Re: [PATCH v2] fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Nov 19, 2019 at 02:24:47PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Extend the FS_IOC_ADD_ENCRYPTION_KEY ioctl to allow the raw key to be
> specified by a Linux keyring key, rather than specified directly.
> 
> This is useful because fscrypt keys belong to a particular filesystem
> instance, so they are destroyed when that filesystem is unmounted.
> Usually this is desired.  But in some cases, userspace may need to
> unmount and re-mount the filesystem while keeping the keys, e.g. during
> a system update.  This requires keeping the keys somewhere else too.
> 
> The keys could be kept in memory in a userspace daemon.  But depending
> on the security architecture and assumptions, it can be preferable to
> keep them only in kernel memory, where they are unreadable by userspace.
> 
> We also can't solve this by going back to the original fscrypt API
> (where for each file, the master key was looked up in the process's
> keyring hierarchy) because that caused lots of problems of its own.
> 
> Therefore, add the ability for FS_IOC_ADD_ENCRYPTION_KEY to accept a
> Linux keyring key.  This solves the problem by allowing userspace to (if
> needed) save the keys securely in a Linux keyring for re-provisioning,
> while still using the new fscrypt key management ioctls.
> 
> This is analogous to how dm-crypt accepts a Linux keyring key, but the
> key is then stored internally in the dm-crypt data structures rather
> than being looked up again each time the dm-crypt device is accessed.
> 
> Use a custom key type "fscrypt-provisioning" rather than one of the
> existing key types such as "logon".  This is strongly desired because it
> enforces that these keys are only usable for a particular purpose: for
> fscrypt as input to a particular KDF.  Otherwise, the keys could also be
> passed to any kernel API that accepts a "logon" key with any service
> prefix, e.g. dm-crypt, UBIFS, or (recently proposed) AF_ALG.  This would
> risk leaking information about the raw key despite it ostensibly being
> unreadable.  Of course, this mistake has already been made for multiple
> kernel APIs; but since this is a new API, let's do it right.
> 
> This patch has been tested using an xfstest which I wrote to test it.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Applied to fscrypt.git#master for 5.6.

- Eric