seccomp: Provide matching filter for introspection

[mirror_ubuntu-artful-kernel.git] / kernel / seccomp.c

/*
 * 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/refcount.h>
#include <linux/audit.h>
#include <linux/compat.h>
#include <linux/coredump.h>
#include <linux/sched.h>
#include <linux/sched/task_stack.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
 * @prog: the BPF program 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 {
        refcount_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 @sd
 * @sd: optional seccomp data to be passed to filters
 * @match: stores struct seccomp_filter that resulted in the return value,
 *         unless filter returned SECCOMP_RET_ALLOW, in which case it will
 *         be unchanged.
 *
 * Returns valid seccomp BPF response codes.
 */
static u32 seccomp_run_filters(const struct seccomp_data *sd,
                               struct seccomp_filter **match)
{
        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, sd);

                if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION)) {
                        ret = cur_ret;
                        *match = f;
                }
        }
        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);
        }

        refcount_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. */
        refcount_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 && refcount_dec_and_test(&orig->usage)) {
                struct seccomp_filter *freeme = orig;
                orig = orig->prev;
                seccomp_filter_free(freeme);
        }
}

static void seccomp_init_siginfo(siginfo_t *info, int syscall, int reason)
{
        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;
}

/**
 * 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;
        seccomp_init_siginfo(&info, syscall, reason);
        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;
        struct seccomp_filter *match = NULL;
        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, &match);
        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:
                /*
                 * Note that the "match" filter will always be NULL for
                 * this action since SECCOMP_RET_ALLOW is the starting
                 * state in seccomp_run_filters().
                 */
                return 0;

        case SECCOMP_RET_KILL:
        default:
                audit_seccomp(this_syscall, SIGSYS, action);
                /* Dump core only if this is the last remaining thread. */
                if (get_nr_threads(current) == 1) {
                        siginfo_t info;

                        /* Show the original registers in the dump. */
                        syscall_rollback(current, task_pt_regs(current));
                        /* Trigger a manual coredump since do_exit skips it. */
                        seccomp_init_siginfo(&info, this_syscall, data);
                        do_coredump(&info);
                }
                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