TOMOYO: Use GFP_NOFS rather than GFP_KERNEL.

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / security / selinux / avc.c

/*
 * Implementation of the kernel access vector cache (AVC).
 *
 * Authors:  Stephen Smalley, <sds@epoch.ncsc.mil>
 *           James Morris <jmorris@redhat.com>
 *
 * Update:   KaiGai, Kohei <kaigai@ak.jp.nec.com>
 *      Replaced the avc_lock spinlock by RCU.
 *
 * Copyright (C) 2003 Red Hat, Inc., James Morris <jmorris@redhat.com>
 *
 *      This program is free software; you can redistribute it and/or modify
 *      it under the terms of the GNU General Public License version 2,
 *      as published by the Free Software Foundation.
 */
#include <linux/types.h>
#include <linux/stddef.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/fs.h>
#include <linux/dcache.h>
#include <linux/init.h>
#include <linux/skbuff.h>
#include <linux/percpu.h>
#include <net/sock.h>
#include <linux/un.h>
#include <net/af_unix.h>
#include <linux/ip.h>
#include <linux/audit.h>
#include <linux/ipv6.h>
#include <net/ipv6.h>
#include "avc.h"
#include "avc_ss.h"
#include "classmap.h"

#define AVC_CACHE_SLOTS                 512
#define AVC_DEF_CACHE_THRESHOLD         512
#define AVC_CACHE_RECLAIM               16

#ifdef CONFIG_SECURITY_SELINUX_AVC_STATS
#define avc_cache_stats_incr(field)                             \
do {                                                            \
        per_cpu(avc_cache_stats, get_cpu()).field++;            \
        put_cpu();                                              \
} while (0)
#else
#define avc_cache_stats_incr(field)     do {} while (0)
#endif

struct avc_entry {
        u32                     ssid;
        u32                     tsid;
        u16                     tclass;
        struct av_decision      avd;
};

struct avc_node {
        struct avc_entry        ae;
        struct hlist_node       list; /* anchored in avc_cache->slots[i] */
        struct rcu_head         rhead;
};

struct avc_cache {
        struct hlist_head       slots[AVC_CACHE_SLOTS]; /* head for avc_node->list */
        spinlock_t              slots_lock[AVC_CACHE_SLOTS]; /* lock for writes */
        atomic_t                lru_hint;       /* LRU hint for reclaim scan */
        atomic_t                active_nodes;
        u32                     latest_notif;   /* latest revocation notification */
};

struct avc_callback_node {
        int (*callback) (u32 event, u32 ssid, u32 tsid,
                         u16 tclass, u32 perms,
                         u32 *out_retained);
        u32 events;
        u32 ssid;
        u32 tsid;
        u16 tclass;
        u32 perms;
        struct avc_callback_node *next;
};

/* Exported via selinufs */
unsigned int avc_cache_threshold = AVC_DEF_CACHE_THRESHOLD;

#ifdef CONFIG_SECURITY_SELINUX_AVC_STATS
DEFINE_PER_CPU(struct avc_cache_stats, avc_cache_stats) = { 0 };
#endif

static struct avc_cache avc_cache;
static struct avc_callback_node *avc_callbacks;
static struct kmem_cache *avc_node_cachep;

static inline int avc_hash(u32 ssid, u32 tsid, u16 tclass)
{
        return (ssid ^ (tsid<<2) ^ (tclass<<4)) & (AVC_CACHE_SLOTS - 1);
}

/**
 * avc_dump_av - Display an access vector in human-readable form.
 * @tclass: target security class
 * @av: access vector
 */
static void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av)
{
        const char **perms;
        int i, perm;

        if (av == 0) {
                audit_log_format(ab, " null");
                return;
        }

        perms = secclass_map[tclass-1].perms;

        audit_log_format(ab, " {");
        i = 0;
        perm = 1;
        while (i < (sizeof(av) * 8)) {
                if ((perm & av) && perms[i]) {
                        audit_log_format(ab, " %s", perms[i]);
                        av &= ~perm;
                }
                i++;
                perm <<= 1;
        }

        if (av)
                audit_log_format(ab, " 0x%x", av);

        audit_log_format(ab, " }");
}

/**
 * avc_dump_query - Display a SID pair and a class in human-readable form.
 * @ssid: source security identifier
 * @tsid: target security identifier
 * @tclass: target security class
 */
static void avc_dump_query(struct audit_buffer *ab, u32 ssid, u32 tsid, u16 tclass)
{
        int rc;
        char *scontext;
        u32 scontext_len;

        rc = security_sid_to_context(ssid, &scontext, &scontext_len);
        if (rc)
                audit_log_format(ab, "ssid=%d", ssid);
        else {
                audit_log_format(ab, "scontext=%s", scontext);
                kfree(scontext);
        }

        rc = security_sid_to_context(tsid, &scontext, &scontext_len);
        if (rc)
                audit_log_format(ab, " tsid=%d", tsid);
        else {
                audit_log_format(ab, " tcontext=%s", scontext);
                kfree(scontext);
        }

        BUG_ON(tclass >= ARRAY_SIZE(secclass_map));
        audit_log_format(ab, " tclass=%s", secclass_map[tclass-1].name);
}

/**
 * avc_init - Initialize the AVC.
 *
 * Initialize the access vector cache.
 */
void __init avc_init(void)
{
        int i;

        for (i = 0; i < AVC_CACHE_SLOTS; i++) {
                INIT_HLIST_HEAD(&avc_cache.slots[i]);
                spin_lock_init(&avc_cache.slots_lock[i]);
        }
        atomic_set(&avc_cache.active_nodes, 0);
        atomic_set(&avc_cache.lru_hint, 0);

        avc_node_cachep = kmem_cache_create("avc_node", sizeof(struct avc_node),
                                             0, SLAB_PANIC, NULL);

        audit_log(current->audit_context, GFP_KERNEL, AUDIT_KERNEL, "AVC INITIALIZED\n");
}

int avc_get_hash_stats(char *page)
{
        int i, chain_len, max_chain_len, slots_used;
        struct avc_node *node;
        struct hlist_head *head;

        rcu_read_lock();

        slots_used = 0;
        max_chain_len = 0;
        for (i = 0; i < AVC_CACHE_SLOTS; i++) {
                head = &avc_cache.slots[i];
                if (!hlist_empty(head)) {
                        struct hlist_node *next;

                        slots_used++;
                        chain_len = 0;
                        hlist_for_each_entry_rcu(node, next, head, list)
                                chain_len++;
                        if (chain_len > max_chain_len)
                                max_chain_len = chain_len;
                }
        }

        rcu_read_unlock();

        return scnprintf(page, PAGE_SIZE, "entries: %d\nbuckets used: %d/%d\n"
                         "longest chain: %d\n",
                         atomic_read(&avc_cache.active_nodes),
                         slots_used, AVC_CACHE_SLOTS, max_chain_len);
}

static void avc_node_free(struct rcu_head *rhead)
{
        struct avc_node *node = container_of(rhead, struct avc_node, rhead);
        kmem_cache_free(avc_node_cachep, node);
        avc_cache_stats_incr(frees);
}

static void avc_node_delete(struct avc_node *node)
{
        hlist_del_rcu(&node->list);
        call_rcu(&node->rhead, avc_node_free);
        atomic_dec(&avc_cache.active_nodes);
}

static void avc_node_kill(struct avc_node *node)
{
        kmem_cache_free(avc_node_cachep, node);
        avc_cache_stats_incr(frees);
        atomic_dec(&avc_cache.active_nodes);
}

static void avc_node_replace(struct avc_node *new, struct avc_node *old)
{
        hlist_replace_rcu(&old->list, &new->list);
        call_rcu(&old->rhead, avc_node_free);
        atomic_dec(&avc_cache.active_nodes);
}

static inline int avc_reclaim_node(void)
{
        struct avc_node *node;
        int hvalue, try, ecx;
        unsigned long flags;
        struct hlist_head *head;
        struct hlist_node *next;
        spinlock_t *lock;

        for (try = 0, ecx = 0; try < AVC_CACHE_SLOTS; try++) {
                hvalue = atomic_inc_return(&avc_cache.lru_hint) & (AVC_CACHE_SLOTS - 1);
                head = &avc_cache.slots[hvalue];
                lock = &avc_cache.slots_lock[hvalue];

                if (!spin_trylock_irqsave(lock, flags))
                        continue;

                rcu_read_lock();
                hlist_for_each_entry(node, next, head, list) {
                        avc_node_delete(node);
                        avc_cache_stats_incr(reclaims);
                        ecx++;
                        if (ecx >= AVC_CACHE_RECLAIM) {
                                rcu_read_unlock();
                                spin_unlock_irqrestore(lock, flags);
                                goto out;
                        }
                }
                rcu_read_unlock();
                spin_unlock_irqrestore(lock, flags);
        }
out:
        return ecx;
}

static struct avc_node *avc_alloc_node(void)
{
        struct avc_node *node;

        node = kmem_cache_zalloc(avc_node_cachep, GFP_ATOMIC);
        if (!node)
                goto out;

        INIT_RCU_HEAD(&node->rhead);
        INIT_HLIST_NODE(&node->list);
        avc_cache_stats_incr(allocations);

        if (atomic_inc_return(&avc_cache.active_nodes) > avc_cache_threshold)
                avc_reclaim_node();

out:
        return node;
}

static void avc_node_populate(struct avc_node *node, u32 ssid, u32 tsid, u16 tclass, struct av_decision *avd)
{
        node->ae.ssid = ssid;
        node->ae.tsid = tsid;
        node->ae.tclass = tclass;
        memcpy(&node->ae.avd, avd, sizeof(node->ae.avd));
}

static inline struct avc_node *avc_search_node(u32 ssid, u32 tsid, u16 tclass)
{
        struct avc_node *node, *ret = NULL;
        int hvalue;
        struct hlist_head *head;
        struct hlist_node *next;

        hvalue = avc_hash(ssid, tsid, tclass);
        head = &avc_cache.slots[hvalue];
        hlist_for_each_entry_rcu(node, next, head, list) {
                if (ssid == node->ae.ssid &&
                    tclass == node->ae.tclass &&
                    tsid == node->ae.tsid) {
                        ret = node;
                        break;
                }
        }

        return ret;
}

/**
 * avc_lookup - Look up an AVC entry.
 * @ssid: source security identifier
 * @tsid: target security identifier
 * @tclass: target security class
 *
 * Look up an AVC entry that is valid for the
 * (@ssid, @tsid), interpreting the permissions
 * based on @tclass.  If a valid AVC entry exists,
 * then this function returns the avc_node.
 * Otherwise, this function returns NULL.
 */
static struct avc_node *avc_lookup(u32 ssid, u32 tsid, u16 tclass)
{
        struct avc_node *node;

        avc_cache_stats_incr(lookups);
        node = avc_search_node(ssid, tsid, tclass);

        if (node)
                avc_cache_stats_incr(hits);
        else
                avc_cache_stats_incr(misses);

        return node;
}

static int avc_latest_notif_update(int seqno, int is_insert)
{
        int ret = 0;
        static DEFINE_SPINLOCK(notif_lock);
        unsigned long flag;

        spin_lock_irqsave(&notif_lock, flag);
        if (is_insert) {
                if (seqno < avc_cache.latest_notif) {
                        printk(KERN_WARNING "SELinux: avc:  seqno %d < latest_notif %d\n",
                               seqno, avc_cache.latest_notif);
                        ret = -EAGAIN;
                }
        } else {
                if (seqno > avc_cache.latest_notif)
                        avc_cache.latest_notif = seqno;
        }
        spin_unlock_irqrestore(&notif_lock, flag);

        return ret;
}

/**
 * avc_insert - Insert an AVC entry.
 * @ssid: source security identifier
 * @tsid: target security identifier
 * @tclass: target security class
 * @avd: resulting av decision
 *
 * Insert an AVC entry for the SID pair
 * (@ssid, @tsid) and class @tclass.
 * The access vectors and the sequence number are
 * normally provided by the security server in
 * response to a security_compute_av() call.  If the
 * sequence number @avd->seqno is not less than the latest
 * revocation notification, then the function copies
 * the access vectors into a cache entry, returns
 * avc_node inserted. Otherwise, this function returns NULL.
 */
static struct avc_node *avc_insert(u32 ssid, u32 tsid, u16 tclass, struct av_decision *avd)
{
        struct avc_node *pos, *node = NULL;
        int hvalue;
        unsigned long flag;

        if (avc_latest_notif_update(avd->seqno, 1))
                goto out;

        node = avc_alloc_node();
        if (node) {
                struct hlist_head *head;
                struct hlist_node *next;
                spinlock_t *lock;

                hvalue = avc_hash(ssid, tsid, tclass);
                avc_node_populate(node, ssid, tsid, tclass, avd);

                head = &avc_cache.slots[hvalue];
                lock = &avc_cache.slots_lock[hvalue];

                spin_lock_irqsave(lock, flag);
                hlist_for_each_entry(pos, next, head, list) {
                        if (pos->ae.ssid == ssid &&
                            pos->ae.tsid == tsid &&
                            pos->ae.tclass == tclass) {
                                avc_node_replace(node, pos);
                                goto found;
                        }
                }
                hlist_add_head_rcu(&node->list, head);
found:
                spin_unlock_irqrestore(lock, flag);
        }
out:
        return node;
}

/**
 * avc_audit_pre_callback - SELinux specific information
 * will be called by generic audit code
 * @ab: the audit buffer
 * @a: audit_data
 */
static void avc_audit_pre_callback(struct audit_buffer *ab, void *a)
{
        struct common_audit_data *ad = a;
        audit_log_format(ab, "avc:  %s ",
                         ad->selinux_audit_data.denied ? "denied" : "granted");
        avc_dump_av(ab, ad->selinux_audit_data.tclass,
                        ad->selinux_audit_data.audited);
        audit_log_format(ab, " for ");
}

/**
 * avc_audit_post_callback - SELinux specific information
 * will be called by generic audit code
 * @ab: the audit buffer
 * @a: audit_data
 */
static void avc_audit_post_callback(struct audit_buffer *ab, void *a)
{
        struct common_audit_data *ad = a;
        audit_log_format(ab, " ");
        avc_dump_query(ab, ad->selinux_audit_data.ssid,
                           ad->selinux_audit_data.tsid,
                           ad->selinux_audit_data.tclass);
}

/**
 * avc_audit - Audit the granting or denial of permissions.
 * @ssid: source security identifier
 * @tsid: target security identifier
 * @tclass: target security class
 * @requested: requested permissions
 * @avd: access vector decisions
 * @result: result from avc_has_perm_noaudit
 * @a:  auxiliary audit data
 *
 * Audit the granting or denial of permissions in accordance
 * with the policy.  This function is typically called by
 * avc_has_perm() after a permission check, but can also be
 * called directly by callers who use avc_has_perm_noaudit()
 * in order to separate the permission check from the auditing.
 * For example, this separation is useful when the permission check must
 * be performed under a lock, to allow the lock to be released
 * before calling the auditing code.
 */
void avc_audit(u32 ssid, u32 tsid,
               u16 tclass, u32 requested,
               struct av_decision *avd, int result, struct common_audit_data *a)
{
        struct common_audit_data stack_data;
        u32 denied, audited;
        denied = requested & ~avd->allowed;
        if (denied)
                audited = denied & avd->auditdeny;
        else if (result)
                audited = denied = requested;
        else
                audited = requested & avd->auditallow;
        if (!audited)
                return;
        if (!a) {
                a = &stack_data;
                COMMON_AUDIT_DATA_INIT(a, NONE);
        }
        a->selinux_audit_data.tclass = tclass;
        a->selinux_audit_data.requested = requested;
        a->selinux_audit_data.ssid = ssid;
        a->selinux_audit_data.tsid = tsid;
        a->selinux_audit_data.audited = audited;
        a->selinux_audit_data.denied = denied;
        a->lsm_pre_audit = avc_audit_pre_callback;
        a->lsm_post_audit = avc_audit_post_callback;
        common_lsm_audit(a);
}

/**
 * avc_add_callback - Register a callback for security events.
 * @callback: callback function
 * @events: security events
 * @ssid: source security identifier or %SECSID_WILD
 * @tsid: target security identifier or %SECSID_WILD
 * @tclass: target security class
 * @perms: permissions
 *
 * Register a callback function for events in the set @events
 * related to the SID pair (@ssid, @tsid) 
 * and the permissions @perms, interpreting
 * @perms based on @tclass.  Returns %0 on success or
 * -%ENOMEM if insufficient memory exists to add the callback.
 */
int avc_add_callback(int (*callback)(u32 event, u32 ssid, u32 tsid,
                                     u16 tclass, u32 perms,
                                     u32 *out_retained),
                     u32 events, u32 ssid, u32 tsid,
                     u16 tclass, u32 perms)
{
        struct avc_callback_node *c;
        int rc = 0;

        c = kmalloc(sizeof(*c), GFP_ATOMIC);
        if (!c) {
                rc = -ENOMEM;
                goto out;
        }

        c->callback = callback;
        c->events = events;
        c->ssid = ssid;
        c->tsid = tsid;
        c->perms = perms;
        c->next = avc_callbacks;
        avc_callbacks = c;
out:
        return rc;
}

static inline int avc_sidcmp(u32 x, u32 y)
{
        return (x == y || x == SECSID_WILD || y == SECSID_WILD);
}

/**
 * avc_update_node Update an AVC entry
 * @event : Updating event
 * @perms : Permission mask bits
 * @ssid,@tsid,@tclass : identifier of an AVC entry
 * @seqno : sequence number when decision was made
 *
 * if a valid AVC entry doesn't exist,this function returns -ENOENT.
 * if kmalloc() called internal returns NULL, this function returns -ENOMEM.
 * otherwise, this function updates the AVC entry. The original AVC-entry object
 * will release later by RCU.
 */
static int avc_update_node(u32 event, u32 perms, u32 ssid, u32 tsid, u16 tclass,
                           u32 seqno)
{
        int hvalue, rc = 0;
        unsigned long flag;
        struct avc_node *pos, *node, *orig = NULL;
        struct hlist_head *head;
        struct hlist_node *next;
        spinlock_t *lock;

        node = avc_alloc_node();
        if (!node) {
                rc = -ENOMEM;
                goto out;
        }

        /* Lock the target slot */
        hvalue = avc_hash(ssid, tsid, tclass);

        head = &avc_cache.slots[hvalue];
        lock = &avc_cache.slots_lock[hvalue];

        spin_lock_irqsave(lock, flag);

        hlist_for_each_entry(pos, next, head, list) {
                if (ssid == pos->ae.ssid &&
                    tsid == pos->ae.tsid &&
                    tclass == pos->ae.tclass &&
                    seqno == pos->ae.avd.seqno){
                        orig = pos;
                        break;
                }
        }

        if (!orig) {
                rc = -ENOENT;
                avc_node_kill(node);
                goto out_unlock;
        }

        /*
         * Copy and replace original node.
         */

        avc_node_populate(node, ssid, tsid, tclass, &orig->ae.avd);

        switch (event) {
        case AVC_CALLBACK_GRANT:
                node->ae.avd.allowed |= perms;
                break;
        case AVC_CALLBACK_TRY_REVOKE:
        case AVC_CALLBACK_REVOKE:
                node->ae.avd.allowed &= ~perms;
                break;
        case AVC_CALLBACK_AUDITALLOW_ENABLE:
                node->ae.avd.auditallow |= perms;
                break;
        case AVC_CALLBACK_AUDITALLOW_DISABLE:
                node->ae.avd.auditallow &= ~perms;
                break;
        case AVC_CALLBACK_AUDITDENY_ENABLE:
                node->ae.avd.auditdeny |= perms;
                break;
        case AVC_CALLBACK_AUDITDENY_DISABLE:
                node->ae.avd.auditdeny &= ~perms;
                break;
        }
        avc_node_replace(node, orig);
out_unlock:
        spin_unlock_irqrestore(lock, flag);
out:
        return rc;
}

/**
 * avc_flush - Flush the cache
 */
static void avc_flush(void)
{
        struct hlist_head *head;
        struct hlist_node *next;
        struct avc_node *node;
        spinlock_t *lock;
        unsigned long flag;
        int i;

        for (i = 0; i < AVC_CACHE_SLOTS; i++) {
                head = &avc_cache.slots[i];
                lock = &avc_cache.slots_lock[i];

                spin_lock_irqsave(lock, flag);
                /*
                 * With preemptable RCU, the outer spinlock does not
                 * prevent RCU grace periods from ending.
                 */
                rcu_read_lock();
                hlist_for_each_entry(node, next, head, list)
                        avc_node_delete(node);
                rcu_read_unlock();
                spin_unlock_irqrestore(lock, flag);
        }
}

/**
 * avc_ss_reset - Flush the cache and revalidate migrated permissions.
 * @seqno: policy sequence number
 */
int avc_ss_reset(u32 seqno)
{
        struct avc_callback_node *c;
        int rc = 0, tmprc;

        avc_flush();

        for (c = avc_callbacks; c; c = c->next) {
                if (c->events & AVC_CALLBACK_RESET) {
                        tmprc = c->callback(AVC_CALLBACK_RESET,
                                            0, 0, 0, 0, NULL);
                        /* save the first error encountered for the return
                           value and continue processing the callbacks */
                        if (!rc)
                                rc = tmprc;
                }
        }

        avc_latest_notif_update(seqno, 0);
        return rc;
}

/**
 * avc_has_perm_noaudit - Check permissions but perform no auditing.
 * @ssid: source security identifier
 * @tsid: target security identifier
 * @tclass: target security class
 * @requested: requested permissions, interpreted based on @tclass
 * @flags:  AVC_STRICT or 0
 * @avd: access vector decisions
 *
 * Check the AVC to determine whether the @requested permissions are granted
 * for the SID pair (@ssid, @tsid), interpreting the permissions
 * based on @tclass, and call the security server on a cache miss to obtain
 * a new decision and add it to the cache.  Return a copy of the decisions
 * in @avd.  Return %0 if all @requested permissions are granted,
 * -%EACCES if any permissions are denied, or another -errno upon
 * other errors.  This function is typically called by avc_has_perm(),
 * but may also be called directly to separate permission checking from
 * auditing, e.g. in cases where a lock must be held for the check but
 * should be released for the auditing.
 */
int avc_has_perm_noaudit(u32 ssid, u32 tsid,
                         u16 tclass, u32 requested,
                         unsigned flags,
                         struct av_decision *in_avd)
{
        struct avc_node *node;
        struct av_decision avd_entry, *avd;
        int rc = 0;
        u32 denied;

        BUG_ON(!requested);

        rcu_read_lock();

        node = avc_lookup(ssid, tsid, tclass);
        if (!node) {
                rcu_read_unlock();

                if (in_avd)
                        avd = in_avd;
                else
                        avd = &avd_entry;

                security_compute_av(ssid, tsid, tclass, avd);
                rcu_read_lock();
                node = avc_insert(ssid, tsid, tclass, avd);
        } else {
                if (in_avd)
                        memcpy(in_avd, &node->ae.avd, sizeof(*in_avd));
                avd = &node->ae.avd;
        }

        denied = requested & ~(avd->allowed);

        if (denied) {
                if (flags & AVC_STRICT)
                        rc = -EACCES;
                else if (!selinux_enforcing || (avd->flags & AVD_FLAGS_PERMISSIVE))
                        avc_update_node(AVC_CALLBACK_GRANT, requested, ssid,
                                        tsid, tclass, avd->seqno);
                else
                        rc = -EACCES;
        }

        rcu_read_unlock();
        return rc;
}

/**
 * avc_has_perm - Check permissions and perform any appropriate auditing.
 * @ssid: source security identifier
 * @tsid: target security identifier
 * @tclass: target security class
 * @requested: requested permissions, interpreted based on @tclass
 * @auditdata: auxiliary audit data
 *
 * Check the AVC to determine whether the @requested permissions are granted
 * for the SID pair (@ssid, @tsid), interpreting the permissions
 * based on @tclass, and call the security server on a cache miss to obtain
 * a new decision and add it to the cache.  Audit the granting or denial of
 * permissions in accordance with the policy.  Return %0 if all @requested
 * permissions are granted, -%EACCES if any permissions are denied, or
 * another -errno upon other errors.
 */
int avc_has_perm(u32 ssid, u32 tsid, u16 tclass,
                 u32 requested, struct common_audit_data *auditdata)
{
        struct av_decision avd;
        int rc;

        rc = avc_has_perm_noaudit(ssid, tsid, tclass, requested, 0, &avd);
        avc_audit(ssid, tsid, tclass, requested, &avd, rc, auditdata);
        return rc;
}

u32 avc_policy_seqno(void)
{
        return avc_cache.latest_notif;
}

void avc_disable(void)
{
        /*
         * If you are looking at this because you have realized that we are
         * not destroying the avc_node_cachep it might be easy to fix, but
         * I don't know the memory barrier semantics well enough to know.  It's
         * possible that some other task dereferenced security_ops when
         * it still pointed to selinux operations.  If that is the case it's
         * possible that it is about to use the avc and is about to need the
         * avc_node_cachep.  I know I could wrap the security.c security_ops call
         * in an rcu_lock, but seriously, it's not worth it.  Instead I just flush
         * the cache and get that memory back.
         */
        if (avc_node_cachep) {
                avc_flush();
                /* kmem_cache_destroy(avc_node_cachep); */
        }
}