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

Eric Biggers
2019-11-13T20:35:55Z

On Wed, Nov 06, 2019 at 04:12:59PM -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.
>
> Signed-off-by: Eric Biggers <ebiggers@google.com>

David and Jarkko, are you okay with this patch from a keyrings subsystem
perspective?

- Eric