keys: fix an RCU warning

Fix the following RCU warning:

  ===================================================
  [ INFO: suspicious rcu_dereference_check() usage. ]
  ---------------------------------------------------
  security/keys/request_key.c:116 invoked rcu_dereference_check() without protection!

This was caused by doing:

	[root@andromeda ~]# keyctl newring fred @s
	539196288
	[root@andromeda ~]# keyctl request2 user a a 539196288
	request_key: Required key not available

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 03fe63ed55b..ea97c3120d6 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -68,7 +68,8 @@ static int call_sbin_request_key(struct key_construction *cons,
 {
         const struct cred *cred = current_cred();
         key_serial_t prkey, sskey;
-        struct key *key = cons->key, *authkey = cons->authkey, *keyring;
+        struct key *key = cons->key, *authkey = cons->authkey, *keyring,
+                *session;
         char *argv[9], *envp[3], uid_str[12], gid_str[12];
         char key_str[12], keyring_str[3][12];
         char desc[20];
@@ -112,10 +113,12 @@ static int call_sbin_request_key(struct key_construction *cons,
         if (cred->tgcred->process_keyring)
                 prkey = cred->tgcred->process_keyring->serial;
 
-        if (cred->tgcred->session_keyring)
-                sskey = rcu_dereference(cred->tgcred->session_keyring)->serial;
-        else
-                sskey = cred->user->session_keyring->serial;
+        rcu_read_lock();
+        session = rcu_dereference(cred->tgcred->session_keyring);
+        if (!session)
+                session = cred->user->session_keyring;
+        sskey = session->serial;
+        rcu_read_unlock();
 
         sprintf(keyring_str[2], "%d", sskey);