/*
   * linux/kernel/seccomp.c
   *
   * Copyright 2004-2005  Andrea Arcangeli <andrea@cpushare.com>
   *

   * Copyright (C) 2012 Google, Inc.
   * Will Drewry <wad@chromium.org>
   *
   * This defines a simple but solid secure-computing facility.
   *
   * Mode 1 uses a fixed list of allowed system calls.
   * Mode 2 allows user-defined system call filters in the form
   *        of Berkeley Packet Filters/Linux Socket Filters.

   */

  #include <linux/atomic.h>

  #include <linux/audit.h>

  #include <linux/compat.h>

  #include <linux/sched.h>
  #include <linux/seccomp.h>

  #include <linux/slab.h>

  #include <linux/syscalls.h>

  #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER

  #include <asm/syscall.h>

  #endif

  
  #ifdef CONFIG_SECCOMP_FILTER

  #include <linux/filter.h>

  #include <linux/pid.h>

  #include <linux/ptrace.h>

  #include <linux/security.h>

  #include <linux/tracehook.h>
  #include <linux/uaccess.h>
  
  /**
   * struct seccomp_filter - container for seccomp BPF programs
   *
   * @usage: reference count to manage the object lifetime.
   *         get/put helpers should be used when accessing an instance
   *         outside of a lifetime-guarded section.  In general, this
   *         is only needed for handling filters shared across tasks.
   * @prev: points to a previously installed, or inherited, filter
   * @len: the number of instructions in the program

   * @insnsi: the BPF program instructions to evaluate

   *
   * seccomp_filter objects are organized in a tree linked via the @prev
   * pointer.  For any task, it appears to be a singly-linked list starting
   * with current->seccomp.filter, the most recently attached or inherited filter.
   * However, multiple filters may share a @prev node, by way of fork(), which
   * results in a unidirectional tree existing in memory.  This is similar to
   * how namespaces work.
   *
   * seccomp_filter objects should never be modified after being attached
   * to a task_struct (other than @usage).
   */
  struct seccomp_filter {
  	atomic_t usage;
  	struct seccomp_filter *prev;

  	struct bpf_prog *prog;

  };
  
  /* Limit any path through the tree to 256KB worth of instructions. */
  #define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter))

  /*

   * Endianness is explicitly ignored and left for BPF program authors to manage
   * as per the specific architecture.
   */

  static void populate_seccomp_data(struct seccomp_data *sd)

  {

  	struct task_struct *task = current;
  	struct pt_regs *regs = task_pt_regs(task);

  	unsigned long args[6];

  	sd->nr = syscall_get_nr(task, regs);

  	sd->arch = syscall_get_arch();

  	syscall_get_arguments(task, regs, 0, 6, args);
  	sd->args[0] = args[0];
  	sd->args[1] = args[1];
  	sd->args[2] = args[2];
  	sd->args[3] = args[3];
  	sd->args[4] = args[4];
  	sd->args[5] = args[5];

  	sd->instruction_pointer = KSTK_EIP(task);

  }
  
  /**
   *	seccomp_check_filter - verify seccomp filter code
   *	@filter: filter to verify
   *	@flen: length of filter
   *

   * Takes a previously checked filter (by bpf_check_classic) and

   * redirects all filter code that loads struct sk_buff data
   * and related data through seccomp_bpf_load.  It also
   * enforces length and alignment checking of those loads.
   *
   * Returns 0 if the rule set is legal or -EINVAL if not.
   */
  static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
  {
  	int pc;
  	for (pc = 0; pc < flen; pc++) {
  		struct sock_filter *ftest = &filter[pc];
  		u16 code = ftest->code;
  		u32 k = ftest->k;
  
  		switch (code) {

  		case BPF_LD | BPF_W | BPF_ABS:

  			ftest->code = BPF_LDX | BPF_W | BPF_ABS;

  			/* 32-bit aligned and not out of bounds. */
  			if (k >= sizeof(struct seccomp_data) || k & 3)
  				return -EINVAL;
  			continue;

  		case BPF_LD | BPF_W | BPF_LEN:

  			ftest->code = BPF_LD | BPF_IMM;

  			ftest->k = sizeof(struct seccomp_data);
  			continue;

  		case BPF_LDX | BPF_W | BPF_LEN:

  			ftest->code = BPF_LDX | BPF_IMM;

  			ftest->k = sizeof(struct seccomp_data);
  			continue;
  		/* Explicitly include allowed calls. */

  		case BPF_RET | BPF_K:
  		case BPF_RET | BPF_A:
  		case BPF_ALU | BPF_ADD | BPF_K:
  		case BPF_ALU | BPF_ADD | BPF_X:
  		case BPF_ALU | BPF_SUB | BPF_K:
  		case BPF_ALU | BPF_SUB | BPF_X:
  		case BPF_ALU | BPF_MUL | BPF_K:
  		case BPF_ALU | BPF_MUL | BPF_X:
  		case BPF_ALU | BPF_DIV | BPF_K:
  		case BPF_ALU | BPF_DIV | BPF_X:
  		case BPF_ALU | BPF_AND | BPF_K:
  		case BPF_ALU | BPF_AND | BPF_X:
  		case BPF_ALU | BPF_OR | BPF_K:
  		case BPF_ALU | BPF_OR | BPF_X:
  		case BPF_ALU | BPF_XOR | BPF_K:
  		case BPF_ALU | BPF_XOR | BPF_X:
  		case BPF_ALU | BPF_LSH | BPF_K:
  		case BPF_ALU | BPF_LSH | BPF_X:
  		case BPF_ALU | BPF_RSH | BPF_K:
  		case BPF_ALU | BPF_RSH | BPF_X:
  		case BPF_ALU | BPF_NEG:
  		case BPF_LD | BPF_IMM:
  		case BPF_LDX | BPF_IMM:
  		case BPF_MISC | BPF_TAX:
  		case BPF_MISC | BPF_TXA:
  		case BPF_LD | BPF_MEM:
  		case BPF_LDX | BPF_MEM:
  		case BPF_ST:
  		case BPF_STX:
  		case BPF_JMP | BPF_JA:
  		case BPF_JMP | BPF_JEQ | BPF_K:
  		case BPF_JMP | BPF_JEQ | BPF_X:
  		case BPF_JMP | BPF_JGE | BPF_K:
  		case BPF_JMP | BPF_JGE | BPF_X:
  		case BPF_JMP | BPF_JGT | BPF_K:
  		case BPF_JMP | BPF_JGT | BPF_X:
  		case BPF_JMP | BPF_JSET | BPF_K:
  		case BPF_JMP | BPF_JSET | BPF_X:

  			continue;
  		default:
  			return -EINVAL;
  		}
  	}
  	return 0;
  }
  
  /**
   * seccomp_run_filters - evaluates all seccomp filters against @syscall
   * @syscall: number of the current system call
   *
   * Returns valid seccomp BPF response codes.
   */

  static u32 seccomp_run_filters(const struct seccomp_data *sd)

  {

  	struct seccomp_data sd_local;

  	u32 ret = SECCOMP_RET_ALLOW;

  	/* Make sure cross-thread synced filter points somewhere sane. */
  	struct seccomp_filter *f =
  			lockless_dereference(current->seccomp.filter);

  
  	/* Ensure unexpected behavior doesn't result in failing open. */

  	if (unlikely(WARN_ON(f == NULL)))

  		return SECCOMP_RET_KILL;

  	if (!sd) {
  		populate_seccomp_data(&sd_local);
  		sd = &sd_local;
  	}

  	/*
  	 * All filters in the list are evaluated and the lowest BPF return

  	 * value always takes priority (ignoring the DATA).

  	 */

  	for (; f; f = f->prev) {

  		u32 cur_ret = BPF_PROG_RUN(f->prog, (void *)sd);

  		if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
  			ret = cur_ret;

  	}
  	return ret;
  }

  #endif /* CONFIG_SECCOMP_FILTER */

  static inline bool seccomp_may_assign_mode(unsigned long seccomp_mode)
  {

  	assert_spin_locked(&current->sighand->siglock);

  	if (current->seccomp.mode && current->seccomp.mode != seccomp_mode)
  		return false;
  
  	return true;
  }

  static inline void seccomp_assign_mode(struct task_struct *task,
  				       unsigned long seccomp_mode)

  {

  	assert_spin_locked(&task->sighand->siglock);

  	task->seccomp.mode = seccomp_mode;
  	/*
  	 * Make sure TIF_SECCOMP cannot be set before the mode (and
  	 * filter) is set.
  	 */
  	smp_mb__before_atomic();
  	set_tsk_thread_flag(task, TIF_SECCOMP);

  }
  
  #ifdef CONFIG_SECCOMP_FILTER

  /* Returns 1 if the parent is an ancestor of the child. */
  static int is_ancestor(struct seccomp_filter *parent,
  		       struct seccomp_filter *child)
  {
  	/* NULL is the root ancestor. */
  	if (parent == NULL)
  		return 1;
  	for (; child; child = child->prev)
  		if (child == parent)
  			return 1;
  	return 0;
  }
  
  /**
   * seccomp_can_sync_threads: checks if all threads can be synchronized
   *
   * Expects sighand and cred_guard_mutex locks to be held.
   *
   * Returns 0 on success, -ve on error, or the pid of a thread which was
   * either not in the correct seccomp mode or it did not have an ancestral
   * seccomp filter.
   */
  static inline pid_t seccomp_can_sync_threads(void)
  {
  	struct task_struct *thread, *caller;
  
  	BUG_ON(!mutex_is_locked(&current->signal->cred_guard_mutex));

  	assert_spin_locked(&current->sighand->siglock);

  
  	/* Validate all threads being eligible for synchronization. */
  	caller = current;
  	for_each_thread(caller, thread) {
  		pid_t failed;
  
  		/* Skip current, since it is initiating the sync. */
  		if (thread == caller)
  			continue;
  
  		if (thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
  		    (thread->seccomp.mode == SECCOMP_MODE_FILTER &&
  		     is_ancestor(thread->seccomp.filter,
  				 caller->seccomp.filter)))
  			continue;
  
  		/* Return the first thread that cannot be synchronized. */
  		failed = task_pid_vnr(thread);
  		/* If the pid cannot be resolved, then return -ESRCH */
  		if (unlikely(WARN_ON(failed == 0)))
  			failed = -ESRCH;
  		return failed;
  	}
  
  	return 0;
  }
  
  /**
   * seccomp_sync_threads: sets all threads to use current's filter
   *
   * Expects sighand and cred_guard_mutex locks to be held, and for
   * seccomp_can_sync_threads() to have returned success already
   * without dropping the locks.
   *
   */
  static inline void seccomp_sync_threads(void)
  {
  	struct task_struct *thread, *caller;
  
  	BUG_ON(!mutex_is_locked(&current->signal->cred_guard_mutex));

  	assert_spin_locked(&current->sighand->siglock);

  
  	/* Synchronize all threads. */
  	caller = current;
  	for_each_thread(caller, thread) {
  		/* Skip current, since it needs no changes. */
  		if (thread == caller)
  			continue;
  
  		/* Get a task reference for the new leaf node. */
  		get_seccomp_filter(caller);
  		/*
  		 * Drop the task reference to the shared ancestor since
  		 * current's path will hold a reference.  (This also
  		 * allows a put before the assignment.)
  		 */
  		put_seccomp_filter(thread);
  		smp_store_release(&thread->seccomp.filter,
  				  caller->seccomp.filter);

  
  		/*
  		 * Don't let an unprivileged task work around
  		 * the no_new_privs restriction by creating
  		 * a thread that sets it up, enters seccomp,
  		 * then dies.
  		 */
  		if (task_no_new_privs(caller))
  			task_set_no_new_privs(thread);

  		/*
  		 * Opt the other thread into seccomp if needed.
  		 * As threads are considered to be trust-realm
  		 * equivalent (see ptrace_may_access), it is safe to
  		 * allow one thread to transition the other.
  		 */

  		if (thread->seccomp.mode == SECCOMP_MODE_DISABLED)

  			seccomp_assign_mode(thread, SECCOMP_MODE_FILTER);

  	}
  }

  /**

   * seccomp_prepare_filter: Prepares a seccomp filter for use.

   * @fprog: BPF program to install
   *

   * Returns filter on success or an ERR_PTR on failure.

   */

  static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)

  {

  	struct seccomp_filter *sfilter;
  	int ret;

  	const bool save_orig = IS_ENABLED(CONFIG_CHECKPOINT_RESTORE);

  
  	if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)

  		return ERR_PTR(-EINVAL);

  	BUG_ON(INT_MAX / fprog->len < sizeof(struct sock_filter));

  
  	/*

  	 * Installing a seccomp filter requires that the task has

  	 * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
  	 * This avoids scenarios where unprivileged tasks can affect the
  	 * behavior of privileged children.
  	 */

  	if (!task_no_new_privs(current) &&

  	    security_capable_noaudit(current_cred(), current_user_ns(),
  				     CAP_SYS_ADMIN) != 0)

  		return ERR_PTR(-EACCES);

  	/* Allocate a new seccomp_filter */

  	sfilter = kzalloc(sizeof(*sfilter), GFP_KERNEL | __GFP_NOWARN);
  	if (!sfilter)

  		return ERR_PTR(-ENOMEM);

  
  	ret = bpf_prog_create_from_user(&sfilter->prog, fprog,

  					seccomp_check_filter, save_orig);

  	if (ret < 0) {
  		kfree(sfilter);
  		return ERR_PTR(ret);

  	}

  	atomic_set(&sfilter->usage, 1);

  	return sfilter;

  }
  
  /**

   * seccomp_prepare_user_filter - prepares a user-supplied sock_fprog

   * @user_filter: pointer to the user data containing a sock_fprog.
   *
   * Returns 0 on success and non-zero otherwise.
   */

  static struct seccomp_filter *
  seccomp_prepare_user_filter(const char __user *user_filter)

  {
  	struct sock_fprog fprog;

  	struct seccomp_filter *filter = ERR_PTR(-EFAULT);

  
  #ifdef CONFIG_COMPAT

  	if (in_compat_syscall()) {

  		struct compat_sock_fprog fprog32;
  		if (copy_from_user(&fprog32, user_filter, sizeof(fprog32)))
  			goto out;
  		fprog.len = fprog32.len;
  		fprog.filter = compat_ptr(fprog32.filter);
  	} else /* falls through to the if below. */
  #endif
  	if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
  		goto out;

  	filter = seccomp_prepare_filter(&fprog);

  out:

  	return filter;
  }
  
  /**
   * seccomp_attach_filter: validate and attach filter
   * @flags:  flags to change filter behavior
   * @filter: seccomp filter to add to the current process
   *

   * Caller must be holding current->sighand->siglock lock.
   *

   * Returns 0 on success, -ve on error.
   */
  static long seccomp_attach_filter(unsigned int flags,
  				  struct seccomp_filter *filter)
  {
  	unsigned long total_insns;
  	struct seccomp_filter *walker;

  	assert_spin_locked(&current->sighand->siglock);

  	/* Validate resulting filter length. */
  	total_insns = filter->prog->len;
  	for (walker = current->seccomp.filter; walker; walker = walker->prev)
  		total_insns += walker->prog->len + 4;  /* 4 instr penalty */
  	if (total_insns > MAX_INSNS_PER_PATH)
  		return -ENOMEM;

  	/* If thread sync has been requested, check that it is possible. */
  	if (flags & SECCOMP_FILTER_FLAG_TSYNC) {
  		int ret;
  
  		ret = seccomp_can_sync_threads();
  		if (ret)
  			return ret;
  	}

  	/*
  	 * If there is an existing filter, make it the prev and don't drop its
  	 * task reference.
  	 */
  	filter->prev = current->seccomp.filter;
  	current->seccomp.filter = filter;

  	/* Now that the new filter is in place, synchronize to all threads. */
  	if (flags & SECCOMP_FILTER_FLAG_TSYNC)
  		seccomp_sync_threads();

  	return 0;

  }
  
  /* get_seccomp_filter - increments the reference count of the filter on @tsk */
  void get_seccomp_filter(struct task_struct *tsk)
  {
  	struct seccomp_filter *orig = tsk->seccomp.filter;
  	if (!orig)
  		return;
  	/* Reference count is bounded by the number of total processes. */
  	atomic_inc(&orig->usage);
  }

  static inline void seccomp_filter_free(struct seccomp_filter *filter)
  {
  	if (filter) {

  		bpf_prog_destroy(filter->prog);

  		kfree(filter);
  	}
  }

  /* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
  void put_seccomp_filter(struct task_struct *tsk)
  {
  	struct seccomp_filter *orig = tsk->seccomp.filter;
  	/* Clean up single-reference branches iteratively. */
  	while (orig && atomic_dec_and_test(&orig->usage)) {
  		struct seccomp_filter *freeme = orig;
  		orig = orig->prev;

  		seccomp_filter_free(freeme);

  	}
  }

  
  /**
   * seccomp_send_sigsys - signals the task to allow in-process syscall emulation
   * @syscall: syscall number to send to userland
   * @reason: filter-supplied reason code to send to userland (via si_errno)
   *
   * Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
   */
  static void seccomp_send_sigsys(int syscall, int reason)
  {
  	struct siginfo info;
  	memset(&info, 0, sizeof(info));
  	info.si_signo = SIGSYS;
  	info.si_code = SYS_SECCOMP;
  	info.si_call_addr = (void __user *)KSTK_EIP(current);
  	info.si_errno = reason;

  	info.si_arch = syscall_get_arch();

  	info.si_syscall = syscall;
  	force_sig_info(SIGSYS, &info, current);
  }

  #endif	/* CONFIG_SECCOMP_FILTER */

  
  /*
   * Secure computing mode 1 allows only read/write/exit/sigreturn.
   * To be fully secure this must be combined with rlimit
   * to limit the stack allocations too.
   */

  static const int mode1_syscalls[] = {

  	__NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
  	0, /* null terminated */
  };

  static void __secure_computing_strict(int this_syscall)

  {

  	const int *syscall_whitelist = mode1_syscalls;

  #ifdef CONFIG_COMPAT

  	if (in_compat_syscall())

  		syscall_whitelist = get_compat_mode1_syscalls();

  #endif
  	do {
  		if (*syscall_whitelist == this_syscall)
  			return;
  	} while (*++syscall_whitelist);
  
  #ifdef SECCOMP_DEBUG
  	dump_stack();
  #endif
  	audit_seccomp(this_syscall, SIGKILL, SECCOMP_RET_KILL);
  	do_exit(SIGKILL);
  }
  
  #ifndef CONFIG_HAVE_ARCH_SECCOMP_FILTER
  void secure_computing_strict(int this_syscall)
  {
  	int mode = current->seccomp.mode;

  	if (IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) &&

  	    unlikely(current->ptrace & PT_SUSPEND_SECCOMP))
  		return;

  	if (mode == SECCOMP_MODE_DISABLED)

  		return;
  	else if (mode == SECCOMP_MODE_STRICT)
  		__secure_computing_strict(this_syscall);
  	else
  		BUG();
  }
  #else

  
  #ifdef CONFIG_SECCOMP_FILTER

  static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
  			    const bool recheck_after_trace)

  {
  	u32 filter_ret, action;
  	int data;

  	/*
  	 * Make sure that any changes to mode from another thread have
  	 * been seen after TIF_SECCOMP was seen.
  	 */
  	rmb();

  	filter_ret = seccomp_run_filters(sd);

  	data = filter_ret & SECCOMP_RET_DATA;
  	action = filter_ret & SECCOMP_RET_ACTION;
  
  	switch (action) {
  	case SECCOMP_RET_ERRNO:

  		/* Set low-order bits as an errno, capped at MAX_ERRNO. */
  		if (data > MAX_ERRNO)
  			data = MAX_ERRNO;

  		syscall_set_return_value(current, task_pt_regs(current),

  					 -data, 0);
  		goto skip;
  
  	case SECCOMP_RET_TRAP:
  		/* Show the handler the original registers. */

  		syscall_rollback(current, task_pt_regs(current));

  		/* Let the filter pass back 16 bits of data. */
  		seccomp_send_sigsys(this_syscall, data);
  		goto skip;
  
  	case SECCOMP_RET_TRACE:

  		/* We've been put in this state by the ptracer already. */
  		if (recheck_after_trace)
  			return 0;

  		/* ENOSYS these calls if there is no tracer attached. */
  		if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP)) {
  			syscall_set_return_value(current,
  						 task_pt_regs(current),
  						 -ENOSYS, 0);
  			goto skip;
  		}
  
  		/* Allow the BPF to provide the event message */
  		ptrace_event(PTRACE_EVENT_SECCOMP, data);
  		/*
  		 * The delivery of a fatal signal during event

  		 * notification may silently skip tracer notification,
  		 * which could leave us with a potentially unmodified
  		 * syscall that the tracer would have liked to have
  		 * changed. Since the process is about to die, we just
  		 * force the syscall to be skipped and let the signal
  		 * kill the process and correctly handle any tracer exit
  		 * notifications.

  		 */
  		if (fatal_signal_pending(current))

  			goto skip;

  		/* Check if the tracer forced the syscall to be skipped. */
  		this_syscall = syscall_get_nr(current, task_pt_regs(current));
  		if (this_syscall < 0)
  			goto skip;

  		/*
  		 * Recheck the syscall, since it may have changed. This
  		 * intentionally uses a NULL struct seccomp_data to force
  		 * a reload of all registers. This does not goto skip since
  		 * a skip would have already been reported.
  		 */
  		if (__seccomp_filter(this_syscall, NULL, true))
  			return -1;

  		return 0;

  
  	case SECCOMP_RET_ALLOW:

  		return 0;

  
  	case SECCOMP_RET_KILL:
  	default:
  		audit_seccomp(this_syscall, SIGSYS, action);
  		do_exit(SIGSYS);
  	}
  
  	unreachable();
  
  skip:
  	audit_seccomp(this_syscall, 0, action);

  	return -1;
  }
  #else

  static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
  			    const bool recheck_after_trace)

  {
  	BUG();

  }

  #endif

  int __secure_computing(const struct seccomp_data *sd)

  {
  	int mode = current->seccomp.mode;

  	int this_syscall;

  	if (IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) &&

  	    unlikely(current->ptrace & PT_SUSPEND_SECCOMP))

  		return 0;
  
  	this_syscall = sd ? sd->nr :
  		syscall_get_nr(current, task_pt_regs(current));

  	switch (mode) {

  	case SECCOMP_MODE_STRICT:

  		__secure_computing_strict(this_syscall);  /* may call do_exit */

  		return 0;

  	case SECCOMP_MODE_FILTER:

  		return __seccomp_filter(this_syscall, sd, false);

  	default:
  		BUG();
  	}

  }

  #endif /* CONFIG_HAVE_ARCH_SECCOMP_FILTER */

  
  long prctl_get_seccomp(void)
  {
  	return current->seccomp.mode;
  }

  /**

   * seccomp_set_mode_strict: internal function for setting strict seccomp

   *
   * Once current->seccomp.mode is non-zero, it may not be changed.
   *
   * Returns 0 on success or -EINVAL on failure.
   */

  static long seccomp_set_mode_strict(void)

  {

  	const unsigned long seccomp_mode = SECCOMP_MODE_STRICT;

  	long ret = -EINVAL;

  	spin_lock_irq(&current->sighand->siglock);

  	if (!seccomp_may_assign_mode(seccomp_mode))

  		goto out;

  #ifdef TIF_NOTSC

  	disable_TSC();

  #endif

  	seccomp_assign_mode(current, seccomp_mode);

  	ret = 0;
  
  out:

  	spin_unlock_irq(&current->sighand->siglock);

  
  	return ret;
  }

  #ifdef CONFIG_SECCOMP_FILTER

  /**
   * seccomp_set_mode_filter: internal function for setting seccomp filter

   * @flags:  flags to change filter behavior

   * @filter: struct sock_fprog containing filter
   *
   * This function may be called repeatedly to install additional filters.
   * Every filter successfully installed will be evaluated (in reverse order)
   * for each system call the task makes.
   *
   * Once current->seccomp.mode is non-zero, it may not be changed.
   *
   * Returns 0 on success or -EINVAL on failure.
   */

  static long seccomp_set_mode_filter(unsigned int flags,
  				    const char __user *filter)

  {
  	const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;

  	struct seccomp_filter *prepared = NULL;

  	long ret = -EINVAL;

  	/* Validate flags. */

  	if (flags & ~SECCOMP_FILTER_FLAG_MASK)

  		return -EINVAL;

  	/* Prepare the new filter before holding any locks. */
  	prepared = seccomp_prepare_user_filter(filter);
  	if (IS_ERR(prepared))
  		return PTR_ERR(prepared);

  	/*
  	 * Make sure we cannot change seccomp or nnp state via TSYNC
  	 * while another thread is in the middle of calling exec.
  	 */
  	if (flags & SECCOMP_FILTER_FLAG_TSYNC &&
  	    mutex_lock_killable(&current->signal->cred_guard_mutex))
  		goto out_free;

  	spin_lock_irq(&current->sighand->siglock);

  	if (!seccomp_may_assign_mode(seccomp_mode))
  		goto out;

  	ret = seccomp_attach_filter(flags, prepared);

  	if (ret)

  		goto out;

  	/* Do not free the successfully attached filter. */
  	prepared = NULL;

  	seccomp_assign_mode(current, seccomp_mode);

  out:

  	spin_unlock_irq(&current->sighand->siglock);

  	if (flags & SECCOMP_FILTER_FLAG_TSYNC)
  		mutex_unlock(&current->signal->cred_guard_mutex);
  out_free:

  	seccomp_filter_free(prepared);

  	return ret;
  }

  #else

  static inline long seccomp_set_mode_filter(unsigned int flags,
  					   const char __user *filter)

  {
  	return -EINVAL;
  }
  #endif

  /* Common entry point for both prctl and syscall. */
  static long do_seccomp(unsigned int op, unsigned int flags,
  		       const char __user *uargs)
  {
  	switch (op) {
  	case SECCOMP_SET_MODE_STRICT:
  		if (flags != 0 || uargs != NULL)
  			return -EINVAL;
  		return seccomp_set_mode_strict();
  	case SECCOMP_SET_MODE_FILTER:
  		return seccomp_set_mode_filter(flags, uargs);
  	default:
  		return -EINVAL;
  	}
  }
  
  SYSCALL_DEFINE3(seccomp, unsigned int, op, unsigned int, flags,
  			 const char __user *, uargs)
  {
  	return do_seccomp(op, flags, uargs);
  }

  /**
   * prctl_set_seccomp: configures current->seccomp.mode
   * @seccomp_mode: requested mode to use
   * @filter: optional struct sock_fprog for use with SECCOMP_MODE_FILTER
   *
   * Returns 0 on success or -EINVAL on failure.
   */
  long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
  {

  	unsigned int op;
  	char __user *uargs;

  	switch (seccomp_mode) {
  	case SECCOMP_MODE_STRICT:

  		op = SECCOMP_SET_MODE_STRICT;
  		/*
  		 * Setting strict mode through prctl always ignored filter,
  		 * so make sure it is always NULL here to pass the internal
  		 * check in do_seccomp().
  		 */
  		uargs = NULL;
  		break;

  	case SECCOMP_MODE_FILTER:

  		op = SECCOMP_SET_MODE_FILTER;
  		uargs = filter;
  		break;

  	default:
  		return -EINVAL;
  	}

  
  	/* prctl interface doesn't have flags, so they are always zero. */
  	return do_seccomp(op, 0, uargs);

  }

  
  #if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
  long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
  			void __user *data)
  {
  	struct seccomp_filter *filter;
  	struct sock_fprog_kern *fprog;
  	long ret;
  	unsigned long count = 0;
  
  	if (!capable(CAP_SYS_ADMIN) ||
  	    current->seccomp.mode != SECCOMP_MODE_DISABLED) {
  		return -EACCES;
  	}
  
  	spin_lock_irq(&task->sighand->siglock);
  	if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
  		ret = -EINVAL;
  		goto out;
  	}
  
  	filter = task->seccomp.filter;
  	while (filter) {
  		filter = filter->prev;
  		count++;
  	}
  
  	if (filter_off >= count) {
  		ret = -ENOENT;
  		goto out;
  	}
  	count -= filter_off;
  
  	filter = task->seccomp.filter;
  	while (filter && count > 1) {
  		filter = filter->prev;
  		count--;
  	}
  
  	if (WARN_ON(count != 1 || !filter)) {
  		/* The filter tree shouldn't shrink while we're using it. */
  		ret = -ENOENT;
  		goto out;
  	}
  
  	fprog = filter->prog->orig_prog;
  	if (!fprog) {

  		/* This must be a new non-cBPF filter, since we save

  		 * every cBPF filter's orig_prog above when
  		 * CONFIG_CHECKPOINT_RESTORE is enabled.
  		 */
  		ret = -EMEDIUMTYPE;
  		goto out;
  	}
  
  	ret = fprog->len;
  	if (!data)
  		goto out;
  
  	get_seccomp_filter(task);
  	spin_unlock_irq(&task->sighand->siglock);
  
  	if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog)))
  		ret = -EFAULT;
  
  	put_seccomp_filter(task);
  	return ret;
  
  out:
  	spin_unlock_irq(&task->sighand->siglock);
  	return ret;
  }
  #endif