[PATCH] eCryptfs: Public key transport mechanism

This is the transport code for public key functionality in eCryptfs.  It
manages encryption/decryption request queues with a transport mechanism.
Currently, netlink is the only implemented transport.

Each inode has a unique File Encryption Key (FEK).  Under passphrase, a File
Encryption Key Encryption Key (FEKEK) is generated from a salt/passphrase
combo on mount.  This FEKEK encrypts each FEK and writes it into the header of
each file using the packet format specified in RFC 2440.  This is all
symmetric key encryption, so it can all be done via the kernel crypto API.

These new patches introduce public key encryption of the FEK.  There is no
asymmetric key encryption support in the kernel crypto API, so eCryptfs pushes
the FEK encryption and decryption out to a userspace daemon.  After
considering our requirements and determining the complexity of using various
transport mechanisms, we settled on netlink for this communication.

eCryptfs stores authentication tokens into the kernel keyring.  These tokens
correlate with individual keys.  For passphrase mode of operation, the
authentication token contains the symmetric FEKEK.  For public key, the
authentication token contains a PKI type and an opaque data blob managed by
individual PKI modules in userspace.

Each user who opens a file under an eCryptfs partition mounted in public key
mode must be running a daemon.  That daemon has the user's credentials and has
access to all of the keys to which the user should have access.  The daemon,
when started, initializes the pluggable PKI modules available on the system
and registers itself with the eCryptfs kernel module.  Userspace utilities
register public key authentication tokens into the user session keyring.
These authentication tokens correlate key signatures with PKI modules and PKI
blobs.  The PKI blobs contain PKI-specific information necessary for the PKI
module to carry out asymmetric key encryption and decryption.

When the eCryptfs module parses the header of an existing file and finds a Tag
1 (Public Key) packet (see RFC 2440), it reads in the public key identifier
(signature).  The asymmetrically encrypted FEK is in the Tag 1 packet;
eCryptfs puts together a decrypt request packet containing the signature and
the encrypted FEK, then it passes it to the daemon registered for the
current->euid via a netlink unicast to the PID of the daemon, which was
registered at the time the daemon was started by the user.

The daemon actually just makes calls to libecryptfs, which implements request
packet parsing and manages PKI modules.  libecryptfs grabs the public key
authentication token for the given signature from the user session keyring.
This auth tok tells libecryptfs which PKI module should receive the request.
libecryptfs then makes a decrypt() call to the PKI module, and it passes along
the PKI block from the auth tok.  The PKI uses the blob to figure out how it
should decrypt the data passed to it; it performs the decryption and passes
the decrypted data back to libecryptfs.  libecryptfs then puts together a
reply packet with the decrypted FEK and passes that back to the eCryptfs
module.

The eCryptfs module manages these request callouts to userspace code via
message context structs.  The module maintains an array of message context
structs and places the elements of the array on two lists: a free and an
allocated list.  When eCryptfs wants to make a request, it moves a msg ctx
from the free list to the allocated list, sets its state to pending, and fires
off the message to the user's registered daemon.

When eCryptfs receives a netlink message (via the callback), it correlates the
msg ctx struct in the alloc list with the data in the message itself.  The
msg->index contains the offset of the array of msg ctx structs.  It verifies
that the registered daemon PID is the same as the PID of the process that sent
the message.  It also validates a sequence number between the received packet
and the msg ctx.  Then, it copies the contents of the message (the reply
packet) into the msg ctx struct, sets the state in the msg ctx to done, and
wakes up the process that was sleeping while waiting for the reply.

The sleeping process was whatever was performing the sys_open().  This process
originally called ecryptfs_send_message(); it is now in
ecryptfs_wait_for_response().  When it wakes up and sees that the msg ctx
state was set to done, it returns a pointer to the message contents (the reply
packet) and returns.  If all went well, this packet contains the decrypted
FEK, which is then copied into the crypt_stat struct, and life continues as
normal.

The case for creation of a new file is very similar, only instead of a decrypt
request, eCryptfs sends out an encrypt request.

> - We have a great clod of key mangement code in-kernel.  Why is that
>   not suitable (or growable) for public key management?

eCryptfs uses Howells' keyring to store persistent key data and PKI state
information.  It defers public key cryptographic transformations to userspace
code.  The userspace data manipulation request really is orthogonal to key
management in and of itself.  What eCryptfs basically needs is a secure way to
communicate with a particular daemon for a particular task doing a syscall,
based on the UID.  Nothing running under another UID should be able to access
that channel of communication.

> - Is it appropriate that new infrastructure for public key
> management be private to a particular fs?

The messaging.c file contains a lot of code that, perhaps, could be extracted
into a separate kernel service.  In essence, this would be a sort of
request/reply mechanism that would involve a userspace daemon.  I am not aware
of anything that does quite what eCryptfs does, so I was not aware of any
existing tools to do just what we wanted.

>   What happens if one of these daemons exits without sending a quit
>   message?

There is a stale uid<->pid association in the hash table for that user.  When
the user registers a new daemon, eCryptfs cleans up the old association and
generates a new one.  See ecryptfs_process_helo().

> - _why_ does it use netlink?

Netlink provides the transport mechanism that would minimize the complexity of
the implementation, given that we can have multiple daemons (one per user).  I
explored the possibility of using relayfs, but that would involve having to
introduce control channels and a protocol for creating and tearing down
channels for the daemons.  We do not have to worry about any of that with
netlink.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 255 insertions, 0 deletions

diff --git a/fs/ecryptfs/netlink.c b/fs/ecryptfs/netlink.c
new file mode 100644
index 000000000000..aba061d62118
--- /dev/null
+++ b/fs/ecryptfs/netlink.c
@@ -0,0 +1,255 @@
+/**
+ * eCryptfs: Linux filesystem encryption layer
+ *
+ * Copyright (C) 2004-2006 International Business Machines Corp.
+ *   Author(s): Michael A. Halcrow <mhalcrow@us.ibm.com>
+ *              Tyler Hicks <tyhicks@ou.edu>
+ *
+ * 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.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ */
+
+#include <net/sock.h>
+#include <linux/hash.h>
+#include <linux/random.h>
+#include "ecryptfs_kernel.h"
+
+static struct sock *ecryptfs_nl_sock;
+
+/**
+ * ecryptfs_send_netlink
+ * @data: The data to include as the payload
+ * @data_len: The byte count of the data
+ * @msg_ctx: The netlink context that will be used to handle the
+ *          response message
+ * @msg_type: The type of netlink message to send
+ * @msg_flags: The flags to include in the netlink header
+ * @daemon_pid: The process id of the daemon to send the message to
+ *
+ * Sends the data to the specified daemon pid and uses the netlink
+ * context element to store the data needed for validation upon
+ * receiving the response.  The data and the netlink context can be
+ * null if just sending a netlink header is sufficient.  Returns zero
+ * upon sending the message; non-zero upon error.
+ */
+int ecryptfs_send_netlink(char *data, int data_len,
+                          struct ecryptfs_msg_ctx *msg_ctx, u16 msg_type,
+                          u16 msg_flags, pid_t daemon_pid)
+{
+        struct sk_buff *skb;
+        struct nlmsghdr *nlh;
+        struct ecryptfs_message *msg;
+        size_t payload_len;
+        int rc;
+
+        payload_len = ((data && data_len) ? (sizeof(*msg) + data_len) : 0);
+        skb = alloc_skb(NLMSG_SPACE(payload_len), GFP_KERNEL);
+        if (!skb) {
+                rc = -ENOMEM;
+                ecryptfs_printk(KERN_ERR, "Failed to allocate socket buffer\n");
+                goto out;
+        }
+        nlh = NLMSG_PUT(skb, daemon_pid, msg_ctx ? msg_ctx->counter : 0,
+                        msg_type, payload_len);
+        nlh->nlmsg_flags = msg_flags;
+        if (msg_ctx && payload_len) {
+                msg = (struct ecryptfs_message *)NLMSG_DATA(nlh);
+                msg->index = msg_ctx->index;
+                msg->data_len = data_len;
+                memcpy(msg->data, data, data_len);
+        }
+        rc = netlink_unicast(ecryptfs_nl_sock, skb, daemon_pid, 0);
+        if (rc < 0) {
+                ecryptfs_printk(KERN_ERR, "Failed to send eCryptfs netlink "
+                                "message; rc = [%d]\n", rc);
+                goto out;
+        }
+        rc = 0;
+        goto out;
+nlmsg_failure:
+        rc = -EMSGSIZE;
+        kfree_skb(skb);
+out:
+        return rc;
+}
+
+/**
+ * ecryptfs_process_nl_reponse
+ * @skb: The socket buffer containing the netlink message of state
+ *       RESPONSE
+ *
+ * Processes a response message after sending a operation request to
+ * userspace.  Attempts to assign the msg to a netlink context element
+ * at the index specified in the msg.  The sk_buff and nlmsghdr must
+ * be validated before this function. Returns zero upon delivery to
+ * desired context element; non-zero upon delivery failure or error.
+ */
+static int ecryptfs_process_nl_response(struct sk_buff *skb)
+{
+        struct nlmsghdr *nlh = (struct nlmsghdr*)skb->data;
+        struct ecryptfs_message *msg = NLMSG_DATA(nlh);
+        int rc;
+
+        if (skb->len - NLMSG_HDRLEN - sizeof(*msg) != msg->data_len) {
+                rc = -EINVAL;
+                ecryptfs_printk(KERN_ERR, "Received netlink message with "
+                                "incorrectly specified data length\n");
+                goto out;
+        }
+        rc = ecryptfs_process_response(msg, NETLINK_CREDS(skb)->pid,
+                                       nlh->nlmsg_seq);
+        if (rc)
+                printk(KERN_ERR
+                       "Error processing response message; rc = [%d]\n", rc);
+out:
+        return rc;
+}
+
+/**
+ * ecryptfs_process_nl_helo
+ * @skb: The socket buffer containing the nlmsghdr in HELO state
+ *
+ * Gets uid and pid of the skb and adds the values to the daemon id
+ * hash. Returns zero after adding a new daemon id to the hash list;
+ * non-zero otherwise.
+ */
+static int ecryptfs_process_nl_helo(struct sk_buff *skb)
+{
+        int rc;
+
+        rc = ecryptfs_process_helo(ECRYPTFS_TRANSPORT_NETLINK,
+                                   NETLINK_CREDS(skb)->uid,
+                                   NETLINK_CREDS(skb)->pid);
+        if (rc)
+                printk(KERN_WARNING "Error processing HELO; rc = [%d]\n", rc);
+        return rc;
+}
+
+/**
+ * ecryptfs_process_nl_quit
+ * @skb: The socket buffer containing the nlmsghdr in QUIT state
+ *
+ * Gets uid and pid of the skb and deletes the corresponding daemon
+ * id, if it is the registered that is requesting the
+ * deletion. Returns zero after deleting the desired daemon id;
+ * non-zero otherwise.
+ */
+static int ecryptfs_process_nl_quit(struct sk_buff *skb)
+{
+        int rc;
+
+        rc = ecryptfs_process_quit(NETLINK_CREDS(skb)->uid,
+                                   NETLINK_CREDS(skb)->pid);
+        if (rc)
+                printk(KERN_WARNING
+                       "Error processing QUIT message; rc = [%d]\n", rc);
+        return rc;
+}
+
+/**
+ * ecryptfs_receive_nl_message
+ *
+ * Callback function called by netlink system when a message arrives.
+ * If the message looks to be valid, then an attempt is made to assign
+ * it to its desired netlink context element and wake up the process
+ * that is waiting for a response.
+ */
+static void ecryptfs_receive_nl_message(struct sock *sk, int len)
+{
+        struct sk_buff *skb;
+        struct nlmsghdr *nlh;
+        int rc = 0;     /* skb_recv_datagram requires this */
+
+receive:
+        skb = skb_recv_datagram(sk, 0, 0, &rc);
+        if (rc == -EINTR)
+                goto receive;
+        else if (rc < 0) {
+                ecryptfs_printk(KERN_ERR, "Error occurred while "
+                                "receiving eCryptfs netlink message; "
+                                "rc = [%d]\n", rc);
+                return;
+        }
+        nlh = (struct nlmsghdr *)skb->data;
+        if (!NLMSG_OK(nlh, skb->len)) {
+                ecryptfs_printk(KERN_ERR, "Received corrupt netlink "
+                                "message\n");
+                goto free;
+        }
+        switch (nlh->nlmsg_type) {
+                case ECRYPTFS_NLMSG_RESPONSE:
+                        if (ecryptfs_process_nl_response(skb)) {
+                                ecryptfs_printk(KERN_WARNING, "Failed to "
+                                                "deliver netlink response to "
+                                                "requesting operation\n");
+                        }
+                        break;
+                case ECRYPTFS_NLMSG_HELO:
+                        if (ecryptfs_process_nl_helo(skb)) {
+                                ecryptfs_printk(KERN_WARNING, "Failed to "
+                                                "fulfill HELO request\n");
+                        }
+                        break;
+                case ECRYPTFS_NLMSG_QUIT:
+                        if (ecryptfs_process_nl_quit(skb)) {
+                                ecryptfs_printk(KERN_WARNING, "Failed to "
+                                                "fulfill QUIT request\n");
+                        }
+                        break;
+                default:
+                        ecryptfs_printk(KERN_WARNING, "Dropping netlink "
+                                        "message of unrecognized type [%d]\n",
+                                        nlh->nlmsg_type);
+                        break;
+        }
+free:
+        kfree_skb(skb);
+}
+
+/**
+ * ecryptfs_init_netlink
+ *
+ * Initializes the daemon id hash list, netlink context array, and
+ * necessary locks.  Returns zero upon success; non-zero upon error.
+ */
+int ecryptfs_init_netlink(void)
+{
+        int rc;
+
+        ecryptfs_nl_sock = netlink_kernel_create(NETLINK_ECRYPTFS, 0,
+                                                 ecryptfs_receive_nl_message,
+                                                 THIS_MODULE);
+        if (!ecryptfs_nl_sock) {
+                rc = -EIO;
+                ecryptfs_printk(KERN_ERR, "Failed to create netlink socket\n");
+                goto out;
+        }
+        ecryptfs_nl_sock->sk_sndtimeo = ECRYPTFS_DEFAULT_SEND_TIMEOUT;
+        rc = 0;
+out:
+        return rc;
+}
+
+/**
+ * ecryptfs_release_netlink
+ *
+ * Frees all memory used by the netlink context array and releases the
+ * netlink socket.
+ */
+void ecryptfs_release_netlink(void)
+{
+        if (ecryptfs_nl_sock && ecryptfs_nl_sock->sk_socket)
+                sock_release(ecryptfs_nl_sock->sk_socket);
+        ecryptfs_nl_sock = NULL;
+}