CRED: Inaugurate COW credentials

Inaugurate copy-on-write credentials management.  This uses RCU to manage the
credentials pointer in the task_struct with respect to accesses by other tasks.
A process may only modify its own credentials, and so does not need locking to
access or modify its own credentials.

A mutex (cred_replace_mutex) is added to the task_struct to control the effect
of PTRACE_ATTACHED on credential calculations, particularly with respect to
execve().

With this patch, the contents of an active credentials struct may not be
changed directly; rather a new set of credentials must be prepared, modified
and committed using something like the following sequence of events:

	struct cred *new = prepare_creds();
	int ret = blah(new);
	if (ret < 0) {
		abort_creds(new);
		return ret;
	}
	return commit_creds(new);

There are some exceptions to this rule: the keyrings pointed to by the active
credentials may be instantiated - keyrings violate the COW rule as managing
COW keyrings is tricky, given that it is possible for a task to directly alter
the keys in a keyring in use by another task.

To help enforce this, various pointers to sets of credentials, such as those in
the task_struct, are declared const.  The purpose of this is compile-time
discouragement of altering credentials through those pointers.  Once a set of
credentials has been made public through one of these pointers, it may not be
modified, except under special circumstances:

  (1) Its reference count may incremented and decremented.

  (2) The keyrings to which it points may be modified, but not replaced.

The only safe way to modify anything else is to create a replacement and commit
using the functions described in Documentation/credentials.txt (which will be
added by a later patch).

This patch and the preceding patches have been tested with the LTP SELinux
testsuite.

This patch makes several logical sets of alteration:

 (1) execve().

     This now prepares and commits credentials in various places in the
     security code rather than altering the current creds directly.

 (2) Temporary credential overrides.

     do_coredump() and sys_faccessat() now prepare their own credentials and
     temporarily override the ones currently on the acting thread, whilst
     preventing interference from other threads by holding cred_replace_mutex
     on the thread being dumped.

     This will be replaced in a future patch by something that hands down the
     credentials directly to the functions being called, rather than altering
     the task's objective credentials.

 (3) LSM interface.

     A number of functions have been changed, added or removed:

     (*) security_capset_check(), ->capset_check()
     (*) security_capset_set(), ->capset_set()

     	 Removed in favour of security_capset().

     (*) security_capset(), ->capset()

     	 New.  This is passed a pointer to the new creds, a pointer to the old
     	 creds and the proposed capability sets.  It should fill in the new
     	 creds or return an error.  All pointers, barring the pointer to the
     	 new creds, are now const.

     (*) security_bprm_apply_creds(), ->bprm_apply_creds()

     	 Changed; now returns a value, which will cause the process to be
     	 killed if it's an error.

     (*) security_task_alloc(), ->task_alloc_security()

     	 Removed in favour of security_prepare_creds().

     (*) security_cred_free(), ->cred_free()

     	 New.  Free security data attached to cred->security.

     (*) security_prepare_creds(), ->cred_prepare()

     	 New. Duplicate any security data attached to cred->security.

     (*) security_commit_creds(), ->cred_commit()

     	 New. Apply any security effects for the upcoming installation of new
     	 security by commit_creds().

     (*) security_task_post_setuid(), ->task_post_setuid()

     	 Removed in favour of security_task_fix_setuid().

     (*) security_task_fix_setuid(), ->task_fix_setuid()

     	 Fix up the proposed new credentials for setuid().  This is used by
     	 cap_set_fix_setuid() to implicitly adjust capabilities in line with
     	 setuid() changes.  Changes are made to the new credentials, rather
     	 than the task itself as in security_task_post_setuid().

     (*) security_task_reparent_to_init(), ->task_reparent_to_init()

     	 Removed.  Instead the task being reparented to init is referred
     	 directly to init's credentials.

	 NOTE!  This results in the loss of some state: SELinux's osid no
	 longer records the sid of the thread that forked it.

     (*) security_key_alloc(), ->key_alloc()
     (*) security_key_permission(), ->key_permission()

     	 Changed.  These now take cred pointers rather than task pointers to
     	 refer to the security context.

 (4) sys_capset().

     This has been simplified and uses less locking.  The LSM functions it
     calls have been merged.

 (5) reparent_to_kthreadd().

     This gives the current thread the same credentials as init by simply using
     commit_thread() to point that way.

 (6) __sigqueue_alloc() and switch_uid()

     __sigqueue_alloc() can't stop the target task from changing its creds
     beneath it, so this function gets a reference to the currently applicable
     user_struct which it then passes into the sigqueue struct it returns if
     successful.

     switch_uid() is now called from commit_creds(), and possibly should be
     folded into that.  commit_creds() should take care of protecting
     __sigqueue_alloc().

 (7) [sg]et[ug]id() and co and [sg]et_current_groups.

     The set functions now all use prepare_creds(), commit_creds() and
     abort_creds() to build and check a new set of credentials before applying
     it.

     security_task_set[ug]id() is called inside the prepared section.  This
     guarantees that nothing else will affect the creds until we've finished.

     The calling of set_dumpable() has been moved into commit_creds().

     Much of the functionality of set_user() has been moved into
     commit_creds().

     The get functions all simply access the data directly.

 (8) security_task_prctl() and cap_task_prctl().

     security_task_prctl() has been modified to return -ENOSYS if it doesn't
     want to handle a function, or otherwise return the return value directly
     rather than through an argument.

     Additionally, cap_task_prctl() now prepares a new set of credentials, even
     if it doesn't end up using it.

 (9) Keyrings.

     A number of changes have been made to the keyrings code:

     (a) switch_uid_keyring(), copy_keys(), exit_keys() and suid_keys() have
     	 all been dropped and built in to the credentials functions directly.
     	 They may want separating out again later.

     (b) key_alloc() and search_process_keyrings() now take a cred pointer
     	 rather than a task pointer to specify the security context.

     (c) copy_creds() gives a new thread within the same thread group a new
     	 thread keyring if its parent had one, otherwise it discards the thread
     	 keyring.

     (d) The authorisation key now points directly to the credentials to extend
     	 the search into rather pointing to the task that carries them.

     (e) Installing thread, process or session keyrings causes a new set of
     	 credentials to be created, even though it's not strictly necessary for
     	 process or session keyrings (they're shared).

(10) Usermode helper.

     The usermode helper code now carries a cred struct pointer in its
     subprocess_info struct instead of a new session keyring pointer.  This set
     of credentials is derived from init_cred and installed on the new process
     after it has been cloned.

     call_usermodehelper_setup() allocates the new credentials and
     call_usermodehelper_freeinfo() discards them if they haven't been used.  A
     special cred function (prepare_usermodeinfo_creds()) is provided
     specifically for call_usermodehelper_setup() to call.

     call_usermodehelper_setkeys() adjusts the credentials to sport the
     supplied keyring as the new session keyring.

(11) SELinux.

     SELinux has a number of changes, in addition to those to support the LSM
     interface changes mentioned above:

     (a) selinux_setprocattr() no longer does its check for whether the
     	 current ptracer can access processes with the new SID inside the lock
     	 that covers getting the ptracer's SID.  Whilst this lock ensures that
     	 the check is done with the ptracer pinned, the result is only valid
     	 until the lock is released, so there's no point doing it inside the
     	 lock.

(12) is_single_threaded().

     This function has been extracted from selinux_setprocattr() and put into
     a file of its own in the lib/ directory as join_session_keyring() now
     wants to use it too.

     The code in SELinux just checked to see whether a task shared mm_structs
     with other tasks (CLONE_VM), but that isn't good enough.  We really want
     to know if they're part of the same thread group (CLONE_THREAD).

(13) nfsd.

     The NFS server daemon now has to use the COW credentials to set the
     credentials it is going to use.  It really needs to pass the credentials
     down to the functions it calls, but it can't do that until other patches
     in this series have been applied.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 164 insertions, 169 deletions

diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 70ee93406f30..df329f684a65 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -42,11 +42,15 @@ struct key_user root_key_user = {
  */
 int install_user_keyrings(void)
 {
-	struct user_struct *user = current->cred->user;
+	struct user_struct *user;
+	const struct cred *cred;
 	struct key *uid_keyring, *session_keyring;
 	char buf[20];
 	int ret;
 
+	cred = current_cred();
+	user = cred->user;
+
 	kenter("%p{%u}", user, user->uid);
 
 	if (user->uid_keyring) {
@@ -67,7 +71,7 @@ int install_user_keyrings(void)
 		uid_keyring = find_keyring_by_name(buf, true);
 		if (IS_ERR(uid_keyring)) {
 			uid_keyring = keyring_alloc(buf, user->uid, (gid_t) -1,
-						    current, KEY_ALLOC_IN_QUOTA,
+						    cred, KEY_ALLOC_IN_QUOTA,
 						    NULL);
 			if (IS_ERR(uid_keyring)) {
 				ret = PTR_ERR(uid_keyring);
@@ -83,8 +87,7 @@ int install_user_keyrings(void)
 		if (IS_ERR(session_keyring)) {
 			session_keyring =
 				keyring_alloc(buf, user->uid, (gid_t) -1,
-					      current, KEY_ALLOC_IN_QUOTA,
-					      NULL);
+					      cred, KEY_ALLOC_IN_QUOTA, NULL);
 			if (IS_ERR(session_keyring)) {
 				ret = PTR_ERR(session_keyring);
 				goto error_release;
@@ -116,142 +119,128 @@ error:
 	return ret;
 }
 
-/*****************************************************************************/
 /*
- * deal with the UID changing
+ * install a fresh thread keyring directly to new credentials
  */
-void switch_uid_keyring(struct user_struct *new_user)
+int install_thread_keyring_to_cred(struct cred *new)
 {
-#if 0 /* do nothing for now */
-	struct key *old;
-
-	/* switch to the new user's session keyring if we were running under
-	 * root's default session keyring */
-	if (new_user->uid != 0 &&
-	    current->session_keyring == &root_session_keyring
-	    ) {
-		atomic_inc(&new_user->session_keyring->usage);
-
-		task_lock(current);
-		old = current->session_keyring;
-		current->session_keyring = new_user->session_keyring;
-		task_unlock(current);
+	struct key *keyring;
 
-		key_put(old);
-	}
-#endif
+	keyring = keyring_alloc("_tid", new->uid, new->gid, new,
+				KEY_ALLOC_QUOTA_OVERRUN, NULL);
+	if (IS_ERR(keyring))
+		return PTR_ERR(keyring);
 
-} /* end switch_uid_keyring() */
+	new->thread_keyring = keyring;
+	return 0;
+}
 
-/*****************************************************************************/
 /*
  * install a fresh thread keyring, discarding the old one
  */
-int install_thread_keyring(void)
+static int install_thread_keyring(void)
 {
-	struct task_struct *tsk = current;
-	struct key *keyring, *old;
-	char buf[20];
+	struct cred *new;
 	int ret;
 
-	sprintf(buf, "_tid.%u", tsk->pid);
+	new = prepare_creds();
+	if (!new)
+		return -ENOMEM;
 
-	keyring = keyring_alloc(buf, tsk->cred->uid, tsk->cred->gid, tsk,
-				KEY_ALLOC_QUOTA_OVERRUN, NULL);
-	if (IS_ERR(keyring)) {
-		ret = PTR_ERR(keyring);
-		goto error;
+	BUG_ON(new->thread_keyring);
+
+	ret = install_thread_keyring_to_cred(new);
+	if (ret < 0) {
+		abort_creds(new);
+		return ret;
 	}
 
-	task_lock(tsk);
-	old = tsk->cred->thread_keyring;
-	tsk->cred->thread_keyring = keyring;
-	task_unlock(tsk);
+	return commit_creds(new);
+}
 
-	ret = 0;
+/*
+ * install a process keyring directly to a credentials struct
+ * - returns -EEXIST if there was already a process keyring, 0 if one installed,
+ *   and other -ve on any other error
+ */
+int install_process_keyring_to_cred(struct cred *new)
+{
+	struct key *keyring;
+	int ret;
 
-	key_put(old);
-error:
+	if (new->tgcred->process_keyring)
+		return -EEXIST;
+
+	keyring = keyring_alloc("_pid", new->uid, new->gid,
+				new, KEY_ALLOC_QUOTA_OVERRUN, NULL);
+	if (IS_ERR(keyring))
+		return PTR_ERR(keyring);
+
+	spin_lock_irq(&new->tgcred->lock);
+	if (!new->tgcred->process_keyring) {
+		new->tgcred->process_keyring = keyring;
+		keyring = NULL;
+		ret = 0;
+	} else {
+		ret = -EEXIST;
+	}
+	spin_unlock_irq(&new->tgcred->lock);
+	key_put(keyring);
 	return ret;
+}
 
-} /* end install_thread_keyring() */
-
-/*****************************************************************************/
 /*
  * make sure a process keyring is installed
+ * - we
  */
-int install_process_keyring(void)
+static int install_process_keyring(void)
 {
-	struct task_struct *tsk = current;
-	struct key *keyring;
-	char buf[20];
+	struct cred *new;
 	int ret;
 
-	might_sleep();
-
-	if (!tsk->cred->tgcred->process_keyring) {
-		sprintf(buf, "_pid.%u", tsk->tgid);
-
-		keyring = keyring_alloc(buf, tsk->cred->uid, tsk->cred->gid, tsk,
-					KEY_ALLOC_QUOTA_OVERRUN, NULL);
-		if (IS_ERR(keyring)) {
-			ret = PTR_ERR(keyring);
-			goto error;
-		}
-
-		/* attach keyring */
-		spin_lock_irq(&tsk->cred->tgcred->lock);
-		if (!tsk->cred->tgcred->process_keyring) {
-			tsk->cred->tgcred->process_keyring = keyring;
-			keyring = NULL;
-		}
-		spin_unlock_irq(&tsk->cred->tgcred->lock);
+	new = prepare_creds();
+	if (!new)
+		return -ENOMEM;
 
-		key_put(keyring);
+	ret = install_process_keyring_to_cred(new);
+	if (ret < 0) {
+		abort_creds(new);
+		return ret != -EEXIST ?: 0;
 	}
 
-	ret = 0;
-error:
-	return ret;
-
-} /* end install_process_keyring() */
+	return commit_creds(new);
+}
 
-/*****************************************************************************/
 /*
- * install a session keyring, discarding the old one
- * - if a keyring is not supplied, an empty one is invented
+ * install a session keyring directly to a credentials struct
  */
-static int install_session_keyring(struct key *keyring)
+static int install_session_keyring_to_cred(struct cred *cred,
+					   struct key *keyring)
 {
-	struct task_struct *tsk = current;
 	unsigned long flags;
 	struct key *old;
-	char buf[20];
 
 	might_sleep();
 
 	/* create an empty session keyring */
 	if (!keyring) {
-		sprintf(buf, "_ses.%u", tsk->tgid);
-
 		flags = KEY_ALLOC_QUOTA_OVERRUN;
-		if (tsk->cred->tgcred->session_keyring)
+		if (cred->tgcred->session_keyring)
 			flags = KEY_ALLOC_IN_QUOTA;
 
-		keyring = keyring_alloc(buf, tsk->cred->uid, tsk->cred->gid,
-					tsk, flags, NULL);
+		keyring = keyring_alloc("_ses", cred->uid, cred->gid,
+					cred, flags, NULL);
 		if (IS_ERR(keyring))
 			return PTR_ERR(keyring);
-	}
-	else {
+	} else {
 		atomic_inc(&keyring->usage);
 	}
 
 	/* install the keyring */
-	spin_lock_irq(&tsk->cred->tgcred->lock);
-	old = tsk->cred->tgcred->session_keyring;
-	rcu_assign_pointer(tsk->cred->tgcred->session_keyring, keyring);
-	spin_unlock_irq(&tsk->cred->tgcred->lock);
+	spin_lock_irq(&cred->tgcred->lock);
+	old = cred->tgcred->session_keyring;
+	rcu_assign_pointer(cred->tgcred->session_keyring, keyring);
+	spin_unlock_irq(&cred->tgcred->lock);
 
 	/* we're using RCU on the pointer, but there's no point synchronising
 	 * on it if it didn't previously point to anything */
@@ -261,38 +250,29 @@ static int install_session_keyring(struct key *keyring)
 	}
 
 	return 0;
+}
 
-} /* end install_session_keyring() */
-
-/*****************************************************************************/
 /*
- * copy the keys for fork
+ * install a session keyring, discarding the old one
+ * - if a keyring is not supplied, an empty one is invented
  */
-int copy_keys(unsigned long clone_flags, struct task_struct *tsk)
+static int install_session_keyring(struct key *keyring)
 {
-	key_check(tsk->cred->thread_keyring);
-	key_check(tsk->cred->request_key_auth);
-
-	/* no thread keyring yet */
-	tsk->cred->thread_keyring = NULL;
-
-	/* copy the request_key() authorisation for this thread */
-	key_get(tsk->cred->request_key_auth);
-
-	return 0;
+	struct cred *new;
+	int ret;
 
-} /* end copy_keys() */
+	new = prepare_creds();
+	if (!new)
+		return -ENOMEM;
 
-/*****************************************************************************/
-/*
- * dispose of per-thread keys upon thread exit
- */
-void exit_keys(struct task_struct *tsk)
-{
-	key_put(tsk->cred->thread_keyring);
-	key_put(tsk->cred->request_key_auth);
+	ret = install_session_keyring_to_cred(new, NULL);
+	if (ret < 0) {
+		abort_creds(new);
+		return ret;
+	}
 
-} /* end exit_keys() */
+	return commit_creds(new);
+}
 
 /*****************************************************************************/
 /*
@@ -300,38 +280,41 @@ void exit_keys(struct task_struct *tsk)
  */
 int exec_keys(struct task_struct *tsk)
 {
-	struct key *old;
+	struct thread_group_cred *tgcred = NULL;
+	struct cred *new;
 
-	/* newly exec'd tasks don't get a thread keyring */
-	task_lock(tsk);
-	old = tsk->cred->thread_keyring;
-	tsk->cred->thread_keyring = NULL;
-	task_unlock(tsk);
+#ifdef CONFIG_KEYS
+	tgcred = kmalloc(sizeof(*tgcred), GFP_KERNEL);
+	if (!tgcred)
+		return -ENOMEM;
+#endif
 
-	key_put(old);
+	new = prepare_creds();
+	if (new < 0)
+		return -ENOMEM;
 
-	/* discard the process keyring from a newly exec'd task */
-	spin_lock_irq(&tsk->cred->tgcred->lock);
-	old = tsk->cred->tgcred->process_keyring;
-	tsk->cred->tgcred->process_keyring = NULL;
-	spin_unlock_irq(&tsk->cred->tgcred->lock);
+	/* newly exec'd tasks don't get a thread keyring */
+	key_put(new->thread_keyring);
+	new->thread_keyring = NULL;
 
-	key_put(old);
+	/* create a new per-thread-group creds for all this set of threads to
+	 * share */
+	memcpy(tgcred, new->tgcred, sizeof(struct thread_group_cred));
 
-	return 0;
+	atomic_set(&tgcred->usage, 1);
+	spin_lock_init(&tgcred->lock);
 
-} /* end exec_keys() */
+	/* inherit the session keyring; new process keyring */
+	key_get(tgcred->session_keyring);
+	tgcred->process_keyring = NULL;
 
-/*****************************************************************************/
-/*
- * deal with SUID programs
- * - we might want to make this invent a new session keyring
- */
-int suid_keys(struct task_struct *tsk)
-{
+	release_tgcred(new);
+	new->tgcred = tgcred;
+
+	commit_creds(new);
 	return 0;
 
-} /* end suid_keys() */
+} /* end exec_keys() */
 
 /*****************************************************************************/
 /*
@@ -376,16 +359,13 @@ void key_fsgid_changed(struct task_struct *tsk)
 key_ref_t search_process_keyrings(struct key_type *type,
 				  const void *description,
 				  key_match_func_t match,
-				  struct task_struct *context)
+				  const struct cred *cred)
 {
 	struct request_key_auth *rka;
-	struct cred *cred;
 	key_ref_t key_ref, ret, err;
 
 	might_sleep();
 
-	cred = get_task_cred(context);
-
 	/* we want to return -EAGAIN or -ENOKEY if any of the keyrings were
 	 * searchable, but we failed to find a key or we found a negative key;
 	 * otherwise we want to return a sample error (probably -EACCES) if
@@ -401,7 +381,7 @@ key_ref_t search_process_keyrings(struct key_type *type,
 	if (cred->thread_keyring) {
 		key_ref = keyring_search_aux(
 			make_key_ref(cred->thread_keyring, 1),
-			context, type, description, match);
+			cred, type, description, match);
 		if (!IS_ERR(key_ref))
 			goto found;
 
@@ -422,7 +402,7 @@ key_ref_t search_process_keyrings(struct key_type *type,
 	if (cred->tgcred->process_keyring) {
 		key_ref = keyring_search_aux(
 			make_key_ref(cred->tgcred->process_keyring, 1),
-			context, type, description, match);
+			cred, type, description, match);
 		if (!IS_ERR(key_ref))
 			goto found;
 
@@ -446,7 +426,7 @@ key_ref_t search_process_keyrings(struct key_type *type,
 			make_key_ref(rcu_dereference(
 					     cred->tgcred->session_keyring),
 				     1),
-			context, type, description, match);
+			cred, type, description, match);
 		rcu_read_unlock();
 
 		if (!IS_ERR(key_ref))
@@ -468,7 +448,7 @@ key_ref_t search_process_keyrings(struct key_type *type,
 	else if (cred->user->session_keyring) {
 		key_ref = keyring_search_aux(
 			make_key_ref(cred->user->session_keyring, 1),
-			context, type, description, match);
+			cred, type, description, match);
 		if (!IS_ERR(key_ref))
 			goto found;
 
@@ -490,7 +470,7 @@ key_ref_t search_process_keyrings(struct key_type *type,
 	 * - we don't permit access to request_key auth keys via this method
 	 */
 	if (cred->request_key_auth &&
-	    context == current &&
+	    cred == current_cred() &&
 	    type != &key_type_request_key_auth
 	    ) {
 		/* defend against the auth key being revoked */
@@ -500,7 +480,7 @@ key_ref_t search_process_keyrings(struct key_type *type,
 			rka = cred->request_key_auth->payload.data;
 
 			key_ref = search_process_keyrings(type, description,
-							  match, rka->context);
+							  match, rka->cred);
 
 			up_read(&cred->request_key_auth->sem);
 
@@ -527,7 +507,6 @@ key_ref_t search_process_keyrings(struct key_type *type,
 	key_ref = ret ? ret : err;
 
 found:
-	put_cred(cred);
 	return key_ref;
 
 } /* end search_process_keyrings() */
@@ -552,8 +531,7 @@ key_ref_t lookup_user_key(key_serial_t id, int create, int partial,
 			  key_perm_t perm)
 {
 	struct request_key_auth *rka;
-	struct task_struct *t = current;
-	struct cred *cred;
+	const struct cred *cred;
 	struct key *key;
 	key_ref_t key_ref, skey_ref;
 	int ret;
@@ -608,6 +586,7 @@ try_again:
 				goto error;
 			ret = install_session_keyring(
 				cred->user->session_keyring);
+
 			if (ret < 0)
 				goto error;
 			goto reget_creds;
@@ -693,7 +672,7 @@ try_again:
 		/* check to see if we possess the key */
 		skey_ref = search_process_keyrings(key->type, key,
 						   lookup_user_key_possessed,
-						   current);
+						   cred);
 
 		if (!IS_ERR(skey_ref)) {
 			key_put(key);
@@ -725,7 +704,7 @@ try_again:
 		goto invalid_key;
 
 	/* check the permissions */
-	ret = key_task_permission(key_ref, t, perm);
+	ret = key_task_permission(key_ref, cred, perm);
 	if (ret < 0)
 		goto invalid_key;
 
@@ -755,21 +734,33 @@ reget_creds:
  */
 long join_session_keyring(const char *name)
 {
-	struct task_struct *tsk = current;
-	struct cred *cred = current->cred;
+	const struct cred *old;
+	struct cred *new;
 	struct key *keyring;
-	long ret;
+	long ret, serial;
+
+	/* only permit this if there's a single thread in the thread group -
+	 * this avoids us having to adjust the creds on all threads and risking
+	 * ENOMEM */
+	if (!is_single_threaded(current))
+		return -EMLINK;
+
+	new = prepare_creds();
+	if (!new)
+		return -ENOMEM;
+	old = current_cred();
 
 	/* if no name is provided, install an anonymous keyring */
 	if (!name) {
-		ret = install_session_keyring(NULL);
+		ret = install_session_keyring_to_cred(new, NULL);
 		if (ret < 0)
 			goto error;
 
-		rcu_read_lock();
-		ret = rcu_dereference(cred->tgcred->session_keyring)->serial;
-		rcu_read_unlock();
-		goto error;
+		serial = new->tgcred->session_keyring->serial;
+		ret = commit_creds(new);
+		if (ret == 0)
+			ret = serial;
+		goto okay;
 	}
 
 	/* allow the user to join or create a named keyring */
@@ -779,29 +770,33 @@ long join_session_keyring(const char *name)
 	keyring = find_keyring_by_name(name, false);
 	if (PTR_ERR(keyring) == -ENOKEY) {
 		/* not found - try and create a new one */
-		keyring = keyring_alloc(name, cred->uid, cred->gid, tsk,
+		keyring = keyring_alloc(name, old->uid, old->gid, old,
 					KEY_ALLOC_IN_QUOTA, NULL);
 		if (IS_ERR(keyring)) {
 			ret = PTR_ERR(keyring);
 			goto error2;
 		}
-	}
-	else if (IS_ERR(keyring)) {
+	} else if (IS_ERR(keyring)) {
 		ret = PTR_ERR(keyring);
 		goto error2;
 	}
 
 	/* we've got a keyring - now to install it */
-	ret = install_session_keyring(keyring);
+	ret = install_session_keyring_to_cred(new, keyring);
 	if (ret < 0)
 		goto error2;
 
+	commit_creds(new);
+	mutex_unlock(&key_session_mutex);
+
 	ret = keyring->serial;
 	key_put(keyring);
+okay:
+	return ret;
 
 error2:
 	mutex_unlock(&key_session_mutex);
 error:
+	abort_creds(new);
 	return ret;
-
-} /* end join_session_keyring() */
+}