return ret;
  
  	if (current->rseq_sig != sig) {
  		printk_ratelimited(KERN_WARNING
  			"Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).
  ",
  			sig, current->rseq_sig, current->pid, usig);

  		return -EINVAL;

  	}
  	return 0;
  }
  
  static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
  {
  	u32 flags, event_mask;
  	int ret;
  
  	/* Get thread flags. */

  	ret = get_user(flags, &t->rseq->flags);

  	if (ret)
  		return ret;
  
  	/* Take critical section flags into account. */
  	flags |= cs_flags;
  
  	/*
  	 * Restart on signal can only be inhibited when restart on
  	 * preempt and restart on migrate are inhibited too. Otherwise,
  	 * a preempted signal handler could fail to restart the prior
  	 * execution context on sigreturn.
  	 */
  	if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) &&
  		     (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) !=
  		     RSEQ_CS_PREEMPT_MIGRATE_FLAGS))
  		return -EINVAL;
  
  	/*
  	 * Load and clear event mask atomically with respect to
  	 * scheduler preemption.
  	 */
  	preempt_disable();
  	event_mask = t->rseq_event_mask;
  	t->rseq_event_mask = 0;
  	preempt_enable();
  
  	return !!(event_mask & ~flags);
  }
  
  static int clear_rseq_cs(struct task_struct *t)
  {
  	/*
  	 * The rseq_cs field is set to NULL on preemption or signal
  	 * delivery on top of rseq assembly block, as well as on top
  	 * of code outside of the rseq assembly block. This performs
  	 * a lazy clear of the rseq_cs field.
  	 *

  	 * Set rseq_cs to NULL.

  	 */

  	if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
  		return -EFAULT;
  	return 0;

  }
  
  /*
   * Unsigned comparison will be true when ip >= start_ip, and when
   * ip < start_ip + post_commit_offset.
   */
  static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
  {
  	return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
  }
  
  static int rseq_ip_fixup(struct pt_regs *regs)
  {
  	unsigned long ip = instruction_pointer(regs);
  	struct task_struct *t = current;
  	struct rseq_cs rseq_cs;
  	int ret;
  
  	ret = rseq_get_rseq_cs(t, &rseq_cs);
  	if (ret)
  		return ret;
  
  	/*
  	 * Handle potentially not being within a critical section.
  	 * If not nested over a rseq critical section, restart is useless.
  	 * Clear the rseq_cs pointer and return.
  	 */
  	if (!in_rseq_cs(ip, &rseq_cs))
  		return clear_rseq_cs(t);
  	ret = rseq_need_restart(t, rseq_cs.flags);
  	if (ret <= 0)
  		return ret;
  	ret = clear_rseq_cs(t);
  	if (ret)
  		return ret;
  	trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
  			    rseq_cs.abort_ip);
  	instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
  	return 0;
  }
  
  /*
   * This resume handler must always be executed between any of:
   * - preemption,
   * - signal delivery,
   * and return to user-space.
   *

   * This is how we can ensure that the entire rseq critical section

   * will issue the commit instruction only if executed atomically with
   * respect to other threads scheduled on the same CPU, and with respect
   * to signal handlers.
   */

  void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)

  {
  	struct task_struct *t = current;

  	int ret, sig;

  
  	if (unlikely(t->flags & PF_EXITING))
  		return;

  	if (unlikely(!access_ok(t->rseq, sizeof(*t->rseq))))

  		goto error;
  	ret = rseq_ip_fixup(regs);
  	if (unlikely(ret < 0))
  		goto error;
  	if (unlikely(rseq_update_cpu_id(t)))
  		goto error;
  	return;
  
  error:

  	sig = ksig ? ksig->sig : 0;

  	force_sigsegv(sig);

  }
  
  #ifdef CONFIG_DEBUG_RSEQ
  
  /*
   * Terminate the process if a syscall is issued within a restartable
   * sequence.
   */
  void rseq_syscall(struct pt_regs *regs)
  {
  	unsigned long ip = instruction_pointer(regs);
  	struct task_struct *t = current;
  	struct rseq_cs rseq_cs;
  
  	if (!t->rseq)
  		return;

  	if (!access_ok(t->rseq, sizeof(*t->rseq)) ||

  	    rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))

  		force_sig(SIGSEGV);

  }
  
  #endif
  
  /*
   * sys_rseq - setup restartable sequences for caller thread.
   */
  SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
  		int, flags, u32, sig)
  {
  	int ret;
  
  	if (flags & RSEQ_FLAG_UNREGISTER) {
  		/* Unregister rseq for current thread. */
  		if (current->rseq != rseq || !current->rseq)
  			return -EINVAL;

  		if (rseq_len != sizeof(*rseq))

  			return -EINVAL;
  		if (current->rseq_sig != sig)
  			return -EPERM;
  		ret = rseq_reset_rseq_cpu_id(current);
  		if (ret)
  			return ret;
  		current->rseq = NULL;

  		current->rseq_sig = 0;
  		return 0;
  	}
  
  	if (unlikely(flags))
  		return -EINVAL;
  
  	if (current->rseq) {
  		/*
  		 * If rseq is already registered, check whether
  		 * the provided address differs from the prior
  		 * one.
  		 */

  		if (current->rseq != rseq || rseq_len != sizeof(*rseq))

  			return -EINVAL;
  		if (current->rseq_sig != sig)
  			return -EPERM;
  		/* Already registered. */
  		return -EBUSY;
  	}
  
  	/*
  	 * If there was no rseq previously registered,
  	 * ensure the provided rseq is properly aligned and valid.
  	 */
  	if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
  	    rseq_len != sizeof(*rseq))
  		return -EINVAL;

  	if (!access_ok(rseq, rseq_len))

  		return -EFAULT;
  	current->rseq = rseq;

  	current->rseq_sig = sig;
  	/*
  	 * If rseq was previously inactive, and has just been
  	 * registered, ensure the cpu_id_start and cpu_id fields
  	 * are updated before returning to user-space.
  	 */
  	rseq_set_notify_resume(current);
  
  	return 0;
  }