• Michael Halcrow's avatar
    [PATCH] eCryptfs: Public key transport mechanism · 88b4a07e
    Michael Halcrow authored
    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: default avatarMichael Halcrow <mhalcrow@us.ibm.com>
    Cc: David Howells <dhowells@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    88b4a07e
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