fscrypt: add FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl

Add a new fscrypt ioctl, FS_IOC_GET_ENCRYPTION_KEY_STATUS.  Given a key
specified by 'struct fscrypt_key_specifier' (the same way a key is
specified for the other fscrypt key management ioctls), it returns
status information in a 'struct fscrypt_get_key_status_arg'.

The main motivation for this is that applications need to be able to
check whether an encrypted directory is "unlocked" or not, so that they
can add the key if it is not, and avoid adding the key (which may
involve prompting the user for a passphrase) if it already is.

It's possible to use some workarounds such as checking whether opening a
regular file fails with ENOKEY, or checking whether the filenames "look
like gibberish" or not.  However, no workaround is usable in all cases.

Like the other key management ioctls, the keyrings syscalls may seem at
first to be a good fit for this.  Unfortunately, they are not.  Even if
we exposed the keyring ID of the ->s_master_keys keyring and gave
everyone Search permission on it (note: currently the keyrings
permission system would also allow everyone to "invalidate" the keyring
too), the fscrypt keys have an additional state that doesn't map cleanly
to the keyrings API: the secret can be removed, but we can be still
tracking the files that were using the key, and the removal can be
re-attempted or the secret added again.

After later patches, some applications will also need a way to determine
whether a key was added by the current user vs. by some other user.
Reserved fields are included in fscrypt_get_key_status_arg for this and
other future extensions.

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

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index b494c5f9c01f..6628d09585bd 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -142,6 +142,7 @@ extern int fscrypt_inherit_context(struct inode *, struct inode *,
 extern void fscrypt_sb_free(struct super_block *sb);
 extern int fscrypt_ioctl_add_key(struct file *filp, void __user *arg);
 extern int fscrypt_ioctl_remove_key(struct file *filp, void __user *arg);
+extern int fscrypt_ioctl_get_key_status(struct file *filp, void __user *arg);
 
 /* keysetup.c */
 extern int fscrypt_get_encryption_info(struct inode *);
@@ -388,6 +389,12 @@ static inline int fscrypt_ioctl_remove_key(struct file *filp, void __user *arg)
 	return -EOPNOTSUPP;
 }
 
+static inline int fscrypt_ioctl_get_key_status(struct file *filp,
+					       void __user *arg)
+{
+	return -EOPNOTSUPP;
+}
+
 /* keysetup.c */
 static inline int fscrypt_get_encryption_info(struct inode *inode)
 {