Commit 5ab7189a authored by Eric Biggers's avatar Eric Biggers
Browse files

fscrypt: require that key be added when setting a v2 encryption policy 



By looking up the master keys in a filesystem-level keyring rather than
in the calling processes' key hierarchy, it becomes possible for a user
to set an encryption policy which refers to some key they don't actually
know, then encrypt their files using that key.  Cryptographically this
isn't much of a problem, but the semantics of this would be a bit weird.
Thus, enforce that a v2 encryption policy can only be set if the user
has previously added the key, or has capable(CAP_FOWNER).

We tolerate that this problem will continue to exist for v1 encryption
policies, however; there is no way around that.

Reviewed-by: default avatarTheodore Ts'o <tytso@mit.edu>
Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
parent 78a1b96b

fs/crypto/fscrypt_private.h

+3 −0
Original line number Diff line number Diff line
@@ -431,6 +431,9 @@ extern struct key * 
fscrypt_find_master_key(struct super_block *sb,

			const struct fscrypt_key_specifier *mk_spec);



extern int fscrypt_verify_key_added(struct super_block *sb,

				    const u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]);



extern int __init fscrypt_init_keyring(void);



/* keysetup.c */

fs/crypto/keyring.c

+47 −0
Original line number Diff line number Diff line
@@ -562,6 +562,53 @@ out_wipe_secret: 
}

EXPORT_SYMBOL_GPL(fscrypt_ioctl_add_key);



/*

 * Verify that the current user has added a master key with the given identifier

 * (returns -ENOKEY if not).  This is needed to prevent a user from encrypting

 * their files using some other user's key which they don't actually know.

 * Cryptographically this isn't much of a problem, but the semantics of this

 * would be a bit weird, so it's best to just forbid it.

 *

 * The system administrator (CAP_FOWNER) can override this, which should be

 * enough for any use cases where encryption policies are being set using keys

 * that were chosen ahead of time but aren't available at the moment.

 *

 * Note that the key may have already removed by the time this returns, but

 * that's okay; we just care whether the key was there at some point.

 *

 * Return: 0 if the key is added, -ENOKEY if it isn't, or another -errno code

 */

int fscrypt_verify_key_added(struct super_block *sb,

			     const u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE])

{

	struct fscrypt_key_specifier mk_spec;

	struct key *key, *mk_user;

	struct fscrypt_master_key *mk;

	int err;



	mk_spec.type = FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER;

	memcpy(mk_spec.u.identifier, identifier, FSCRYPT_KEY_IDENTIFIER_SIZE);



	key = fscrypt_find_master_key(sb, &mk_spec);

	if (IS_ERR(key)) {

		err = PTR_ERR(key);

		goto out;

	}

	mk = key->payload.data[0];

	mk_user = find_master_key_user(mk);

	if (IS_ERR(mk_user)) {

		err = PTR_ERR(mk_user);

	} else {

		key_put(mk_user);

		err = 0;

	}

	key_put(key);

out:

	if (err == -ENOKEY && capable(CAP_FOWNER))

		err = 0;

	return err;

}



/*

 * 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

fs/crypto/policy.c

+13 −1
Original line number Diff line number Diff line
@@ -233,11 +233,13 @@ static int set_encryption_policy(struct inode *inode, 
{

	union fscrypt_context ctx;

	int ctxsize;

	int err;



	if (!fscrypt_supported_policy(policy, inode))

		return -EINVAL;



	if (policy->version == FSCRYPT_POLICY_V1) {

	switch (policy->version) {

	case FSCRYPT_POLICY_V1:

		/*

		 * The original encryption policy version provided no way of

		 * verifying that the correct master key was supplied, which was
@@ -251,6 +253,16 @@ static int set_encryption_policy(struct inode *inode, 
		 */

		pr_warn_once("%s (pid %d) is setting deprecated v1 encryption policy; recommend upgrading to v2.\n",

			     current->comm, current->pid);

		break;

	case FSCRYPT_POLICY_V2:

		err = fscrypt_verify_key_added(inode->i_sb,

					       policy->v2.master_key_identifier);

		if (err)

			return err;

		break;

	default:

		WARN_ON(1);

		return -EINVAL;

	}



	ctxsize = fscrypt_new_context_from_policy(&ctx, policy);

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