CRED: Allow kernel services to override LSM settings for task actions

[deliverable/linux.git] / kernel / cred.c

/* Task credentials management - see Documentation/credentials.txt
 *
 * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */
#include <linux/module.h>
#include <linux/cred.h>
#include <linux/sched.h>
#include <linux/key.h>
#include <linux/keyctl.h>
#include <linux/init_task.h>
#include <linux/security.h>
#include <linux/cn_proc.h>
#include "cred-internals.h"

static struct kmem_cache *cred_jar;

/*
 * The common credentials for the initial task's thread group
 */
#ifdef CONFIG_KEYS
static struct thread_group_cred init_tgcred = {
        .usage  = ATOMIC_INIT(2),
        .tgid   = 0,
        .lock   = SPIN_LOCK_UNLOCKED,
};
#endif

/*
 * The initial credentials for the initial task
 */
struct cred init_cred = {
        .usage                  = ATOMIC_INIT(4),
        .securebits             = SECUREBITS_DEFAULT,
        .cap_inheritable        = CAP_INIT_INH_SET,
        .cap_permitted          = CAP_FULL_SET,
        .cap_effective          = CAP_INIT_EFF_SET,
        .cap_bset               = CAP_INIT_BSET,
        .user                   = INIT_USER,
        .group_info             = &init_groups,
#ifdef CONFIG_KEYS
        .tgcred                 = &init_tgcred,
#endif
};

/*
 * Dispose of the shared task group credentials
 */
#ifdef CONFIG_KEYS
static void release_tgcred_rcu(struct rcu_head *rcu)
{
        struct thread_group_cred *tgcred =
                container_of(rcu, struct thread_group_cred, rcu);

        BUG_ON(atomic_read(&tgcred->usage) != 0);

        key_put(tgcred->session_keyring);
        key_put(tgcred->process_keyring);
        kfree(tgcred);
}
#endif

/*
 * Release a set of thread group credentials.
 */
static void release_tgcred(struct cred *cred)
{
#ifdef CONFIG_KEYS
        struct thread_group_cred *tgcred = cred->tgcred;

        if (atomic_dec_and_test(&tgcred->usage))
                call_rcu(&tgcred->rcu, release_tgcred_rcu);
#endif
}

/*
 * The RCU callback to actually dispose of a set of credentials
 */
static void put_cred_rcu(struct rcu_head *rcu)
{
        struct cred *cred = container_of(rcu, struct cred, rcu);

        if (atomic_read(&cred->usage) != 0)
                panic("CRED: put_cred_rcu() sees %p with usage %d\n",
                      cred, atomic_read(&cred->usage));

        security_cred_free(cred);
        key_put(cred->thread_keyring);
        key_put(cred->request_key_auth);
        release_tgcred(cred);
        put_group_info(cred->group_info);
        free_uid(cred->user);
        kmem_cache_free(cred_jar, cred);
}

/**
 * __put_cred - Destroy a set of credentials
 * @cred: The record to release
 *
 * Destroy a set of credentials on which no references remain.
 */
void __put_cred(struct cred *cred)
{
        BUG_ON(atomic_read(&cred->usage) != 0);

        call_rcu(&cred->rcu, put_cred_rcu);
}
EXPORT_SYMBOL(__put_cred);

/**
 * prepare_creds - Prepare a new set of credentials for modification
 *
 * Prepare a new set of task credentials for modification.  A task's creds
 * shouldn't generally be modified directly, therefore this function is used to
 * prepare a new copy, which the caller then modifies and then commits by
 * calling commit_creds().
 *
 * Preparation involves making a copy of the objective creds for modification.
 *
 * Returns a pointer to the new creds-to-be if successful, NULL otherwise.
 *
 * Call commit_creds() or abort_creds() to clean up.
 */
struct cred *prepare_creds(void)
{
        struct task_struct *task = current;
        const struct cred *old;
        struct cred *new;

        BUG_ON(atomic_read(&task->real_cred->usage) < 1);

        new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
        if (!new)
                return NULL;

        old = task->cred;
        memcpy(new, old, sizeof(struct cred));

        atomic_set(&new->usage, 1);
        get_group_info(new->group_info);
        get_uid(new->user);

#ifdef CONFIG_KEYS
        key_get(new->thread_keyring);
        key_get(new->request_key_auth);
        atomic_inc(&new->tgcred->usage);
#endif

#ifdef CONFIG_SECURITY
        new->security = NULL;
#endif

        if (security_prepare_creds(new, old, GFP_KERNEL) < 0)
                goto error;
        return new;

error:
        abort_creds(new);
        return NULL;
}
EXPORT_SYMBOL(prepare_creds);

/*
 * Prepare credentials for current to perform an execve()
 * - The caller must hold current->cred_exec_mutex
 */
struct cred *prepare_exec_creds(void)
{
        struct thread_group_cred *tgcred = NULL;
        struct cred *new;

#ifdef CONFIG_KEYS
        tgcred = kmalloc(sizeof(*tgcred), GFP_KERNEL);
        if (!tgcred)
                return NULL;
#endif

        new = prepare_creds();
        if (!new) {
                kfree(tgcred);
                return new;
        }

#ifdef CONFIG_KEYS
        /* newly exec'd tasks don't get a thread keyring */
        key_put(new->thread_keyring);
        new->thread_keyring = NULL;

        /* create a new per-thread-group creds for all this set of threads to
         * share */
        memcpy(tgcred, new->tgcred, sizeof(struct thread_group_cred));

        atomic_set(&tgcred->usage, 1);
        spin_lock_init(&tgcred->lock);

        /* inherit the session keyring; new process keyring */
        key_get(tgcred->session_keyring);
        tgcred->process_keyring = NULL;

        release_tgcred(new);
        new->tgcred = tgcred;
#endif

        return new;
}

/*
 * prepare new credentials for the usermode helper dispatcher
 */
struct cred *prepare_usermodehelper_creds(void)
{
#ifdef CONFIG_KEYS
        struct thread_group_cred *tgcred = NULL;
#endif
        struct cred *new;

#ifdef CONFIG_KEYS
        tgcred = kzalloc(sizeof(*new->tgcred), GFP_ATOMIC);
        if (!tgcred)
                return NULL;
#endif

        new = kmem_cache_alloc(cred_jar, GFP_ATOMIC);
        if (!new)
                return NULL;

        memcpy(new, &init_cred, sizeof(struct cred));

        atomic_set(&new->usage, 1);
        get_group_info(new->group_info);
        get_uid(new->user);

#ifdef CONFIG_KEYS
        new->thread_keyring = NULL;
        new->request_key_auth = NULL;
        new->jit_keyring = KEY_REQKEY_DEFL_DEFAULT;

        atomic_set(&tgcred->usage, 1);
        spin_lock_init(&tgcred->lock);
        new->tgcred = tgcred;
#endif

#ifdef CONFIG_SECURITY
        new->security = NULL;
#endif
        if (security_prepare_creds(new, &init_cred, GFP_ATOMIC) < 0)
                goto error;

        BUG_ON(atomic_read(&new->usage) != 1);
        return new;

error:
        put_cred(new);
        return NULL;
}

/*
 * Copy credentials for the new process created by fork()
 *
 * We share if we can, but under some circumstances we have to generate a new
 * set.
 *
 * The new process gets the current process's subjective credentials as its
 * objective and subjective credentials
 */
int copy_creds(struct task_struct *p, unsigned long clone_flags)
{
#ifdef CONFIG_KEYS
        struct thread_group_cred *tgcred;
#endif
        struct cred *new;

        mutex_init(&p->cred_exec_mutex);

        if (
#ifdef CONFIG_KEYS
                !p->cred->thread_keyring &&
#endif
                clone_flags & CLONE_THREAD
            ) {
                p->real_cred = get_cred(p->cred);
                get_cred(p->cred);
                atomic_inc(&p->cred->user->processes);
                return 0;
        }

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

#ifdef CONFIG_KEYS
        /* new threads get their own thread keyrings if their parent already
         * had one */
        if (new->thread_keyring) {
                key_put(new->thread_keyring);
                new->thread_keyring = NULL;
                if (clone_flags & CLONE_THREAD)
                        install_thread_keyring_to_cred(new);
        }

        /* we share the process and session keyrings between all the threads in
         * a process - this is slightly icky as we violate COW credentials a
         * bit */
        if (!(clone_flags & CLONE_THREAD)) {
                tgcred = kmalloc(sizeof(*tgcred), GFP_KERNEL);
                if (!tgcred) {
                        put_cred(new);
                        return -ENOMEM;
                }
                atomic_set(&tgcred->usage, 1);
                spin_lock_init(&tgcred->lock);
                tgcred->process_keyring = NULL;
                tgcred->session_keyring = key_get(new->tgcred->session_keyring);

                release_tgcred(new);
                new->tgcred = tgcred;
        }
#endif

        atomic_inc(&new->user->processes);
        p->cred = p->real_cred = get_cred(new);
        return 0;
}

/**
 * commit_creds - Install new credentials upon the current task
 * @new: The credentials to be assigned
 *
 * Install a new set of credentials to the current task, using RCU to replace
 * the old set.  Both the objective and the subjective credentials pointers are
 * updated.  This function may not be called if the subjective credentials are
 * in an overridden state.
 *
 * This function eats the caller's reference to the new credentials.
 *
 * Always returns 0 thus allowing this function to be tail-called at the end
 * of, say, sys_setgid().
 */
int commit_creds(struct cred *new)
{
        struct task_struct *task = current;
        const struct cred *old;

        BUG_ON(task->cred != task->real_cred);
        BUG_ON(atomic_read(&task->real_cred->usage) < 2);
        BUG_ON(atomic_read(&new->usage) < 1);

        old = task->real_cred;
        security_commit_creds(new, old);

        get_cred(new); /* we will require a ref for the subj creds too */

        /* dumpability changes */
        if (old->euid != new->euid ||
            old->egid != new->egid ||
            old->fsuid != new->fsuid ||
            old->fsgid != new->fsgid ||
            !cap_issubset(new->cap_permitted, old->cap_permitted)) {
                set_dumpable(task->mm, suid_dumpable);
                task->pdeath_signal = 0;
                smp_wmb();
        }

        /* alter the thread keyring */
        if (new->fsuid != old->fsuid)
                key_fsuid_changed(task);
        if (new->fsgid != old->fsgid)
                key_fsgid_changed(task);

        /* do it
         * - What if a process setreuid()'s and this brings the
         *   new uid over his NPROC rlimit?  We can check this now
         *   cheaply with the new uid cache, so if it matters
         *   we should be checking for it.  -DaveM
         */
        if (new->user != old->user)
                atomic_inc(&new->user->processes);
        rcu_assign_pointer(task->real_cred, new);
        rcu_assign_pointer(task->cred, new);
        if (new->user != old->user)
                atomic_dec(&old->user->processes);

        sched_switch_user(task);

        /* send notifications */
        if (new->uid   != old->uid  ||
            new->euid  != old->euid ||
            new->suid  != old->suid ||
            new->fsuid != old->fsuid)
                proc_id_connector(task, PROC_EVENT_UID);

        if (new->gid   != old->gid  ||
            new->egid  != old->egid ||
            new->sgid  != old->sgid ||
            new->fsgid != old->fsgid)
                proc_id_connector(task, PROC_EVENT_GID);

        /* release the old obj and subj refs both */
        put_cred(old);
        put_cred(old);
        return 0;
}
EXPORT_SYMBOL(commit_creds);

/**
 * abort_creds - Discard a set of credentials and unlock the current task
 * @new: The credentials that were going to be applied
 *
 * Discard a set of credentials that were under construction and unlock the
 * current task.
 */
void abort_creds(struct cred *new)
{
        BUG_ON(atomic_read(&new->usage) < 1);
        put_cred(new);
}
EXPORT_SYMBOL(abort_creds);

/**
 * override_creds - Override the current process's subjective credentials
 * @new: The credentials to be assigned
 *
 * Install a set of temporary override subjective credentials on the current
 * process, returning the old set for later reversion.
 */
const struct cred *override_creds(const struct cred *new)
{
        const struct cred *old = current->cred;

        rcu_assign_pointer(current->cred, get_cred(new));
        return old;
}
EXPORT_SYMBOL(override_creds);

/**
 * revert_creds - Revert a temporary subjective credentials override
 * @old: The credentials to be restored
 *
 * Revert a temporary set of override subjective credentials to an old set,
 * discarding the override set.
 */
void revert_creds(const struct cred *old)
{
        const struct cred *override = current->cred;

        rcu_assign_pointer(current->cred, old);
        put_cred(override);
}
EXPORT_SYMBOL(revert_creds);

/*
 * initialise the credentials stuff
 */
void __init cred_init(void)
{
        /* allocate a slab in which we can store credentials */
        cred_jar = kmem_cache_create("cred_jar", sizeof(struct cred),
                                     0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
}

/**
 * prepare_kernel_cred - Prepare a set of credentials for a kernel service
 * @daemon: A userspace daemon to be used as a reference
 *
 * Prepare a set of credentials for a kernel service.  This can then be used to
 * override a task's own credentials so that work can be done on behalf of that
 * task that requires a different subjective context.
 *
 * @daemon is used to provide a base for the security record, but can be NULL.
 * If @daemon is supplied, then the security data will be derived from that;
 * otherwise they'll be set to 0 and no groups, full capabilities and no keys.
 *
 * The caller may change these controls afterwards if desired.
 *
 * Returns the new credentials or NULL if out of memory.
 *
 * Does not take, and does not return holding current->cred_replace_mutex.
 */
struct cred *prepare_kernel_cred(struct task_struct *daemon)
{
        const struct cred *old;
        struct cred *new;

        new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
        if (!new)
                return NULL;

        if (daemon)
                old = get_task_cred(daemon);
        else
                old = get_cred(&init_cred);

        get_uid(new->user);
        get_group_info(new->group_info);

#ifdef CONFIG_KEYS
        atomic_inc(&init_tgcred.usage);
        new->tgcred = &init_tgcred;
        new->request_key_auth = NULL;
        new->thread_keyring = NULL;
        new->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
#endif

#ifdef CONFIG_SECURITY
        new->security = NULL;
#endif
        if (security_prepare_creds(new, old, GFP_KERNEL) < 0)
                goto error;

        atomic_set(&new->usage, 1);
        put_cred(old);
        return new;

error:
        put_cred(new);
        return NULL;
}
EXPORT_SYMBOL(prepare_kernel_cred);

/**
 * set_security_override - Set the security ID in a set of credentials
 * @new: The credentials to alter
 * @secid: The LSM security ID to set
 *
 * Set the LSM security ID in a set of credentials so that the subjective
 * security is overridden when an alternative set of credentials is used.
 */
int set_security_override(struct cred *new, u32 secid)
{
        return security_kernel_act_as(new, secid);
}
EXPORT_SYMBOL(set_security_override);

/**
 * set_security_override_from_ctx - Set the security ID in a set of credentials
 * @new: The credentials to alter
 * @secctx: The LSM security context to generate the security ID from.
 *
 * Set the LSM security ID in a set of credentials so that the subjective
 * security is overridden when an alternative set of credentials is used.  The
 * security ID is specified in string form as a security context to be
 * interpreted by the LSM.
 */
int set_security_override_from_ctx(struct cred *new, const char *secctx)
{
        u32 secid;
        int ret;

        ret = security_secctx_to_secid(secctx, strlen(secctx), &secid);
        if (ret < 0)
                return ret;

        return set_security_override(new, secid);
}
EXPORT_SYMBOL(set_security_override_from_ctx);

/**
 * set_create_files_as - Set the LSM file create context in a set of credentials
 * @new: The credentials to alter
 * @inode: The inode to take the context from
 *
 * Change the LSM file creation context in a set of credentials to be the same
 * as the object context of the specified inode, so that the new inodes have
 * the same MAC context as that inode.
 */
int set_create_files_as(struct cred *new, struct inode *inode)
{
        new->fsuid = inode->i_uid;
        new->fsgid = inode->i_gid;
        return security_kernel_create_files_as(new, inode);
}
EXPORT_SYMBOL(set_create_files_as);