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authorEric Biggers <ebiggers@google.com>2019-08-04 19:35:48 -0700
committerEric Biggers <ebiggers@google.com>2019-08-12 19:18:50 -0700
commit5ab7189a31bad40e4b44020cae6e56c8074721a1 (patch)
treed9cea4d978ce732bd9e97ded7782ebb2ec600829 /fs/crypto
parent78a1b96bcf7a0721c7852bb1475218c3cbef884a (diff)
downloadlinux-5ab7189a31bad40e4b44020cae6e56c8074721a1.tar.bz2
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 <tytso@mit.edu> Signed-off-by: Eric Biggers <ebiggers@google.com>
Diffstat (limited to 'fs/crypto')
-rw-r--r--fs/crypto/fscrypt_private.h3
-rw-r--r--fs/crypto/keyring.c47
-rw-r--r--fs/crypto/policy.c14
3 files changed, 63 insertions, 1 deletions
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
@@ -563,6 +563,53 @@ 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
* pinned by child dentries, so first try to evict the children too.
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);