From: Eric Biggers <ebiggers@xxxxxxxxxx>
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.
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
fs/crypto/fscrypt_private.h | 3 +++
fs/crypto/keyring.c | 47 +++++++++++++++++++++++++++++++++++++
fs/crypto/policy.c | 6 +++++
3 files changed, 56 insertions(+)
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index 2445b0fc4f01c..8d61f410aa677 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -408,6 +408,9 @@ 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 307533d4d7c51..258409d0c7409 100644
--- a/fs/crypto/keyring.c
+++ b/fs/crypto/keyring.c
@@ -562,6 +562,53 @@ int fscrypt_ioctl_add_key(struct file *filp, void __user *_uarg)
}
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;
+}
+
static void shrink_dcache_inode(struct inode *inode)
{
struct dentry *dentry;
diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index 0141d338c1fdb..60de9048ed4d1 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -233,6 +233,7 @@ static int set_encryption_policy(struct inode *inode,
{
union fscrypt_context ctx;
int ctxsize;
+ int err;
if (!fscrypt_supported_policy(policy, inode))
return -EINVAL;
@@ -251,6 +252,11 @@ 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);
+ } else {
+ err = fscrypt_verify_key_added(inode->i_sb,
+ policy->v2.master_key_identifier);
+ if (err)
+ return err;
}
ctxsize = fscrypt_new_context_from_policy(&ctx, policy);
--
2.22.0
