From 5ab7189a31bad40e4b44020cae6e56c8074721a1 Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Sun, 4 Aug 2019 19:35:48 -0700 Subject: [PATCH] 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: Theodore Ts'o Signed-off-by: Eric Biggers --- fs/crypto/fscrypt_private.h | 3 +++ fs/crypto/keyring.c | 47 +++++++++++++++++++++++++++++++++++++ fs/crypto/policy.c | 14 ++++++++++- 3 files changed, 63 insertions(+), 1 deletion(-) diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h index d0e238234234..e84efc01512e 100644 --- a/fs/crypto/fscrypt_private.h +++ b/fs/crypto/fscrypt_private.h @@ -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 */ diff --git a/fs/crypto/keyring.c b/fs/crypto/keyring.c index 8c600ead0e2e..c34fa7c61b43 100644 --- a/fs/crypto/keyring.c +++ b/fs/crypto/keyring.c @@ -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 diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c index 0141d338c1fd..4072ba644595 100644 --- a/fs/crypto/policy.c +++ b/fs/crypto/policy.c @@ -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);