user namespace: fix incorrect memory barriers

[mirror_ubuntu-bionic-kernel.git] / kernel / user_namespace.c

/*
 *  This program is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU General Public License as
 *  published by the Free Software Foundation, version 2 of the
 *  License.
 */

#include <linux/export.h>
#include <linux/nsproxy.h>
#include <linux/slab.h>
#include <linux/user_namespace.h>
#include <linux/proc_ns.h>
#include <linux/highuid.h>
#include <linux/cred.h>
#include <linux/securebits.h>
#include <linux/keyctl.h>
#include <linux/key-type.h>
#include <keys/user-type.h>
#include <linux/seq_file.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#include <linux/ctype.h>
#include <linux/projid.h>
#include <linux/fs_struct.h>

static struct kmem_cache *user_ns_cachep __read_mostly;

static bool new_idmap_permitted(const struct file *file,
                                struct user_namespace *ns, int cap_setid,
                                struct uid_gid_map *map);

static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
{
        /* Start with the same capabilities as init but useless for doing
         * anything as the capabilities are bound to the new user namespace.
         */
        cred->securebits = SECUREBITS_DEFAULT;
        cred->cap_inheritable = CAP_EMPTY_SET;
        cred->cap_permitted = CAP_FULL_SET;
        cred->cap_effective = CAP_FULL_SET;
        cred->cap_bset = CAP_FULL_SET;
#ifdef CONFIG_KEYS
        key_put(cred->request_key_auth);
        cred->request_key_auth = NULL;
#endif
        /* tgcred will be cleared in our caller bc CLONE_THREAD won't be set */
        cred->user_ns = user_ns;
}

/*
 * Create a new user namespace, deriving the creator from the user in the
 * passed credentials, and replacing that user with the new root user for the
 * new namespace.
 *
 * This is called by copy_creds(), which will finish setting the target task's
 * credentials.
 */
int create_user_ns(struct cred *new)
{
        struct user_namespace *ns, *parent_ns = new->user_ns;
        kuid_t owner = new->euid;
        kgid_t group = new->egid;
        int ret;

        if (parent_ns->level > 32)
                return -EUSERS;

        /*
         * Verify that we can not violate the policy of which files
         * may be accessed that is specified by the root directory,
         * by verifing that the root directory is at the root of the
         * mount namespace which allows all files to be accessed.
         */
        if (current_chrooted())
                return -EPERM;

        /* The creator needs a mapping in the parent user namespace
         * or else we won't be able to reasonably tell userspace who
         * created a user_namespace.
         */
        if (!kuid_has_mapping(parent_ns, owner) ||
            !kgid_has_mapping(parent_ns, group))
                return -EPERM;

        ns = kmem_cache_zalloc(user_ns_cachep, GFP_KERNEL);
        if (!ns)
                return -ENOMEM;

        ret = proc_alloc_inum(&ns->proc_inum);
        if (ret) {
                kmem_cache_free(user_ns_cachep, ns);
                return ret;
        }

        atomic_set(&ns->count, 1);
        /* Leave the new->user_ns reference with the new user namespace. */
        ns->parent = parent_ns;
        ns->level = parent_ns->level + 1;
        ns->owner = owner;
        ns->group = group;

        set_cred_user_ns(new, ns);

#ifdef CONFIG_PERSISTENT_KEYRINGS
        init_rwsem(&ns->persistent_keyring_register_sem);
#endif
        return 0;
}

int unshare_userns(unsigned long unshare_flags, struct cred **new_cred)
{
        struct cred *cred;
        int err = -ENOMEM;

        if (!(unshare_flags & CLONE_NEWUSER))
                return 0;

        cred = prepare_creds();
        if (cred) {
                err = create_user_ns(cred);
                if (err)
                        put_cred(cred);
                else
                        *new_cred = cred;
        }

        return err;
}

void free_user_ns(struct user_namespace *ns)
{
        struct user_namespace *parent;

        do {
                parent = ns->parent;
#ifdef CONFIG_PERSISTENT_KEYRINGS
                key_put(ns->persistent_keyring_register);
#endif
                proc_free_inum(ns->proc_inum);
                kmem_cache_free(user_ns_cachep, ns);
                ns = parent;
        } while (atomic_dec_and_test(&parent->count));
}
EXPORT_SYMBOL(free_user_ns);

static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
{
        unsigned idx, extents;
        u32 first, last, id2;

        id2 = id + count - 1;

        /* Find the matching extent */
        extents = map->nr_extents;
        smp_rmb();
        for (idx = 0; idx < extents; idx++) {
                first = map->extent[idx].first;
                last = first + map->extent[idx].count - 1;
                if (id >= first && id <= last &&
                    (id2 >= first && id2 <= last))
                        break;
        }
        /* Map the id or note failure */
        if (idx < extents)
                id = (id - first) + map->extent[idx].lower_first;
        else
                id = (u32) -1;

        return id;
}

static u32 map_id_down(struct uid_gid_map *map, u32 id)
{
        unsigned idx, extents;
        u32 first, last;

        /* Find the matching extent */
        extents = map->nr_extents;
        smp_rmb();
        for (idx = 0; idx < extents; idx++) {
                first = map->extent[idx].first;
                last = first + map->extent[idx].count - 1;
                if (id >= first && id <= last)
                        break;
        }
        /* Map the id or note failure */
        if (idx < extents)
                id = (id - first) + map->extent[idx].lower_first;
        else
                id = (u32) -1;

        return id;
}

static u32 map_id_up(struct uid_gid_map *map, u32 id)
{
        unsigned idx, extents;
        u32 first, last;

        /* Find the matching extent */
        extents = map->nr_extents;
        smp_rmb();
        for (idx = 0; idx < extents; idx++) {
                first = map->extent[idx].lower_first;
                last = first + map->extent[idx].count - 1;
                if (id >= first && id <= last)
                        break;
        }
        /* Map the id or note failure */
        if (idx < extents)
                id = (id - first) + map->extent[idx].first;
        else
                id = (u32) -1;

        return id;
}

/**
 *      make_kuid - Map a user-namespace uid pair into a kuid.
 *      @ns:  User namespace that the uid is in
 *      @uid: User identifier
 *
 *      Maps a user-namespace uid pair into a kernel internal kuid,
 *      and returns that kuid.
 *
 *      When there is no mapping defined for the user-namespace uid
 *      pair INVALID_UID is returned.  Callers are expected to test
 *      for and handle INVALID_UID being returned.  INVALID_UID
 *      may be tested for using uid_valid().
 */
kuid_t make_kuid(struct user_namespace *ns, uid_t uid)
{
        /* Map the uid to a global kernel uid */
        return KUIDT_INIT(map_id_down(&ns->uid_map, uid));
}
EXPORT_SYMBOL(make_kuid);

/**
 *      from_kuid - Create a uid from a kuid user-namespace pair.
 *      @targ: The user namespace we want a uid in.
 *      @kuid: The kernel internal uid to start with.
 *
 *      Map @kuid into the user-namespace specified by @targ and
 *      return the resulting uid.
 *
 *      There is always a mapping into the initial user_namespace.
 *
 *      If @kuid has no mapping in @targ (uid_t)-1 is returned.
 */
uid_t from_kuid(struct user_namespace *targ, kuid_t kuid)
{
        /* Map the uid from a global kernel uid */
        return map_id_up(&targ->uid_map, __kuid_val(kuid));
}
EXPORT_SYMBOL(from_kuid);

/**
 *      from_kuid_munged - Create a uid from a kuid user-namespace pair.
 *      @targ: The user namespace we want a uid in.
 *      @kuid: The kernel internal uid to start with.
 *
 *      Map @kuid into the user-namespace specified by @targ and
 *      return the resulting uid.
 *
 *      There is always a mapping into the initial user_namespace.
 *
 *      Unlike from_kuid from_kuid_munged never fails and always
 *      returns a valid uid.  This makes from_kuid_munged appropriate
 *      for use in syscalls like stat and getuid where failing the
 *      system call and failing to provide a valid uid are not an
 *      options.
 *
 *      If @kuid has no mapping in @targ overflowuid is returned.
 */
uid_t from_kuid_munged(struct user_namespace *targ, kuid_t kuid)
{
        uid_t uid;
        uid = from_kuid(targ, kuid);

        if (uid == (uid_t) -1)
                uid = overflowuid;
        return uid;
}
EXPORT_SYMBOL(from_kuid_munged);

/**
 *      make_kgid - Map a user-namespace gid pair into a kgid.
 *      @ns:  User namespace that the gid is in
 *      @uid: group identifier
 *
 *      Maps a user-namespace gid pair into a kernel internal kgid,
 *      and returns that kgid.
 *
 *      When there is no mapping defined for the user-namespace gid
 *      pair INVALID_GID is returned.  Callers are expected to test
 *      for and handle INVALID_GID being returned.  INVALID_GID may be
 *      tested for using gid_valid().
 */
kgid_t make_kgid(struct user_namespace *ns, gid_t gid)
{
        /* Map the gid to a global kernel gid */
        return KGIDT_INIT(map_id_down(&ns->gid_map, gid));
}
EXPORT_SYMBOL(make_kgid);

/**
 *      from_kgid - Create a gid from a kgid user-namespace pair.
 *      @targ: The user namespace we want a gid in.
 *      @kgid: The kernel internal gid to start with.
 *
 *      Map @kgid into the user-namespace specified by @targ and
 *      return the resulting gid.
 *
 *      There is always a mapping into the initial user_namespace.
 *
 *      If @kgid has no mapping in @targ (gid_t)-1 is returned.
 */
gid_t from_kgid(struct user_namespace *targ, kgid_t kgid)
{
        /* Map the gid from a global kernel gid */
        return map_id_up(&targ->gid_map, __kgid_val(kgid));
}
EXPORT_SYMBOL(from_kgid);

/**
 *      from_kgid_munged - Create a gid from a kgid user-namespace pair.
 *      @targ: The user namespace we want a gid in.
 *      @kgid: The kernel internal gid to start with.
 *
 *      Map @kgid into the user-namespace specified by @targ and
 *      return the resulting gid.
 *
 *      There is always a mapping into the initial user_namespace.
 *
 *      Unlike from_kgid from_kgid_munged never fails and always
 *      returns a valid gid.  This makes from_kgid_munged appropriate
 *      for use in syscalls like stat and getgid where failing the
 *      system call and failing to provide a valid gid are not options.
 *
 *      If @kgid has no mapping in @targ overflowgid is returned.
 */
gid_t from_kgid_munged(struct user_namespace *targ, kgid_t kgid)
{
        gid_t gid;
        gid = from_kgid(targ, kgid);

        if (gid == (gid_t) -1)
                gid = overflowgid;
        return gid;
}
EXPORT_SYMBOL(from_kgid_munged);

/**
 *      make_kprojid - Map a user-namespace projid pair into a kprojid.
 *      @ns:  User namespace that the projid is in
 *      @projid: Project identifier
 *
 *      Maps a user-namespace uid pair into a kernel internal kuid,
 *      and returns that kuid.
 *
 *      When there is no mapping defined for the user-namespace projid
 *      pair INVALID_PROJID is returned.  Callers are expected to test
 *      for and handle handle INVALID_PROJID being returned.  INVALID_PROJID
 *      may be tested for using projid_valid().
 */
kprojid_t make_kprojid(struct user_namespace *ns, projid_t projid)
{
        /* Map the uid to a global kernel uid */
        return KPROJIDT_INIT(map_id_down(&ns->projid_map, projid));
}
EXPORT_SYMBOL(make_kprojid);

/**
 *      from_kprojid - Create a projid from a kprojid user-namespace pair.
 *      @targ: The user namespace we want a projid in.
 *      @kprojid: The kernel internal project identifier to start with.
 *
 *      Map @kprojid into the user-namespace specified by @targ and
 *      return the resulting projid.
 *
 *      There is always a mapping into the initial user_namespace.
 *
 *      If @kprojid has no mapping in @targ (projid_t)-1 is returned.
 */
projid_t from_kprojid(struct user_namespace *targ, kprojid_t kprojid)
{
        /* Map the uid from a global kernel uid */
        return map_id_up(&targ->projid_map, __kprojid_val(kprojid));
}
EXPORT_SYMBOL(from_kprojid);

/**
 *      from_kprojid_munged - Create a projiid from a kprojid user-namespace pair.
 *      @targ: The user namespace we want a projid in.
 *      @kprojid: The kernel internal projid to start with.
 *
 *      Map @kprojid into the user-namespace specified by @targ and
 *      return the resulting projid.
 *
 *      There is always a mapping into the initial user_namespace.
 *
 *      Unlike from_kprojid from_kprojid_munged never fails and always
 *      returns a valid projid.  This makes from_kprojid_munged
 *      appropriate for use in syscalls like stat and where
 *      failing the system call and failing to provide a valid projid are
 *      not an options.
 *
 *      If @kprojid has no mapping in @targ OVERFLOW_PROJID is returned.
 */
projid_t from_kprojid_munged(struct user_namespace *targ, kprojid_t kprojid)
{
        projid_t projid;
        projid = from_kprojid(targ, kprojid);

        if (projid == (projid_t) -1)
                projid = OVERFLOW_PROJID;
        return projid;
}
EXPORT_SYMBOL(from_kprojid_munged);


static int uid_m_show(struct seq_file *seq, void *v)
{
        struct user_namespace *ns = seq->private;
        struct uid_gid_extent *extent = v;
        struct user_namespace *lower_ns;
        uid_t lower;

        lower_ns = seq_user_ns(seq);
        if ((lower_ns == ns) && lower_ns->parent)
                lower_ns = lower_ns->parent;

        lower = from_kuid(lower_ns, KUIDT_INIT(extent->lower_first));

        seq_printf(seq, "%10u %10u %10u\n",
                extent->first,
                lower,
                extent->count);

        return 0;
}

static int gid_m_show(struct seq_file *seq, void *v)
{
        struct user_namespace *ns = seq->private;
        struct uid_gid_extent *extent = v;
        struct user_namespace *lower_ns;
        gid_t lower;

        lower_ns = seq_user_ns(seq);
        if ((lower_ns == ns) && lower_ns->parent)
                lower_ns = lower_ns->parent;

        lower = from_kgid(lower_ns, KGIDT_INIT(extent->lower_first));

        seq_printf(seq, "%10u %10u %10u\n",
                extent->first,
                lower,
                extent->count);

        return 0;
}

static int projid_m_show(struct seq_file *seq, void *v)
{
        struct user_namespace *ns = seq->private;
        struct uid_gid_extent *extent = v;
        struct user_namespace *lower_ns;
        projid_t lower;

        lower_ns = seq_user_ns(seq);
        if ((lower_ns == ns) && lower_ns->parent)
                lower_ns = lower_ns->parent;

        lower = from_kprojid(lower_ns, KPROJIDT_INIT(extent->lower_first));

        seq_printf(seq, "%10u %10u %10u\n",
                extent->first,
                lower,
                extent->count);

        return 0;
}

static void *m_start(struct seq_file *seq, loff_t *ppos, struct uid_gid_map *map)
{
        struct uid_gid_extent *extent = NULL;
        loff_t pos = *ppos;

        if (pos < map->nr_extents)
                extent = &map->extent[pos];

        return extent;
}

static void *uid_m_start(struct seq_file *seq, loff_t *ppos)
{
        struct user_namespace *ns = seq->private;

        return m_start(seq, ppos, &ns->uid_map);
}

static void *gid_m_start(struct seq_file *seq, loff_t *ppos)
{
        struct user_namespace *ns = seq->private;

        return m_start(seq, ppos, &ns->gid_map);
}

static void *projid_m_start(struct seq_file *seq, loff_t *ppos)
{
        struct user_namespace *ns = seq->private;

        return m_start(seq, ppos, &ns->projid_map);
}

static void *m_next(struct seq_file *seq, void *v, loff_t *pos)
{
        (*pos)++;
        return seq->op->start(seq, pos);
}

static void m_stop(struct seq_file *seq, void *v)
{
        return;
}

struct seq_operations proc_uid_seq_operations = {
        .start = uid_m_start,
        .stop = m_stop,
        .next = m_next,
        .show = uid_m_show,
};

struct seq_operations proc_gid_seq_operations = {
        .start = gid_m_start,
        .stop = m_stop,
        .next = m_next,
        .show = gid_m_show,
};

struct seq_operations proc_projid_seq_operations = {
        .start = projid_m_start,
        .stop = m_stop,
        .next = m_next,
        .show = projid_m_show,
};

static bool mappings_overlap(struct uid_gid_map *new_map, struct uid_gid_extent *extent)
{
        u32 upper_first, lower_first, upper_last, lower_last;
        unsigned idx;

        upper_first = extent->first;
        lower_first = extent->lower_first;
        upper_last = upper_first + extent->count - 1;
        lower_last = lower_first + extent->count - 1;

        for (idx = 0; idx < new_map->nr_extents; idx++) {
                u32 prev_upper_first, prev_lower_first;
                u32 prev_upper_last, prev_lower_last;
                struct uid_gid_extent *prev;

                prev = &new_map->extent[idx];

                prev_upper_first = prev->first;
                prev_lower_first = prev->lower_first;
                prev_upper_last = prev_upper_first + prev->count - 1;
                prev_lower_last = prev_lower_first + prev->count - 1;

                /* Does the upper range intersect a previous extent? */
                if ((prev_upper_first <= upper_last) &&
                    (prev_upper_last >= upper_first))
                        return true;

                /* Does the lower range intersect a previous extent? */
                if ((prev_lower_first <= lower_last) &&
                    (prev_lower_last >= lower_first))
                        return true;
        }
        return false;
}


static DEFINE_MUTEX(id_map_mutex);

static ssize_t map_write(struct file *file, const char __user *buf,
                         size_t count, loff_t *ppos,
                         int cap_setid,
                         struct uid_gid_map *map,
                         struct uid_gid_map *parent_map)
{
        struct seq_file *seq = file->private_data;
        struct user_namespace *ns = seq->private;
        struct uid_gid_map new_map;
        unsigned idx;
        struct uid_gid_extent *extent = NULL;
        unsigned long page = 0;
        char *kbuf, *pos, *next_line;
        ssize_t ret = -EINVAL;

        /*
         * The id_map_mutex serializes all writes to any given map.
         *
         * Any map is only ever written once.
         *
         * An id map fits within 1 cache line on most architectures.
         *
         * On read nothing needs to be done unless you are on an
         * architecture with a crazy cache coherency model like alpha.
         *
         * There is a one time data dependency between reading the
         * count of the extents and the values of the extents.  The
         * desired behavior is to see the values of the extents that
         * were written before the count of the extents.
         *
         * To achieve this smp_wmb() is used on guarantee the write
         * order and smp_rmb() is guaranteed that we don't have crazy
         * architectures returning stale data.
         */
        mutex_lock(&id_map_mutex);

        ret = -EPERM;
        /* Only allow one successful write to the map */
        if (map->nr_extents != 0)
                goto out;

        /*
         * Adjusting namespace settings requires capabilities on the target.
         */
        if (cap_valid(cap_setid) && !file_ns_capable(file, ns, CAP_SYS_ADMIN))
                goto out;

        /* Get a buffer */
        ret = -ENOMEM;
        page = __get_free_page(GFP_TEMPORARY);
        kbuf = (char *) page;
        if (!page)
                goto out;

        /* Only allow <= page size writes at the beginning of the file */
        ret = -EINVAL;
        if ((*ppos != 0) || (count >= PAGE_SIZE))
                goto out;

        /* Slurp in the user data */
        ret = -EFAULT;
        if (copy_from_user(kbuf, buf, count))
                goto out;
        kbuf[count] = '\0';

        /* Parse the user data */
        ret = -EINVAL;
        pos = kbuf;
        new_map.nr_extents = 0;
        for (;pos; pos = next_line) {
                extent = &new_map.extent[new_map.nr_extents];

                /* Find the end of line and ensure I don't look past it */
                next_line = strchr(pos, '\n');
                if (next_line) {
                        *next_line = '\0';
                        next_line++;
                        if (*next_line == '\0')
                                next_line = NULL;
                }

                pos = skip_spaces(pos);
                extent->first = simple_strtoul(pos, &pos, 10);
                if (!isspace(*pos))
                        goto out;

                pos = skip_spaces(pos);
                extent->lower_first = simple_strtoul(pos, &pos, 10);
                if (!isspace(*pos))
                        goto out;

                pos = skip_spaces(pos);
                extent->count = simple_strtoul(pos, &pos, 10);
                if (*pos && !isspace(*pos))
                        goto out;

                /* Verify there is not trailing junk on the line */
                pos = skip_spaces(pos);
                if (*pos != '\0')
                        goto out;

                /* Verify we have been given valid starting values */
                if ((extent->first == (u32) -1) ||
                    (extent->lower_first == (u32) -1 ))
                        goto out;

                /* Verify count is not zero and does not cause the extent to wrap */
                if ((extent->first + extent->count) <= extent->first)
                        goto out;
                if ((extent->lower_first + extent->count) <= extent->lower_first)
                        goto out;

                /* Do the ranges in extent overlap any previous extents? */
                if (mappings_overlap(&new_map, extent))
                        goto out;

                new_map.nr_extents++;

                /* Fail if the file contains too many extents */
                if ((new_map.nr_extents == UID_GID_MAP_MAX_EXTENTS) &&
                    (next_line != NULL))
                        goto out;
        }
        /* Be very certaint the new map actually exists */
        if (new_map.nr_extents == 0)
                goto out;

        ret = -EPERM;
        /* Validate the user is allowed to use user id's mapped to. */
        if (!new_idmap_permitted(file, ns, cap_setid, &new_map))
                goto out;

        /* Map the lower ids from the parent user namespace to the
         * kernel global id space.
         */
        for (idx = 0; idx < new_map.nr_extents; idx++) {
                u32 lower_first;
                extent = &new_map.extent[idx];

                lower_first = map_id_range_down(parent_map,
                                                extent->lower_first,
                                                extent->count);

                /* Fail if we can not map the specified extent to
                 * the kernel global id space.
                 */
                if (lower_first == (u32) -1)
                        goto out;

                extent->lower_first = lower_first;
        }

        /* Install the map */
        memcpy(map->extent, new_map.extent,
                new_map.nr_extents*sizeof(new_map.extent[0]));
        smp_wmb();
        map->nr_extents = new_map.nr_extents;

        *ppos = count;
        ret = count;
out:
        mutex_unlock(&id_map_mutex);
        if (page)
                free_page(page);
        return ret;
}

ssize_t proc_uid_map_write(struct file *file, const char __user *buf, size_t size, loff_t *ppos)
{
        struct seq_file *seq = file->private_data;
        struct user_namespace *ns = seq->private;
        struct user_namespace *seq_ns = seq_user_ns(seq);

        if (!ns->parent)
                return -EPERM;

        if ((seq_ns != ns) && (seq_ns != ns->parent))
                return -EPERM;

        return map_write(file, buf, size, ppos, CAP_SETUID,
                         &ns->uid_map, &ns->parent->uid_map);
}

ssize_t proc_gid_map_write(struct file *file, const char __user *buf, size_t size, loff_t *ppos)
{
        struct seq_file *seq = file->private_data;
        struct user_namespace *ns = seq->private;
        struct user_namespace *seq_ns = seq_user_ns(seq);

        if (!ns->parent)
                return -EPERM;

        if ((seq_ns != ns) && (seq_ns != ns->parent))
                return -EPERM;

        return map_write(file, buf, size, ppos, CAP_SETGID,
                         &ns->gid_map, &ns->parent->gid_map);
}

ssize_t proc_projid_map_write(struct file *file, const char __user *buf, size_t size, loff_t *ppos)
{
        struct seq_file *seq = file->private_data;
        struct user_namespace *ns = seq->private;
        struct user_namespace *seq_ns = seq_user_ns(seq);

        if (!ns->parent)
                return -EPERM;

        if ((seq_ns != ns) && (seq_ns != ns->parent))
                return -EPERM;

        /* Anyone can set any valid project id no capability needed */
        return map_write(file, buf, size, ppos, -1,
                         &ns->projid_map, &ns->parent->projid_map);
}

static bool new_idmap_permitted(const struct file *file, 
                                struct user_namespace *ns, int cap_setid,
                                struct uid_gid_map *new_map)
{
        /* Allow mapping to your own filesystem ids */
        if ((new_map->nr_extents == 1) && (new_map->extent[0].count == 1)) {
                u32 id = new_map->extent[0].lower_first;
                if (cap_setid == CAP_SETUID) {
                        kuid_t uid = make_kuid(ns->parent, id);
                        if (uid_eq(uid, file->f_cred->fsuid))
                                return true;
                }
                else if (cap_setid == CAP_SETGID) {
                        kgid_t gid = make_kgid(ns->parent, id);
                        if (gid_eq(gid, file->f_cred->fsgid))
                                return true;
                }
        }

        /* Allow anyone to set a mapping that doesn't require privilege */
        if (!cap_valid(cap_setid))
                return true;

        /* Allow the specified ids if we have the appropriate capability
         * (CAP_SETUID or CAP_SETGID) over the parent user namespace.
         * And the opener of the id file also had the approprpiate capability.
         */
        if (ns_capable(ns->parent, cap_setid) &&
            file_ns_capable(file, ns->parent, cap_setid))
                return true;

        return false;
}

static void *userns_get(struct task_struct *task)
{
        struct user_namespace *user_ns;

        rcu_read_lock();
        user_ns = get_user_ns(__task_cred(task)->user_ns);
        rcu_read_unlock();

        return user_ns;
}

static void userns_put(void *ns)
{
        put_user_ns(ns);
}

static int userns_install(struct nsproxy *nsproxy, void *ns)
{
        struct user_namespace *user_ns = ns;
        struct cred *cred;

        /* Don't allow gaining capabilities by reentering
         * the same user namespace.
         */
        if (user_ns == current_user_ns())
                return -EINVAL;

        /* Threaded processes may not enter a different user namespace */
        if (atomic_read(&current->mm->mm_users) > 1)
                return -EINVAL;

        if (current->fs->users != 1)
                return -EINVAL;

        if (!ns_capable(user_ns, CAP_SYS_ADMIN))
                return -EPERM;

        cred = prepare_creds();
        if (!cred)
                return -ENOMEM;

        put_user_ns(cred->user_ns);
        set_cred_user_ns(cred, get_user_ns(user_ns));

        return commit_creds(cred);
}

static unsigned int userns_inum(void *ns)
{
        struct user_namespace *user_ns = ns;
        return user_ns->proc_inum;
}

const struct proc_ns_operations userns_operations = {
        .name           = "user",
        .type           = CLONE_NEWUSER,
        .get            = userns_get,
        .put            = userns_put,
        .install        = userns_install,
        .inum           = userns_inum,
};

static __init int user_namespaces_init(void)
{
        user_ns_cachep = KMEM_CACHE(user_namespace, SLAB_PANIC);
        return 0;
}
subsys_initcall(user_namespaces_init);