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>

include/linux/cred.h

@@ -138,7 +138,7 @@ struct cred {
 #ifdef CONFIG_KEYS
 	unsigned char	jit_keyring;	/* default keyring to attach requested
 					 * keys to */
-	struct key __rcu *session_keyring; /* keyring inherited over fork */
+	struct key	*session_keyring; /* keyring inherited over fork */
 	struct key	*process_keyring; /* keyring private to this process */
 	struct key	*thread_keyring; /* keyring private to this thread */
 	struct key	*request_key_auth; /* assumed request_key authority */

security/keys/process_keys.c

@@ -227,6 +227,7 @@ static int install_process_keyring(void)
  * Install the given keyring as the session keyring of the given credentials
  * struct, replacing the existing one if any.  If the given keyring is NULL,
  * then install a new anonymous session keyring.
+ * @cred can not be in use by any task yet.
  *
  * Return: 0 on success; -errno on failure.
  */
@@ -254,7 +255,7 @@ int install_session_keyring_to_cred(struct cred *cred, struct key *keyring)
 
 	/* install the keyring */
 	old = cred->session_keyring;
-	rcu_assign_pointer(cred->session_keyring, keyring);
+	cred->session_keyring = keyring;
 
 	if (old)
 		key_put(old);
@@ -392,11 +393,8 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
 
 	/* search the session keyring */
 	if (ctx->cred->session_keyring) {
-		rcu_read_lock();
 		key_ref = keyring_search_aux(
-			make_key_ref(rcu_dereference(ctx->cred->session_keyring), 1),
+			make_key_ref(ctx->cred->session_keyring, 1), ctx);
-			ctx);
-		rcu_read_unlock();
 
 		if (!IS_ERR(key_ref))
 			goto found;
@@ -612,10 +610,8 @@ try_again:
 			goto reget_creds;
 		}
 
-		rcu_read_lock();
+		key = ctx.cred->session_keyring;
-		key = rcu_dereference(ctx.cred->session_keyring);
 		__key_get(key);
-		rcu_read_unlock();
 		key_ref = make_key_ref(key, 1);
 		break;
 

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 = cred->session_keyring;
-	session = rcu_dereference(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(cred->session_keyring);
-			dest_keyring = key_get(
-				rcu_dereference(cred->session_keyring));
-			rcu_read_unlock();
 
 			if (dest_keyring)
 				break;