fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl

fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl

Add a new fscrypt ioctl, FS_IOC_REMOVE_ENCRYPTION_KEY.  This ioctl
removes an encryption key that was added by FS_IOC_ADD_ENCRYPTION_KEY.
It wipes the secret key itself, then "locks" the encrypted files and
directories that had been unlocked using that key -- implemented by
evicting the relevant dentries and inodes from the VFS caches.

The problem this solves is that many fscrypt users want the ability to
remove encryption keys, causing the corresponding encrypted directories
to appear "locked" (presented in ciphertext form) again.  Moreover,
users want removing an encryption key to *really* remove it, in the
sense that the removed keys cannot be recovered even if kernel memory is
compromised, e.g. by the exploit of a kernel security vulnerability or
by a physical attack.  This is desirable after a user logs out of the
system, for example.  In many cases users even already assume this to be
the case and are surprised to hear when it's not.

It is not sufficient to simply unlink the master key from the keyring
(or to revoke or invalidate it), since the actual encryption transform
objects are still pinned in memory by their inodes.  Therefore, to
really remove a key we must also evict the relevant inodes.

Currently one workaround is to run 'sync && echo 2 >
/proc/sys/vm/drop_caches'.  But, that evicts all unused inodes in the
system rather than just the inodes associated with the key being
removed, causing severe performance problems.  Moreover, it requires
root privileges, so regular users can't "lock" their encrypted files.

Another workaround, used in Chromium OS kernels, is to add a new
VFS-level ioctl FS_IOC_DROP_CACHE which is a more restricted version of
drop_caches that operates on a single super_block.  It does:

        shrink_dcache_sb(sb);
        invalidate_inodes(sb, false);

But it's still a hack.  Yet, the major users of filesystem encryption
want this feature badly enough that they are actually using these hacks.

To properly solve the problem, start maintaining a list of the inodes
which have been "unlocked" using each master key.  Originally this
wasn't possible because the kernel didn't keep track of in-use master
keys at all.  But, with the ->s_master_keys keyring it is now possible.

Then, add an ioctl FS_IOC_REMOVE_ENCRYPTION_KEY.  It finds the specified
master key in ->s_master_keys, then wipes the secret key itself, which
prevents any additional inodes from being unlocked with the key.  Then,
it syncs the filesystem and evicts the inodes in the key's list.  The
normal inode eviction code will free and wipe the per-file keys (in
->i_crypt_info).  Note that freeing ->i_crypt_info without evicting the
inodes was also considered, but would have been racy.

Some inodes may still be in use when a master key is removed, and we
can't simply revoke random file descriptors, mmap's, etc.  Thus, the
ioctl simply skips in-use inodes, and returns -EBUSY to indicate that
some inodes weren't evicted.  The master key *secret* is still removed,
but the fscrypt_master_key struct remains to keep track of the remaining
inodes.  Userspace can then retry the ioctl to evict the remaining
inodes.  Alternatively, if userspace adds the key again, the refreshed
secret will be associated with the existing list of inodes so they
remain correctly tracked for future key removals.

The ioctl doesn't wipe pagecache pages.  Thus, we tolerate that after a
kernel compromise some portions of plaintext file contents may still be
recoverable from memory.  This can be solved by enabling page poisoning
system-wide, which security conscious users may choose to do.  But it's
very difficult to solve otherwise, e.g. note that plaintext file
contents may have been read in other places than pagecache pages.

Like FS_IOC_ADD_ENCRYPTION_KEY, FS_IOC_REMOVE_ENCRYPTION_KEY is
initially restricted to privileged users only.  This is sufficient for
some use cases, but not all.  A later patch will relax this restriction,
but it will require introducing key hashes, among other changes.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>