path: root/keys-krb-support.patch

From 64160c504842a359801cff17464931fa028ff164 Mon Sep 17 00:00:00 2001
From: David Howells <dhowells@redhat.com>
Date: Fri, 30 Aug 2013 15:37:54 +0100
Subject: [PATCH 1/2] KEYS: Implement a big key type that can save to tmpfs

Implement a big key type that can save its contents to tmpfs and thus
swapspace when memory is tight.  This is useful for Kerberos ticket caches.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Simo Sorce <simo@redhat.com>
---
 include/keys/big_key-type.h |  25 ++++++
 include/linux/key.h         |   1 +
 security/keys/Kconfig       |  11 +++
 security/keys/Makefile      |   1 +
 security/keys/big_key.c     | 204 ++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 242 insertions(+)
 create mode 100644 include/keys/big_key-type.h
 create mode 100644 security/keys/big_key.c

diff --git a/include/keys/big_key-type.h b/include/keys/big_key-type.h
new file mode 100644
index 0000000..d69bc8a
--- /dev/null
+++ b/include/keys/big_key-type.h
@@ -0,0 +1,25 @@
+/* Big capacity key type.
+ *
+ * Copyright (C) 2013 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 License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+
+#ifndef _KEYS_BIG_KEY_TYPE_H
+#define _KEYS_BIG_KEY_TYPE_H
+
+#include <linux/key-type.h>
+
+extern struct key_type key_type_big_key;
+
+extern int big_key_instantiate(struct key *key, struct key_preparsed_payload *prep);
+extern void big_key_revoke(struct key *key);
+extern void big_key_destroy(struct key *key);
+extern void big_key_describe(const struct key *big_key, struct seq_file *m);
+extern long big_key_read(const struct key *key, char __user *buffer, size_t buflen);
+
+#endif /* _KEYS_BIG_KEY_TYPE_H */
diff --git a/include/linux/key.h b/include/linux/key.h
index 2417f78..010dbb6 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -201,6 +201,7 @@ struct key {
 			unsigned long		value;
 			void __rcu		*rcudata;
 			void			*data;
+			void			*data2[2];
 		} payload;
 		struct assoc_array keys;
 	};
diff --git a/security/keys/Kconfig b/security/keys/Kconfig
index 15e0dfe..b563622 100644
--- a/security/keys/Kconfig
+++ b/security/keys/Kconfig
@@ -20,6 +20,17 @@ config KEYS
 
 	  If you are unsure as to whether this is required, answer N.
 
+config BIG_KEYS
+	tristate "Large payload keys"
+	depends on KEYS
+	depends on TMPFS
+	help
+	  This option provides support for holding large keys within the kernel
+	  (for example Kerberos ticket caches).  The data may be stored out to
+	  swapspace by tmpfs.
+
+	  If you are unsure as to whether this is required, answer N.
+
 config TRUSTED_KEYS
 	tristate "TRUSTED KEYS"
 	depends on KEYS && TCG_TPM
diff --git a/security/keys/Makefile b/security/keys/Makefile
index 504aaa0..c487c77 100644
--- a/security/keys/Makefile
+++ b/security/keys/Makefile
@@ -22,5 +22,6 @@ obj-$(CONFIG_SYSCTL) += sysctl.o
 #
 # Key types
 #
+obj-$(CONFIG_BIG_KEYS) += big_key.o
 obj-$(CONFIG_TRUSTED_KEYS) += trusted.o
 obj-$(CONFIG_ENCRYPTED_KEYS) += encrypted-keys/
diff --git a/security/keys/big_key.c b/security/keys/big_key.c
new file mode 100644
index 0000000..5f9defc
--- /dev/null
+++ b/security/keys/big_key.c
@@ -0,0 +1,204 @@
+/* Large capacity key type
+ *
+ * Copyright (C) 2013 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/init.h>
+#include <linux/seq_file.h>
+#include <linux/file.h>
+#include <linux/shmem_fs.h>
+#include <linux/err.h>
+#include <keys/user-type.h>
+#include <keys/big_key-type.h>
+
+MODULE_LICENSE("GPL");
+
+/*
+ * If the data is under this limit, there's no point creating a shm file to
+ * hold it as the permanently resident metadata for the shmem fs will be at
+ * least as large as the data.
+ */
+#define BIG_KEY_FILE_THRESHOLD (sizeof(struct inode) + sizeof(struct dentry))
+
+/*
+ * big_key defined keys take an arbitrary string as the description and an
+ * arbitrary blob of data as the payload
+ */
+struct key_type key_type_big_key = {
+	.name			= "big_key",
+	.def_lookup_type	= KEYRING_SEARCH_LOOKUP_DIRECT,
+	.instantiate		= big_key_instantiate,
+	.match			= user_match,
+	.revoke			= big_key_revoke,
+	.destroy		= big_key_destroy,
+	.describe		= big_key_describe,
+	.read			= big_key_read,
+};
+
+/*
+ * Instantiate a big key
+ */
+int big_key_instantiate(struct key *key, struct key_preparsed_payload *prep)
+{
+	struct path *path = (struct path *)&key->payload.data2;
+	struct file *file;
+	ssize_t written;
+	size_t datalen = prep->datalen;
+	int ret;
+
+	ret = -EINVAL;
+	if (datalen <= 0 || datalen > 1024 * 1024 || !prep->data)
+		goto error;
+
+	/* Set an arbitrary quota */
+	ret = key_payload_reserve(key, 16);
+	if (ret < 0)
+		goto error;
+
+	key->type_data.x[1] = datalen;
+
+	if (datalen > BIG_KEY_FILE_THRESHOLD) {
+		/* Create a shmem file to store the data in.  This will permit the data
+		 * to be swapped out if needed.
+		 *
+		 * TODO: Encrypt the stored data with a temporary key.
+		 */
+		file = shmem_file_setup("", datalen, 0);
+		if (IS_ERR(file))
+			goto err_quota;
+
+		written = kernel_write(file, prep->data, prep->datalen, 0);
+		if (written != datalen) {
+			if (written >= 0)
+				ret = -ENOMEM;
+			goto err_fput;
+		}
+
+		/* Pin the mount and dentry to the key so that we can open it again
+		 * later
+		 */
+		*path = file->f_path;
+		path_get(path);
+		fput(file);
+	} else {
+		/* Just store the data in a buffer */
+		void *data = kmalloc(datalen, GFP_KERNEL);
+		if (!data) {
+			ret = -ENOMEM;
+			goto err_quota;
+		}
+
+		key->payload.data = memcpy(data, prep->data, prep->datalen);
+	}
+	return 0;
+
+err_fput:
+	fput(file);
+err_quota:
+	key_payload_reserve(key, 0);
+error:
+	return ret;
+}
+
+/*
+ * dispose of the links from a revoked keyring
+ * - called with the key sem write-locked
+ */
+void big_key_revoke(struct key *key)
+{
+	struct path *path = (struct path *)&key->payload.data2;
+
+	/* clear the quota */
+	key_payload_reserve(key, 0);
+	if (key_is_instantiated(key) && key->type_data.x[1] > BIG_KEY_FILE_THRESHOLD)
+		vfs_truncate(path, 0);
+}
+
+/*
+ * dispose of the data dangling from the corpse of a big_key key
+ */
+void big_key_destroy(struct key *key)
+{
+	if (key->type_data.x[1] > BIG_KEY_FILE_THRESHOLD) {
+		struct path *path = (struct path *)&key->payload.data2;
+		path_put(path);
+		path->mnt = NULL;
+		path->dentry = NULL;
+	} else {
+		kfree(key->payload.data);
+		key->payload.data = NULL;
+	}
+}
+
+/*
+ * describe the big_key key
+ */
+void big_key_describe(const struct key *key, struct seq_file *m)
+{
+	unsigned long datalen = key->type_data.x[1];
+
+	seq_puts(m, key->description);
+
+	if (key_is_instantiated(key))
+		seq_printf(m, ": %lu [%s]",
+			   datalen,
+			   datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
+}
+
+/*
+ * read the key data
+ * - the key's semaphore is read-locked
+ */
+long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
+{
+	unsigned long datalen = key->type_data.x[1];
+	long ret;
+
+	if (!buffer || buflen < datalen)
+		return datalen;
+
+	if (datalen > BIG_KEY_FILE_THRESHOLD) {
+		struct path *path = (struct path *)&key->payload.data2;
+		struct file *file;
+		loff_t pos;
+
+		file = dentry_open(path, O_RDONLY, current_cred());
+		if (IS_ERR(file))
+			return PTR_ERR(file);
+
+		pos = 0;
+		ret = vfs_read(file, buffer, datalen, &pos);
+		fput(file);
+		if (ret >= 0 && ret != datalen)
+			ret = -EIO;
+	} else {
+		ret = datalen;
+		if (copy_to_user(buffer, key->payload.data, datalen) != 0)
+			ret = -EFAULT;
+	}
+
+	return ret;
+}
+
+/*
+ * Module stuff
+ */
+static int __init big_key_init(void)
+{
+	return register_key_type(&key_type_big_key);
+}
+
+static void __exit big_key_cleanup(void)
+{
+	unregister_key_type(&key_type_big_key);
+}
+
+module_init(big_key_init);
+module_exit(big_key_cleanup);
-- 
1.8.3.1


From b1e5b74e060add16de8d6005802644fa1700167f Mon Sep 17 00:00:00 2001
From: David Howells <dhowells@redhat.com>
Date: Fri, 30 Aug 2013 15:37:54 +0100
Subject: [PATCH 2/2] KEYS: Add per-user_namespace registers for persistent
 per-UID kerberos caches

Add support for per-user_namespace registers of persistent per-UID kerberos
caches held within the kernel.

This allows the kerberos cache to be retained beyond the life of all a user's
processes so that the user's cron jobs can work.

The kerberos cache is envisioned as a keyring/key tree looking something like:

	struct user_namespace
	  \___ .krb_cache keyring		- The register
		\___ _krb.0 keyring		- Root's Kerberos cache
		\___ _krb.5000 keyring		- User 5000's Kerberos cache
		\___ _krb.5001 keyring		- User 5001's Kerberos cache
			\___ tkt785 big_key	- A ccache blob
			\___ tkt12345 big_key	- Another ccache blob

Or possibly:

	struct user_namespace
	  \___ .krb_cache keyring		- The register
		\___ _krb.0 keyring		- Root's Kerberos cache
		\___ _krb.5000 keyring		- User 5000's Kerberos cache
		\___ _krb.5001 keyring		- User 5001's Kerberos cache
			\___ tkt785 keyring	- A ccache
				\___ krbtgt/REDHAT.COM@REDHAT.COM big_key
				\___ http/REDHAT.COM@REDHAT.COM user
				\___ afs/REDHAT.COM@REDHAT.COM user
				\___ nfs/REDHAT.COM@REDHAT.COM user
				\___ krbtgt/KERNEL.ORG@KERNEL.ORG big_key
				\___ http/KERNEL.ORG@KERNEL.ORG big_key

What goes into a particular Kerberos cache is entirely up to userspace.  Kernel
support is limited to giving you the Kerberos cache keyring that you want.

The user asks for their Kerberos cache by:

	krb_cache = keyctl_get_krbcache(uid, dest_keyring);

The uid is -1 or the user's own UID for the user's own cache or the uid of some
other user's cache (requires CAP_SETUID).  This permits rpc.gssd or whatever to
mess with the cache.

The cache returned is a keyring named "_krb.<uid>" that the possessor can read,
search, clear, invalidate, unlink from and add links to.  Active LSMs get a
chance to rule on whether the caller is permitted to make a link.

Each uid's cache keyring is created when it first accessed and is given a
timeout that is extended each time this function is called so that the keyring
goes away after a while.  The timeout is configurable by sysctl but defaults to
three days.

Each user_namespace struct gets a lazily-created keyring that serves as the
register.  The cache keyrings are added to it.  This means that standard key
search and garbage collection facilities are available.

The user_namespace struct's register goes away when it does and anything left
in it is then automatically gc'd.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Simo Sorce <simo@redhat.com>
cc: Serge E. Hallyn <serge.hallyn@ubuntu.com>
cc: Eric W. Biederman <ebiederm@xmission.com>
---
 include/linux/user_namespace.h |   6 ++
 include/uapi/linux/keyctl.h    |   1 +
 kernel/user.c                  |   4 +
 kernel/user_namespace.c        |   6 ++
 security/keys/Kconfig          |  17 +++++
 security/keys/Makefile         |   1 +
 security/keys/compat.c         |   3 +
 security/keys/internal.h       |   9 +++
 security/keys/keyctl.c         |   3 +
 security/keys/persistent.c     | 169 +++++++++++++++++++++++++++++++++++++++++
 security/keys/sysctl.c         |  11 +++
 11 files changed, 230 insertions(+)
 create mode 100644 security/keys/persistent.c

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index b6b215f..cf21958 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -28,6 +28,12 @@ struct user_namespace {
 	unsigned int		proc_inum;
 	bool			may_mount_sysfs;
 	bool			may_mount_proc;
+
+	/* Register of per-UID persistent keyrings for this namespace */
+#ifdef CONFIG_PERSISTENT_KEYRINGS
+	struct key		*persistent_keyring_register;
+	struct rw_semaphore	persistent_keyring_register_sem;
+#endif
 };
 
 extern struct user_namespace init_user_ns;
diff --git a/include/uapi/linux/keyctl.h b/include/uapi/linux/keyctl.h
index c9b7f4fa..840cb99 100644
--- a/include/uapi/linux/keyctl.h
+++ b/include/uapi/linux/keyctl.h
@@ -56,5 +56,6 @@
 #define KEYCTL_REJECT			19	/* reject a partially constructed key */
 #define KEYCTL_INSTANTIATE_IOV		20	/* instantiate a partially constructed key */
 #define KEYCTL_INVALIDATE		21	/* invalidate a key */
+#define KEYCTL_GET_PERSISTENT		22	/* get a user's persistent keyring */
 
 #endif /*  _LINUX_KEYCTL_H */
diff --git a/kernel/user.c b/kernel/user.c
index 69b4c3d..6c9e1b9 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -53,6 +53,10 @@ struct user_namespace init_user_ns = {
 	.proc_inum = PROC_USER_INIT_INO,
 	.may_mount_sysfs = true,
 	.may_mount_proc = true,
+#ifdef CONFIG_KEYS_KERBEROS_CACHE
+	.krb_cache_register_sem =
+	__RWSEM_INITIALIZER(init_user_ns.krb_cache_register_sem),
+#endif
 };
 EXPORT_SYMBOL_GPL(init_user_ns);
 
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index d8c30db..ef7985e 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -99,6 +99,9 @@ int create_user_ns(struct cred *new)
 
 	update_mnt_policy(ns);
 
+#ifdef CONFIG_PERSISTENT_KEYRINGS
+	rwsem_init(&ns->persistent_keyring_register_sem);
+#endif
 	return 0;
 }
 
@@ -123,6 +126,9 @@ void free_user_ns(struct user_namespace *ns)
 
 	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;
diff --git a/security/keys/Kconfig b/security/keys/Kconfig
index b563622..53d8748 100644
--- a/security/keys/Kconfig
+++ b/security/keys/Kconfig
@@ -20,6 +20,23 @@ config KEYS
 
 	  If you are unsure as to whether this is required, answer N.
 
+config PERSISTENT_KEYRINGS
+	bool "Enable register of persistent per-UID keyrings"
+	depends on KEYS
+	help
+	  This option provides a register of persistent per-UID keyrings,
+	  primarily aimed at Kerberos key storage.  The keyrings are persistent
+	  in the sense that they stay around after all processes of that UID
+	  have exited, not that they survive the machine being rebooted.
+
+	  A particular keyring may be accessed by either the user whose keyring
+	  it is or by a process with administrative privileges.  The active
+	  LSMs gets to rule on which admin-level processes get to access the
+	  cache.
+
+	  Keyrings are created and added into the register upon demand and get
+	  removed if they expire (a default timeout is set upon creation).
+
 config BIG_KEYS
 	tristate "Large payload keys"
 	depends on KEYS
diff --git a/security/keys/Makefile b/security/keys/Makefile
index c487c77..dfb3a7b 100644
--- a/security/keys/Makefile
+++ b/security/keys/Makefile
@@ -18,6 +18,7 @@ obj-y := \
 obj-$(CONFIG_KEYS_COMPAT) += compat.o
 obj-$(CONFIG_PROC_FS) += proc.o
 obj-$(CONFIG_SYSCTL) += sysctl.o
+obj-$(CONFIG_PERSISTENT_KEYRINGS) += persistent.o
 
 #
 # Key types
diff --git a/security/keys/compat.c b/security/keys/compat.c
index d65fa7f..bbd32c7 100644
--- a/security/keys/compat.c
+++ b/security/keys/compat.c
@@ -138,6 +138,9 @@ asmlinkage long compat_sys_keyctl(u32 option,
 	case KEYCTL_INVALIDATE:
 		return keyctl_invalidate_key(arg2);
 
+	case KEYCTL_GET_PERSISTENT:
+		return keyctl_get_persistent(arg2, arg3);
+
 	default:
 		return -EOPNOTSUPP;
 	}
diff --git a/security/keys/internal.h b/security/keys/internal.h
index 581c6f6..80b2aac 100644
--- a/security/keys/internal.h
+++ b/security/keys/internal.h
@@ -255,6 +255,15 @@ extern long keyctl_invalidate_key(key_serial_t);
 extern long keyctl_instantiate_key_common(key_serial_t,
 					  const struct iovec *,
 					  unsigned, size_t, key_serial_t);
+#ifdef CONFIG_PERSISTENT_KEYRINGS
+extern long keyctl_get_persistent(uid_t, key_serial_t);
+extern unsigned persistent_keyring_expiry;
+#else
+static inline long keyctl_get_persistent(uid_t uid, key_serial_t destring)
+{
+	return -EOPNOTSUPP;
+}
+#endif
 
 /*
  * Debugging key validation
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 33cfd27..cee72ce 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1667,6 +1667,9 @@ SYSCALL_DEFINE5(keyctl, int, option, unsigned long, arg2, unsigned long, arg3,
 	case KEYCTL_INVALIDATE:
 		return keyctl_invalidate_key((key_serial_t) arg2);
 
+	case KEYCTL_GET_PERSISTENT:
+		return keyctl_get_persistent((uid_t)arg2, (key_serial_t)arg3);
+
 	default:
 		return -EOPNOTSUPP;
 	}
diff --git a/security/keys/persistent.c b/security/keys/persistent.c
new file mode 100644
index 0000000..631a022
--- /dev/null
+++ b/security/keys/persistent.c
@@ -0,0 +1,169 @@
+/* General persistent per-UID keyrings register
+ *
+ * Copyright (C) 2013 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/user_namespace.h>
+#include "internal.h"
+
+unsigned persistent_keyring_expiry = 3 * 24 * 3600; /* Expire after 3 days of non-use */
+
+/*
+ * Create the persistent keyring register for the current user namespace.
+ *
+ * Called with the namespace's sem locked for writing.
+ */
+static int key_create_persistent_register(struct user_namespace *ns)
+{
+	struct key *reg = keyring_alloc(".persistent_register",
+					KUIDT_INIT(0), KGIDT_INIT(0),
+					current_cred(),
+					((KEY_POS_ALL & ~KEY_POS_SETATTR) |
+					 KEY_USR_VIEW | KEY_USR_READ),
+					KEY_ALLOC_NOT_IN_QUOTA, NULL);
+	if (IS_ERR(reg))
+		return PTR_ERR(reg);
+
+	ns->persistent_keyring_register = reg;
+	return 0;
+}
+
+/*
+ * Create the persistent keyring for the specified user.
+ *
+ * Called with the namespace's sem locked for writing.
+ */
+static key_ref_t key_create_persistent(struct user_namespace *ns, kuid_t uid,
+				       struct keyring_index_key *index_key)
+{
+	struct key *persistent;
+	key_ref_t reg_ref, persistent_ref;
+
+	if (!ns->persistent_keyring_register) {
+		long err = key_create_persistent_register(ns);
+		if (err < 0)
+			return ERR_PTR(err);
+	} else {
+		reg_ref = make_key_ref(ns->persistent_keyring_register, true);
+		persistent_ref = find_key_to_update(reg_ref, index_key);
+		if (persistent_ref)
+			return persistent_ref;
+	}
+
+	persistent = keyring_alloc(index_key->description,
+				   uid, INVALID_GID, current_cred(),
+				   ((KEY_POS_ALL & ~KEY_POS_SETATTR) |
+				    KEY_USR_VIEW | KEY_USR_READ),
+				   KEY_ALLOC_NOT_IN_QUOTA,
+				   ns->persistent_keyring_register);
+	if (IS_ERR(persistent))
+		return ERR_CAST(persistent);
+
+	return make_key_ref(persistent, true);
+}
+
+/*
+ * Get the persistent keyring for a specific UID and link it to the nominated
+ * keyring.
+ */
+static long key_get_persistent(struct user_namespace *ns, kuid_t uid,
+			       key_ref_t dest_ref)
+{
+	struct keyring_index_key index_key;
+	struct key *persistent;
+	key_ref_t reg_ref, persistent_ref;
+	char buf[32];
+	long ret;
+
+	/* Look in the register if it exists */
+	index_key.type = &key_type_keyring;
+	index_key.description = buf;
+	index_key.desc_len = sprintf(buf, "_persistent.%u", from_kuid(ns, uid));
+
+	if (ns->persistent_keyring_register) {
+		reg_ref = make_key_ref(ns->persistent_keyring_register, true);
+		down_read(&ns->persistent_keyring_register_sem);
+		persistent_ref = find_key_to_update(reg_ref, &index_key);
+		up_read(&ns->persistent_keyring_register_sem);
+
+		if (persistent_ref)
+			goto found;
+	}
+
+	/* It wasn't in the register, so we'll need to create it.  We might
+	 * also need to create the register.
+	 */
+	down_write(&ns->persistent_keyring_register_sem);
+	persistent_ref = key_create_persistent(ns, uid, &index_key);
+	up_write(&ns->persistent_keyring_register_sem);
+	if (!IS_ERR(persistent_ref))
+		goto found;
+
+	return PTR_ERR(persistent_ref);
+
+found:
+	ret = key_task_permission(persistent_ref, current_cred(), KEY_LINK);
+	if (ret == 0) {
+		persistent = key_ref_to_ptr(persistent_ref);
+		ret = key_link(key_ref_to_ptr(dest_ref), persistent);
+		if (ret == 0) {
+			key_set_timeout(persistent, persistent_keyring_expiry);
+			ret = persistent->serial;		
+		}
+	}
+
+	key_ref_put(persistent_ref);
+	return ret;
+}
+
+/*
+ * Get the persistent keyring for a specific UID and link it to the nominated
+ * keyring.
+ */
+long keyctl_get_persistent(uid_t _uid, key_serial_t destid)
+{
+	struct user_namespace *ns = current_user_ns();
+	key_ref_t dest_ref;
+	kuid_t uid;
+	long ret;
+
+	/* -1 indicates the current user */
+	if (_uid == (uid_t)-1) {
+		uid = current_uid();
+	} else {
+		uid = make_kuid(ns, _uid);
+		if (!uid_valid(uid))
+			return -EINVAL;
+
+		/* You can only see your own persistent cache if you're not
+		 * sufficiently privileged.
+		 */
+		if (uid != current_uid() &&
+		    uid != current_suid() &&
+		    uid != current_euid() &&
+		    uid != current_fsuid() &&
+		    !ns_capable(ns, CAP_SETUID))
+			return -EPERM;
+	}
+
+	/* There must be a destination keyring */
+	dest_ref = lookup_user_key(destid, KEY_LOOKUP_CREATE, KEY_WRITE);
+	if (IS_ERR(dest_ref))
+		return PTR_ERR(dest_ref);
+	if (key_ref_to_ptr(dest_ref)->type != &key_type_keyring) {
+		ret = -ENOTDIR;
+		goto out_put_dest;
+	}
+
+	ret = key_get_persistent(ns, uid, dest_ref);
+
+out_put_dest:
+	key_ref_put(dest_ref);
+	return ret;
+}
diff --git a/security/keys/sysctl.c b/security/keys/sysctl.c
index ee32d18..8c0af08 100644
--- a/security/keys/sysctl.c
+++ b/security/keys/sysctl.c
@@ -61,5 +61,16 @@ ctl_table key_sysctls[] = {
 		.extra1 = (void *) &zero,
 		.extra2 = (void *) &max,
 	},
+#ifdef CONFIG_PERSISTENT_KEYRINGS
+	{
+		.procname = "persistent_keyring_expiry",
+		.data = &persistent_keyring_expiry,
+		.maxlen = sizeof(unsigned),
+		.mode = 0644,
+		.proc_handler = proc_dointvec_minmax,
+		.extra1 = (void *) &zero,
+		.extra2 = (void *) &max,
+	},
+#endif
 	{ }
 };
-- 
1.8.3.1