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>

3 files changed, 7 insertions, 16 deletions

diff --git a/include/linux/cred.h b/include/linux/cred.h
index ddd45bb74887..efb6edf32de7 100644
--- a/include/linux/cred.h
+++ b/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 */
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 9320424c4a46..bd7243cb4c85 100644
--- a/security/keys/process_keys.c
+++ b/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),
-                        ctx);
-                rcu_read_unlock();
+                        make_key_ref(ctx->cred->session_keyring, 1), ctx);
 
                 if (!IS_ERR(key_ref))
                         goto found;
@@ -612,10 +610,8 @@ try_again:
                         goto reget_creds;
                 }
 
-                rcu_read_lock();
-                key = rcu_dereference(ctx.cred->session_keyring);
+                key = ctx.cred->session_keyring;
                 __key_get(key);
-                rcu_read_unlock();
                 key_ref = make_key_ref(key, 1);
                 break;
 
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;