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>

1 files changed, 259 insertions, 1 deletions

diff --git a/fs/crypto/keyring.c b/fs/crypto/keyring.c
index bcd7d2836e1e..c3423f0edc70 100644
--- a/fs/crypto/keyring.c
+++ b/fs/crypto/keyring.c
@@ -10,6 +10,7 @@
  * filesystem-level keyring, including the ioctls:
  *
  * - FS_IOC_ADD_ENCRYPTION_KEY
+ * - FS_IOC_REMOVE_ENCRYPTION_KEY
  *
  * See the "User API" section of Documentation/filesystems/fscrypt.rst for more
  * information about these ioctls.
@@ -60,6 +61,13 @@ static void fscrypt_key_destroy(struct key *key)
 static void fscrypt_key_describe(const struct key *key, struct seq_file *m)
 {
 	seq_puts(m, key->description);
+
+	if (key_is_positive(key)) {
+		const struct fscrypt_master_key *mk = key->payload.data[0];
+
+		if (!is_master_key_secret_present(&mk->mk_secret))
+			seq_puts(m, ": secret removed");
+	}
 }
 
 /*
@@ -186,6 +194,10 @@ static int add_new_master_key(struct fscrypt_master_key_secret *secret,
 
 	move_master_key_secret(&mk->mk_secret, secret);
 
+	refcount_set(&mk->mk_refcount, 1); /* secret is present */
+	INIT_LIST_HEAD(&mk->mk_decrypted_inodes);
+	spin_lock_init(&mk->mk_decrypted_inodes_lock);
+
 	format_mk_description(description, mk_spec);
 	key = key_alloc(&key_type_fscrypt, description,
 			GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, current_cred(),
@@ -207,6 +219,21 @@ out_free_mk:
 	return err;
 }
 
+#define KEY_DEAD	1
+
+static int add_existing_master_key(struct fscrypt_master_key *mk,
+				   struct fscrypt_master_key_secret *secret)
+{
+	if (is_master_key_secret_present(&mk->mk_secret))
+		return 0;
+
+	if (!refcount_inc_not_zero(&mk->mk_refcount))
+		return KEY_DEAD;
+
+	move_master_key_secret(&mk->mk_secret, secret);
+	return 0;
+}
+
 static int add_master_key(struct super_block *sb,
 			  struct fscrypt_master_key_secret *secret,
 			  const struct fscrypt_key_specifier *mk_spec)
@@ -216,6 +243,7 @@ static int add_master_key(struct super_block *sb,
 	int err;
 
 	mutex_lock(&fscrypt_add_key_mutex); /* serialize find + link */
+retry:
 	key = fscrypt_find_master_key(sb, mk_spec);
 	if (IS_ERR(key)) {
 		err = PTR_ERR(key);
@@ -227,8 +255,20 @@ static int add_master_key(struct super_block *sb,
 			goto out_unlock;
 		err = add_new_master_key(secret, mk_spec, sb->s_master_keys);
 	} else {
+		/*
+		 * Found the key in ->s_master_keys.  Re-add the secret if
+		 * needed.
+		 */
+		down_write(&key->sem);
+		err = add_existing_master_key(key->payload.data[0], secret);
+		up_write(&key->sem);
+		if (err == KEY_DEAD) {
+			/* Key being removed or needs to be removed */
+			key_invalidate(key);
+			key_put(key);
+			goto retry;
+		}
 		key_put(key);
-		err = 0;
 	}
 out_unlock:
 	mutex_unlock(&fscrypt_add_key_mutex);
@@ -280,6 +320,224 @@ out_wipe_secret:
 }
 EXPORT_SYMBOL_GPL(fscrypt_ioctl_add_key);
 
+/*
+ * Try to evict the inode's dentries from the dentry cache.  If the inode is a
+ * directory, then it can have at most one dentry; however, that dentry may be
+ * pinned by child dentries, so first try to evict the children too.
+ */
+static void shrink_dcache_inode(struct inode *inode)
+{
+	struct dentry *dentry;
+
+	if (S_ISDIR(inode->i_mode)) {
+		dentry = d_find_any_alias(inode);
+		if (dentry) {
+			shrink_dcache_parent(dentry);
+			dput(dentry);
+		}
+	}
+	d_prune_aliases(inode);
+}
+
+static void evict_dentries_for_decrypted_inodes(struct fscrypt_master_key *mk)
+{
+	struct fscrypt_info *ci;
+	struct inode *inode;
+	struct inode *toput_inode = NULL;
+
+	spin_lock(&mk->mk_decrypted_inodes_lock);
+
+	list_for_each_entry(ci, &mk->mk_decrypted_inodes, ci_master_key_link) {
+		inode = ci->ci_inode;
+		spin_lock(&inode->i_lock);
+		if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) {
+			spin_unlock(&inode->i_lock);
+			continue;
+		}
+		__iget(inode);
+		spin_unlock(&inode->i_lock);
+		spin_unlock(&mk->mk_decrypted_inodes_lock);
+
+		shrink_dcache_inode(inode);
+		iput(toput_inode);
+		toput_inode = inode;
+
+		spin_lock(&mk->mk_decrypted_inodes_lock);
+	}
+
+	spin_unlock(&mk->mk_decrypted_inodes_lock);
+	iput(toput_inode);
+}
+
+static int check_for_busy_inodes(struct super_block *sb,
+				 struct fscrypt_master_key *mk)
+{
+	struct list_head *pos;
+	size_t busy_count = 0;
+	unsigned long ino;
+	struct dentry *dentry;
+	char _path[256];
+	char *path = NULL;
+
+	spin_lock(&mk->mk_decrypted_inodes_lock);
+
+	list_for_each(pos, &mk->mk_decrypted_inodes)
+		busy_count++;
+
+	if (busy_count == 0) {
+		spin_unlock(&mk->mk_decrypted_inodes_lock);
+		return 0;
+	}
+
+	{
+		/* select an example file to show for debugging purposes */
+		struct inode *inode =
+			list_first_entry(&mk->mk_decrypted_inodes,
+					 struct fscrypt_info,
+					 ci_master_key_link)->ci_inode;
+		ino = inode->i_ino;
+		dentry = d_find_alias(inode);
+	}
+	spin_unlock(&mk->mk_decrypted_inodes_lock);
+
+	if (dentry) {
+		path = dentry_path(dentry, _path, sizeof(_path));
+		dput(dentry);
+	}
+	if (IS_ERR_OR_NULL(path))
+		path = "(unknown)";
+
+	fscrypt_warn(NULL,
+		     "%s: %zu inode(s) still busy after removing key with %s %*phN, including ino %lu (%s)",
+		     sb->s_id, busy_count, master_key_spec_type(&mk->mk_spec),
+		     master_key_spec_len(&mk->mk_spec), (u8 *)&mk->mk_spec.u,
+		     ino, path);
+	return -EBUSY;
+}
+
+static int try_to_lock_encrypted_files(struct super_block *sb,
+				       struct fscrypt_master_key *mk)
+{
+	int err1;
+	int err2;
+
+	/*
+	 * An inode can't be evicted while it is dirty or has dirty pages.
+	 * Thus, we first have to clean the inodes in ->mk_decrypted_inodes.
+	 *
+	 * Just do it the easy way: call sync_filesystem().  It's overkill, but
+	 * it works, and it's more important to minimize the amount of caches we
+	 * drop than the amount of data we sync.  Also, unprivileged users can
+	 * already call sync_filesystem() via sys_syncfs() or sys_sync().
+	 */
+	down_read(&sb->s_umount);
+	err1 = sync_filesystem(sb);
+	up_read(&sb->s_umount);
+	/* If a sync error occurs, still try to evict as much as possible. */
+
+	/*
+	 * Inodes are pinned by their dentries, so we have to evict their
+	 * dentries.  shrink_dcache_sb() would suffice, but would be overkill
+	 * and inappropriate for use by unprivileged users.  So instead go
+	 * through the inodes' alias lists and try to evict each dentry.
+	 */
+	evict_dentries_for_decrypted_inodes(mk);
+
+	/*
+	 * evict_dentries_for_decrypted_inodes() already iput() each inode in
+	 * the list; any inodes for which that dropped the last reference will
+	 * have been evicted due to fscrypt_drop_inode() detecting the key
+	 * removal and telling the VFS to evict the inode.  So to finish, we
+	 * just need to check whether any inodes couldn't be evicted.
+	 */
+	err2 = check_for_busy_inodes(sb, mk);
+
+	return err1 ?: err2;
+}
+
+/*
+ * Try to remove an fscrypt master encryption key.
+ *
+ * First we wipe the actual master key secret, so that no more inodes can be
+ * unlocked with it.  Then we try to evict all cached inodes that had been
+ * unlocked with the key.
+ *
+ * If all inodes were evicted, then we unlink the fscrypt_master_key from the
+ * keyring.  Otherwise it remains in the keyring in the "incompletely removed"
+ * state (without the actual secret key) where it tracks the list of remaining
+ * inodes.  Userspace can execute the ioctl again later to retry eviction, or
+ * alternatively can re-add the secret key again.
+ *
+ * For more details, see the "Removing keys" section of
+ * Documentation/filesystems/fscrypt.rst.
+ */
+int fscrypt_ioctl_remove_key(struct file *filp, void __user *_uarg)
+{
+	struct super_block *sb = file_inode(filp)->i_sb;
+	struct fscrypt_remove_key_arg __user *uarg = _uarg;
+	struct fscrypt_remove_key_arg arg;
+	struct key *key;
+	struct fscrypt_master_key *mk;
+	u32 status_flags = 0;
+	int err;
+	bool dead;
+
+	if (copy_from_user(&arg, uarg, sizeof(arg)))
+		return -EFAULT;
+
+	if (!valid_key_spec(&arg.key_spec))
+		return -EINVAL;
+
+	if (memchr_inv(arg.__reserved, 0, sizeof(arg.__reserved)))
+		return -EINVAL;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EACCES;
+
+	/* Find the key being removed. */
+	key = fscrypt_find_master_key(sb, &arg.key_spec);
+	if (IS_ERR(key))
+		return PTR_ERR(key);
+	mk = key->payload.data[0];
+
+	down_write(&key->sem);
+
+	/* Wipe the secret. */
+	dead = false;
+	if (is_master_key_secret_present(&mk->mk_secret)) {
+		wipe_master_key_secret(&mk->mk_secret);
+		dead = refcount_dec_and_test(&mk->mk_refcount);
+	}
+	up_write(&key->sem);
+	if (dead) {
+		/*
+		 * No inodes reference the key, and we wiped the secret, so the
+		 * key object is free to be removed from the keyring.
+		 */
+		key_invalidate(key);
+		err = 0;
+	} else {
+		/* Some inodes still reference this key; try to evict them. */
+		err = try_to_lock_encrypted_files(sb, mk);
+		if (err == -EBUSY) {
+			status_flags |=
+				FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY;
+			err = 0;
+		}
+	}
+	/*
+	 * We return 0 if we successfully did something: wiped the secret, or
+	 * tried locking the files again.  Users need to check the informational
+	 * status flags if they care whether the key has been fully removed
+	 * including all files locked.
+	 */
+	key_put(key);
+	if (err == 0)
+		err = put_user(status_flags, &uarg->removal_status_flags);
+	return err;
+}
+EXPORT_SYMBOL_GPL(fscrypt_ioctl_remove_key);
+
 int __init fscrypt_init_keyring(void)
 {
 	return register_key_type(&key_type_fscrypt);