security: don't use RCU accessors for cred->session_keyring

sparse complains that a bunch of places in kernel/cred.c access
cred->session_keyring without the RCU helpers required by the __rcu
annotation.

cred->session_keyring is written in the following places:

 - prepare_kernel_cred() [in a new cred struct]
 - keyctl_session_to_parent() [in a new cred struct]
 - prepare_creds [in a new cred struct, via memcpy]
 - install_session_keyring_to_cred()
  - from install_session_keyring() on new creds
  - from join_session_keyring() on new creds [twice]
  - from umh_keys_init()
   - from call_usermodehelper_exec_async() on new creds

All of these writes are before the creds are committed; therefore,
cred->session_keyring doesn't need RCU protection.

Remove the __rcu annotation and fix up all existing users that use __rcu.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: James Morris <james.morris@microsoft.com>

1 files changed, 2 insertions, 7 deletions

diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 2f17d84d46f1..db72dc4d7639 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -142,12 +142,10 @@ static int call_sbin_request_key(struct key *authkey, void *aux)
 		prkey = cred->process_keyring->serial;
 	sprintf(keyring_str[1], "%d", prkey);
 
-	rcu_read_lock();
-	session = rcu_dereference(cred->session_keyring);
+	session = cred->session_keyring;
 	if (!session)
 		session = cred->user->session_keyring;
 	sskey = session->serial;
-	rcu_read_unlock();
 
 	sprintf(keyring_str[2], "%d", sskey);
 
@@ -287,10 +285,7 @@ static int construct_get_dest_keyring(struct key **_dest_keyring)
 
 			/* fall through */
 		case KEY_REQKEY_DEFL_SESSION_KEYRING:
-			rcu_read_lock();
-			dest_keyring = key_get(
-				rcu_dereference(cred->session_keyring));
-			rcu_read_unlock();
+			dest_keyring = key_get(cred->session_keyring);
 
 			if (dest_keyring)
 				break;