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
shouldn't actually be a major problem; for one, every file will still be
encrypted with a unique derived key, rather than with the master key
directly. But to be on the safe side, 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 | 2 ++
fs/crypto/keyinfo.c | 42 ++++++++++++++++++++++++++++++++++++++++++
fs/crypto/policy.c | 6 ++++++
3 files changed, 50 insertions(+)
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index d0a63086fa95..7a0d5b6c2504 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -234,6 +234,8 @@ extern struct page *fscrypt_alloc_bounce_page(struct fscrypt_ctx *ctx,
gfp_t gfp_flags);
/* keyinfo.c */
+extern int fscrypt_verify_key_added(struct super_block *sb,
+ const u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]);
extern struct key_type key_type_fscrypt_mk;
extern struct key_type key_type_fscrypt_mk_user;
extern void __exit fscrypt_essiv_cleanup(void);
diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
index 1fe44983239a..fd59f37dad10 100644
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -851,6 +851,48 @@ 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 that has 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 speaking, it's debatable how much of a problem this
+ * actually would be, but 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.
+ */
+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.identifier, identifier, FSCRYPT_KEY_IDENTIFIER_SIZE);
+
+ key = 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 evict_dentries_for_decrypted_inodes(struct fscrypt_master_key *mk)
{
struct fscrypt_info *ci;
diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index 27a391038f73..cfb404def9ed 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -170,6 +170,7 @@ static int set_encryption_policy(struct inode *inode,
const union fscrypt_policy *policy)
{
union fscrypt_context ctx;
+ int err;
if (!fscrypt_supported_policy(policy))
return -EINVAL;
@@ -190,6 +191,11 @@ static int set_encryption_policy(struct inode *inode,
*/
pr_warn_once("%s (pid %d) is setting less secure 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;
}
return inode->i_sb->s_cop->set_context(inode, &ctx,
--
2.15.0.rc0.271.g36b669edcc-goog
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