KEYS: Add garbage collection for dead, revoked and expired keys. [try #6]

Add garbage collection for dead, revoked and expired keys.  This involved
erasing all links to such keys from keyrings that point to them.  At that
point, the key will be deleted in the normal manner.

Keyrings from which garbage collection occurs are shrunk and their quota
consumption reduced as appropriate.

Dead keys (for which the key type has been removed) will be garbage collected
immediately.

Revoked and expired keys will hang around for a number of seconds, as set in
/proc/sys/kernel/keys/gc_delay before being automatically removed.  The default
is 5 minutes.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

Documentation/keys.txt

@@ -26,7 +26,7 @@ This document has the following sections:
 	- Notes on accessing payload contents
 	- Defining a key type
 	- Request-key callback service
-	- Key access filesystem
+	- Garbage collection
 
 
 ============
@@ -113,6 +113,9 @@ Each key has a number of attributes:
 
      (*) Dead. The key's type was unregistered, and so the key is now useless.
 
+Keys in the last three states are subject to garbage collection.  See the
+section on "Garbage collection".
+
 
 ====================
 KEY SERVICE OVERVIEW
@@ -1231,3 +1234,17 @@ by executing:
 
 In this case, the program isn't required to actually attach the key to a ring;
 the rings are provided for reference.
+
+
+==================
+GARBAGE COLLECTION
+==================
+
+Dead keys (for which the type has been removed) will be automatically unlinked
+from those keyrings that point to them and deleted as soon as possible by a
+background garbage collector.
+
+Similarly, revoked and expired keys will be garbage collected, but only after a
+certain amount of time has passed.  This time is set as a number of seconds in:
+
+	/proc/sys/kernel/keys/gc_delay

include/linux/key.h

@@ -129,7 +129,10 @@ struct key {
 	struct rw_semaphore	sem;		/* change vs change sem */
 	struct key_user		*user;		/* owner of this key */
 	void			*security;	/* security data for this key */
-	time_t			expiry;		/* time at which key expires (or 0) */
+	union {
+		time_t		expiry;		/* time at which key expires (or 0) */
+		time_t		revoked_at;	/* time at which key was revoked */
+	};
 	uid_t			uid;
 	gid_t			gid;
 	key_perm_t		perm;		/* access permissions */

security/keys/Makefile

@@ -3,6 +3,7 @@
 #
 
 obj-y := \
+	gc.o \
 	key.o \
 	keyring.o \
 	keyctl.o \

security/keys/gc.c

+/* Key garbage collector
+ *
+ * Copyright (C) 2009 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 <keys/keyring-type.h>
+#include "internal.h"
+
+/*
+ * Delay between key revocation/expiry in seconds
+ */
+unsigned key_gc_delay = 5 * 60;
+
+/*
+ * Reaper
+ */
+static void key_gc_timer_func(unsigned long);
+static void key_garbage_collector(struct work_struct *);
+static DEFINE_TIMER(key_gc_timer, key_gc_timer_func, 0, 0);
+static DECLARE_WORK(key_gc_work, key_garbage_collector);
+static key_serial_t key_gc_cursor; /* the last key the gc considered */
+static unsigned long key_gc_executing;
+static time_t key_gc_next_run = LONG_MAX;
+
+/*
+ * Schedule a garbage collection run
+ * - precision isn't particularly important
+ */
+void key_schedule_gc(time_t gc_at)
+{
+	unsigned long expires;
+	time_t now = current_kernel_time().tv_sec;
+
+	kenter("%ld", gc_at - now);
+
+	gc_at += key_gc_delay;
+
+	if (now >= gc_at) {
+		schedule_work(&key_gc_work);
+	} else if (gc_at < key_gc_next_run) {
+		expires = jiffies + (gc_at - now) * HZ;
+		mod_timer(&key_gc_timer, expires);
+	}
+}
+
+/*
+ * The garbage collector timer kicked off
+ */
+static void key_gc_timer_func(unsigned long data)
+{
+	kenter("");
+	key_gc_next_run = LONG_MAX;
+	schedule_work(&key_gc_work);
+}
+
+/*
+ * Garbage collect pointers from a keyring
+ * - return true if we altered the keyring
+ */
+static bool key_gc_keyring(struct key *keyring, time_t limit)
+{
+	struct keyring_list *klist;
+	struct key *key;
+	int loop;
+
+	kenter("%x", key_serial(keyring));
+
+	if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))
+		goto dont_gc;
+
+	/* scan the keyring looking for dead keys */
+	klist = rcu_dereference(keyring->payload.subscriptions);
+	if (!klist)
+		goto dont_gc;
+
+	for (loop = klist->nkeys - 1; loop >= 0; loop--) {
+		key = klist->keys[loop];
+		if (test_bit(KEY_FLAG_DEAD, &key->flags) ||
+		    (key->expiry > 0 && key->expiry <= limit))
+			goto do_gc;
+	}
+
+dont_gc:
+	kleave(" = false");
+	return false;
+
+do_gc:
+	key_gc_cursor = keyring->serial;
+	key_get(keyring);
+	spin_unlock(&key_serial_lock);
+	keyring_gc(keyring, limit);
+	key_put(keyring);
+	kleave(" = true");
+	return true;
+}
+
+/*
+ * Garbage collector for keys
+ * - this involves scanning the keyrings for dead, expired and revoked keys
+ *   that have overstayed their welcome
+ */
+static void key_garbage_collector(struct work_struct *work)
+{
+	struct rb_node *rb;
+	key_serial_t cursor;
+	struct key *key, *xkey;
+	time_t new_timer = LONG_MAX, limit;
+
+	kenter("");
+
+	if (test_and_set_bit(0, &key_gc_executing)) {
+		key_schedule_gc(current_kernel_time().tv_sec);
+		return;
+	}
+
+	limit = current_kernel_time().tv_sec;
+	if (limit > key_gc_delay)
+		limit -= key_gc_delay;
+	else
+		limit = key_gc_delay;
+
+	spin_lock(&key_serial_lock);
+
+	if (RB_EMPTY_ROOT(&key_serial_tree))
+		goto reached_the_end;
+
+	cursor = key_gc_cursor;
+	if (cursor < 0)
+		cursor = 0;
+
+	/* find the first key above the cursor */
+	key = NULL;
+	rb = key_serial_tree.rb_node;
+	while (rb) {
+		xkey = rb_entry(rb, struct key, serial_node);
+		if (cursor < xkey->serial) {
+			key = xkey;
+			rb = rb->rb_left;
+		} else if (cursor > xkey->serial) {
+			rb = rb->rb_right;
+		} else {
+			rb = rb_next(rb);
+			if (!rb)
+				goto reached_the_end;
+			key = rb_entry(rb, struct key, serial_node);
+			break;
+		}
+	}
+
+	if (!key)
+		goto reached_the_end;
+
+	/* trawl through the keys looking for keyrings */
+	for (;;) {
+		if (key->expiry > 0 && key->expiry < new_timer)
+			new_timer = key->expiry;
+
+		if (key->type == &key_type_keyring &&
+		    key_gc_keyring(key, limit)) {
+			/* the gc ate our lock */
+			schedule_work(&key_gc_work);
+			goto no_unlock;
+		}
+
+		rb = rb_next(&key->serial_node);
+		if (!rb) {
+			key_gc_cursor = 0;
+			break;
+		}
+		key = rb_entry(rb, struct key, serial_node);
+	}
+
+out:
+	spin_unlock(&key_serial_lock);
+no_unlock:
+	clear_bit(0, &key_gc_executing);
+	if (new_timer < LONG_MAX)
+		key_schedule_gc(new_timer);
+
+	kleave("");
+	return;
+
+reached_the_end:
+	key_gc_cursor = 0;
+	goto out;
+}

security/keys/internal.h

@@ -132,6 +132,10 @@ extern key_ref_t lookup_user_key(key_serial_t id, unsigned long flags,
 
 extern long join_session_keyring(const char *name);
 
+extern unsigned key_gc_delay;
+extern void keyring_gc(struct key *keyring, time_t limit);
+extern void key_schedule_gc(time_t expiry_at);
+
 /*
  * check to see whether permission is granted to use a key in the desired way
  */

security/keys/key.c

@@ -500,6 +500,7 @@ int key_negate_and_link(struct key *key,
 		set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
 		now = current_kernel_time();
 		key->expiry = now.tv_sec + timeout;
+		key_schedule_gc(key->expiry);
 
 		if (test_and_clear_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags))
 			awaken = 1;
@@ -888,6 +889,9 @@ EXPORT_SYMBOL(key_update);
  */
 void key_revoke(struct key *key)
 {
+	struct timespec now;
+	time_t time;
+
 	key_check(key);
 
 	/* make sure no one's trying to change or use the key when we mark it
@@ -900,6 +904,14 @@ void key_revoke(struct key *key)
 	    key->type->revoke)
 		key->type->revoke(key);
 
+	/* set the death time to no more than the expiry time */
+	now = current_kernel_time();
+	time = now.tv_sec;
+	if (key->revoked_at == 0 || key->revoked_at > time) {
+		key->revoked_at = time;
+		key_schedule_gc(key->revoked_at);
+	}
+
 	up_write(&key->sem);
 
 } /* end key_revoke() */
@@ -984,6 +996,8 @@ void unregister_key_type(struct key_type *ktype)
 	spin_unlock(&key_serial_lock);
 	up_write(&key_types_sem);
 
+	key_schedule_gc(0);
+
 } /* end unregister_key_type() */
 
 EXPORT_SYMBOL(unregister_key_type);

security/keys/keyctl.c

@@ -1115,6 +1115,7 @@ long keyctl_set_timeout(key_serial_t id, unsigned timeout)
 	}
 
 	key->expiry = expiry;
+	key_schedule_gc(key->expiry);
 
 	up_write(&key->sem);
 	key_put(key);

security/keys/keyring.c

@@ -1000,3 +1000,88 @@ static void keyring_revoke(struct key *keyring)
 	}
 
 } /* end keyring_revoke() */
+
+/*
+ * Determine whether a key is dead
+ */
+static bool key_is_dead(struct key *key, time_t limit)
+{
+	return test_bit(KEY_FLAG_DEAD, &key->flags) ||
+		(key->expiry > 0 && key->expiry <= limit);
+}
+
+/*
+ * Collect garbage from the contents of a keyring
+ */
+void keyring_gc(struct key *keyring, time_t limit)
+{
+	struct keyring_list *klist, *new;
+	struct key *key;
+	int loop, keep, max;
+
+	kenter("%x", key_serial(keyring));
+
+	down_write(&keyring->sem);
+
+	klist = keyring->payload.subscriptions;
+	if (!klist)
+		goto just_return;
+
+	/* work out how many subscriptions we're keeping */
+	keep = 0;
+	for (loop = klist->nkeys - 1; loop >= 0; loop--)
+		if (!key_is_dead(klist->keys[loop], limit));
+			keep++;
+
+	if (keep == klist->nkeys)
+		goto just_return;
+
+	/* allocate a new keyring payload */
+	max = roundup(keep, 4);
+	new = kmalloc(sizeof(struct keyring_list) + max * sizeof(struct key *),
+		      GFP_KERNEL);
+	if (!new)
+		goto just_return;
+	new->maxkeys = max;
+	new->nkeys = 0;
+	new->delkey = 0;
+
+	/* install the live keys
+	 * - must take care as expired keys may be updated back to life
+	 */
+	keep = 0;
+	for (loop = klist->nkeys - 1; loop >= 0; loop--) {
+		key = klist->keys[loop];
+		if (!key_is_dead(key, limit)) {
+			if (keep >= max)
+				goto discard_new;
+			new->keys[keep++] = key_get(key);
+		}
+	}
+	new->nkeys = keep;
+
+	/* adjust the quota */
+	key_payload_reserve(keyring,
+			    sizeof(struct keyring_list) +
+			    KEYQUOTA_LINK_BYTES * keep);
+
+	if (keep == 0) {
+		rcu_assign_pointer(keyring->payload.subscriptions, NULL);
+		kfree(new);
+	} else {
+		rcu_assign_pointer(keyring->payload.subscriptions, new);
+	}
+
+	up_write(&keyring->sem);
+
+	call_rcu(&klist->rcu, keyring_clear_rcu_disposal);
+	kleave(" [yes]");
+	return;
+
+discard_new:
+	new->nkeys = keep;
+	keyring_clear_rcu_disposal(&new->rcu);
+just_return:
+	up_write(&keyring->sem);
+	kleave(" [no]");
+}

security/keys/sysctl.c

@@ -13,6 +13,8 @@
 #include <linux/sysctl.h>
 #include "internal.h"
 
+static const int zero, one = 1, max = INT_MAX;
+
 ctl_table key_sysctls[] = {
 	{
 		.ctl_name = CTL_UNNUMBERED,
@@ -20,7 +22,9 @@ ctl_table key_sysctls[] = {
 		.data = &key_quota_maxkeys,
 		.maxlen = sizeof(unsigned),
 		.mode = 0644,
-		.proc_handler = &proc_dointvec,
+		.proc_handler = &proc_dointvec_minmax,
+		.extra1 = (void *) &one,
+		.extra2 = (void *) &max,
 	},
 	{
 		.ctl_name = CTL_UNNUMBERED,
@@ -28,7 +32,9 @@ ctl_table key_sysctls[] = {
 		.data = &key_quota_maxbytes,
 		.maxlen = sizeof(unsigned),
 		.mode = 0644,
-		.proc_handler = &proc_dointvec,
+		.proc_handler = &proc_dointvec_minmax,
+		.extra1 = (void *) &one,
+		.extra2 = (void *) &max,
 	},
 	{
 		.ctl_name = CTL_UNNUMBERED,
@@ -36,7 +42,9 @@ ctl_table key_sysctls[] = {
 		.data = &key_quota_root_maxkeys,
 		.maxlen = sizeof(unsigned),
 		.mode = 0644,
-		.proc_handler = &proc_dointvec,
+		.proc_handler = &proc_dointvec_minmax,
+		.extra1 = (void *) &one,
+		.extra2 = (void *) &max,
 	},
 	{
 		.ctl_name = CTL_UNNUMBERED,
@@ -44,7 +52,19 @@ ctl_table key_sysctls[] = {
 		.data = &key_quota_root_maxbytes,
 		.maxlen = sizeof(unsigned),
 		.mode = 0644,
-		.proc_handler = &proc_dointvec,
+		.proc_handler = &proc_dointvec_minmax,
+		.extra1 = (void *) &one,
+		.extra2 = (void *) &max,
+	},
+	{
+		.ctl_name = CTL_UNNUMBERED,
+		.procname = "gc_delay",
+		.data = &key_gc_delay,
+		.maxlen = sizeof(unsigned),
+		.mode = 0644,
+		.proc_handler = &proc_dointvec_minmax,
+		.extra1 = (void *) &zero,
+		.extra2 = (void *) &max,
 	},
 	{ .ctl_name = 0 }
 };