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authorEric Biggers <ebiggers@google.com>2019-08-04 19:35:47 -0700
committerEric Biggers <ebiggers@google.com>2019-08-12 19:18:50 -0700
commit23c688b54016eed15d39f4387ca9da241e165922 (patch)
tree5420988ce2bd3df0c286e6a8856df462424c9f53 /fs/crypto/fscrypt_private.h
parent5dae460c2292dbbdac3a7a982cd566f470d957a2 (diff)
downloadlinux-23c688b54016eed15d39f4387ca9da241e165922.tar.bz2
fscrypt: allow unprivileged users to add/remove keys for v2 policies
Allow the FS_IOC_ADD_ENCRYPTION_KEY and FS_IOC_REMOVE_ENCRYPTION_KEY ioctls to be used by non-root users to add and remove encryption keys from the filesystem-level crypto keyrings, subject to limitations. Motivation: while privileged fscrypt key management is sufficient for some users (e.g. Android and Chromium OS, where a privileged process manages all keys), the old API by design also allows non-root users to set up and use encrypted directories, and we don't want to regress on that. Especially, we don't want to force users to continue using the old API, running into the visibility mismatch between files and keyrings and being unable to "lock" encrypted directories. Intuitively, the ioctls have to be privileged since they manipulate filesystem-level state. However, it's actually safe to make them unprivileged if we very carefully enforce some specific limitations. First, each key must be identified by a cryptographic hash so that a user can't add the wrong key for another user's files. For v2 encryption policies, we use the key_identifier for this. v1 policies don't have this, so managing keys for them remains privileged. Second, each key a user adds is charged to their quota for the keyrings service. Thus, a user can't exhaust memory by adding a huge number of keys. By default each non-root user is allowed up to 200 keys; this can be changed using the existing sysctl 'kernel.keys.maxkeys'. Third, if multiple users add the same key, we keep track of those users of the key (of which there remains a single copy), and won't really remove the key, i.e. "lock" the encrypted files, until all those users have removed it. This prevents denial of service attacks that would be possible under simpler schemes, such allowing the first user who added a key to remove it -- since that could be a malicious user who has compromised the key. Of course, encryption keys should be kept secret, but the idea is that using encryption should never be *less* secure than not using encryption, even if your key was compromised. We tolerate that a user will be unable to really remove a key, i.e. unable to "lock" their encrypted files, if another user has added the same key. But in a sense, this is actually a good thing because it will avoid providing a false notion of security where a key appears to have been removed when actually it's still in memory, available to any attacker who compromises the operating system kernel. Reviewed-by: Theodore Ts'o <tytso@mit.edu> Signed-off-by: Eric Biggers <ebiggers@google.com>
Diffstat (limited to 'fs/crypto/fscrypt_private.h')
-rw-r--r--fs/crypto/fscrypt_private.h31
1 files changed, 27 insertions, 4 deletions
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index c89e37d38e42..d0e238234234 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -335,9 +335,16 @@ struct fscrypt_master_key {
* FS_IOC_REMOVE_ENCRYPTION_KEY can be retried, or
* FS_IOC_ADD_ENCRYPTION_KEY can add the secret again.
*
- * Locking: protected by key->sem.
+ * Locking: protected by key->sem (outer) and mk_secret_sem (inner).
+ * The reason for two locks is that key->sem also protects modifying
+ * mk_users, which ranks it above the semaphore for the keyring key
+ * type, which is in turn above page faults (via keyring_read). But
+ * sometimes filesystems call fscrypt_get_encryption_info() from within
+ * a transaction, which ranks it below page faults. So we need a
+ * separate lock which protects mk_secret but not also mk_users.
*/
struct fscrypt_master_key_secret mk_secret;
+ struct rw_semaphore mk_secret_sem;
/*
* For v1 policy keys: an arbitrary key descriptor which was assigned by
@@ -348,6 +355,22 @@ struct fscrypt_master_key {
struct fscrypt_key_specifier mk_spec;
/*
+ * Keyring which contains a key of type 'key_type_fscrypt_user' for each
+ * user who has added this key. Normally each key will be added by just
+ * one user, but it's possible that multiple users share a key, and in
+ * that case we need to keep track of those users so that one user can't
+ * remove the key before the others want it removed too.
+ *
+ * This is NULL for v1 policy keys; those can only be added by root.
+ *
+ * Locking: in addition to this keyrings own semaphore, this is
+ * protected by the master key's key->sem, so we can do atomic
+ * search+insert. It can also be searched without taking any locks, but
+ * in that case the returned key may have already been removed.
+ */
+ struct key *mk_users;
+
+ /*
* Length of ->mk_decrypted_inodes, plus one if mk_secret is present.
* Once this goes to 0, the master key is removed from ->s_master_keys.
* The 'struct fscrypt_master_key' will continue to live as long as the
@@ -374,9 +397,9 @@ is_master_key_secret_present(const struct fscrypt_master_key_secret *secret)
/*
* The READ_ONCE() is only necessary for fscrypt_drop_inode() and
* fscrypt_key_describe(). These run in atomic context, so they can't
- * take key->sem and thus 'secret' can change concurrently which would
- * be a data race. But they only need to know whether the secret *was*
- * present at the time of check, so READ_ONCE() suffices.
+ * take ->mk_secret_sem and thus 'secret' can change concurrently which
+ * would be a data race. But they only need to know whether the secret
+ * *was* present at the time of check, so READ_ONCE() suffices.
*/
return READ_ONCE(secret->size) != 0;
}