KEYS: Alter use of key instantiation link-to-keyring argument

Alter the use of the key instantiation and negation functions' link-to-keyring
arguments.  Currently this specifies a keyring in the target process to link
the key into, creating the keyring if it doesn't exist.  This, however, can be
a problem for copy-on-write credentials as it means that the instantiating
process can alter the credentials of the requesting process.

This patch alters the behaviour such that:

 (1) If keyctl_instantiate_key() or keyctl_negate_key() are given a specific
     keyring by ID (ringid >= 0), then that keyring will be used.

 (2) If keyctl_instantiate_key() or keyctl_negate_key() are given one of the
     special constants that refer to the requesting process's keyrings
     (KEY_SPEC_*_KEYRING, all <= 0), then:

     (a) If sys_request_key() was given a keyring to use (destringid) then the
     	 key will be attached to that keyring.

     (b) If sys_request_key() was given a NULL keyring, then the key being
     	 instantiated will be attached to the default keyring as set by
     	 keyctl_set_reqkey_keyring().

 (3) No extra link will be made.

Decision point (1) follows current behaviour, and allows those instantiators
who've searched for a specifically named keyring in the requestor's keyring so
as to partition the keys by type to still have their named keyrings.

Decision point (2) allows the requestor to make sure that the key or keys that
get produced by request_key() go where they want, whilst allowing the
instantiator to request that the key is retained.  This is mainly useful for
situations where the instantiator makes a secondary request, the key for which
should be retained by the initial requestor:

	+-----------+        +--------------+        +--------------+
	|           |        |              |        |              |
	| Requestor |------->| Instantiator |------->| Instantiator |
	|           |        |              |        |              |
	+-----------+        +--------------+        +--------------+
	           request_key()           request_key()

This might be useful, for example, in Kerberos, where the requestor requests a
ticket, and then the ticket instantiator requests the TGT, which someone else
then has to go and fetch.  The TGT, however, should be retained in the
keyrings of the requestor, not the first instantiator.  To make this explict
an extra special keyring constant is also added.

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

1 files changed, 51 insertions, 24 deletions

diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 91953c814497..8e9d93b4a402 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -76,6 +76,10 @@ static int call_sbin_request_key(struct key_construction *cons,
 
         kenter("{%d},{%d},%s", key->serial, authkey->serial, op);
 
+        ret = install_user_keyrings();
+        if (ret < 0)
+                goto error_alloc;
+
         /* allocate a new session keyring */
         sprintf(desc, "_req.%u", key->serial);
 
@@ -165,7 +169,8 @@ error_alloc:
  * - we ignore program failure and go on key status instead
  */
 static int construct_key(struct key *key, const void *callout_info,
-                         size_t callout_len, void *aux)
+                         size_t callout_len, void *aux,
+                         struct key *dest_keyring)
 {
         struct key_construction *cons;
         request_key_actor_t actor;
@@ -179,7 +184,8 @@ static int construct_key(struct key *key, const void *callout_info,
                 return -ENOMEM;
 
         /* allocate an authorisation key */
-        authkey = request_key_auth_new(key, callout_info, callout_len);
+        authkey = request_key_auth_new(key, callout_info, callout_len,
+                                       dest_keyring);
         if (IS_ERR(authkey)) {
                 kfree(cons);
                 ret = PTR_ERR(authkey);
@@ -207,27 +213,48 @@ static int construct_key(struct key *key, const void *callout_info,
 }
 
 /*
- * link a key to the appropriate destination keyring
- * - the caller must hold a write lock on the destination keyring
+ * get the appropriate destination keyring for the request
+ * - we return whatever keyring we select with an extra reference upon it which
+ *   the caller must release
  */
-static void construct_key_make_link(struct key *key, struct key *dest_keyring)
+static void construct_get_dest_keyring(struct key **_dest_keyring)
 {
+        struct request_key_auth *rka;
         struct task_struct *tsk = current;
-        struct key *drop = NULL;
+        struct key *dest_keyring = *_dest_keyring, *authkey;
 
-        kenter("{%d},%p", key->serial, dest_keyring);
+        kenter("%p", dest_keyring);
 
         /* find the appropriate keyring */
-        if (!dest_keyring) {
+        if (dest_keyring) {
+                /* the caller supplied one */
+                key_get(dest_keyring);
+        } else {
+                /* use a default keyring; falling through the cases until we
+                 * find one that we actually have */
                 switch (tsk->jit_keyring) {
                 case KEY_REQKEY_DEFL_DEFAULT:
+                case KEY_REQKEY_DEFL_REQUESTOR_KEYRING:
+                        if (tsk->request_key_auth) {
+                                authkey = tsk->request_key_auth;
+                                down_read(&authkey->sem);
+                                rka = authkey->payload.data;
+                                if (!test_bit(KEY_FLAG_REVOKED,
+                                              &authkey->flags))
+                                        dest_keyring =
+                                                key_get(rka->dest_keyring);
+                                up_read(&authkey->sem);
+                                if (dest_keyring)
+                                        break;
+                        }
+
                 case KEY_REQKEY_DEFL_THREAD_KEYRING:
-                        dest_keyring = tsk->thread_keyring;
+                        dest_keyring = key_get(tsk->thread_keyring);
                         if (dest_keyring)
                                 break;
 
                 case KEY_REQKEY_DEFL_PROCESS_KEYRING:
-                        dest_keyring = tsk->signal->process_keyring;
+                        dest_keyring = key_get(tsk->signal->process_keyring);
                         if (dest_keyring)
                                 break;
 
@@ -236,17 +263,16 @@ static void construct_key_make_link(struct key *key, struct key *dest_keyring)
                         dest_keyring = key_get(
                                 rcu_dereference(tsk->signal->session_keyring));
                         rcu_read_unlock();
-                        drop = dest_keyring;
 
                         if (dest_keyring)
                                 break;
 
                 case KEY_REQKEY_DEFL_USER_SESSION_KEYRING:
-                        dest_keyring = tsk->user->session_keyring;
+                        dest_keyring = key_get(tsk->user->session_keyring);
                         break;
 
                 case KEY_REQKEY_DEFL_USER_KEYRING:
-                        dest_keyring = tsk->user->uid_keyring;
+                        dest_keyring = key_get(tsk->user->uid_keyring);
                         break;
 
                 case KEY_REQKEY_DEFL_GROUP_KEYRING:
@@ -255,10 +281,9 @@ static void construct_key_make_link(struct key *key, struct key *dest_keyring)
                 }
         }
 
-        /* and attach the key to it */
-        __key_link(dest_keyring, key);
-        key_put(drop);
-        kleave("");
+        *_dest_keyring = dest_keyring;
+        kleave(" [dk %d]", key_serial(dest_keyring));
+        return;
 }
 
 /*
@@ -288,8 +313,7 @@ static int construct_alloc_key(struct key_type *type,
 
         set_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags);
 
-        if (dest_keyring)
-                down_write(&dest_keyring->sem);
+        down_write(&dest_keyring->sem);
 
         /* attach the key to the destination keyring under lock, but we do need
          * to do another check just in case someone beat us to it whilst we
@@ -301,12 +325,10 @@ static int construct_alloc_key(struct key_type *type,
         if (!IS_ERR(key_ref))
                 goto key_already_present;
 
-        if (dest_keyring)
-                construct_key_make_link(key, dest_keyring);
+        __key_link(dest_keyring, key);
 
         mutex_unlock(&key_construction_mutex);
-        if (dest_keyring)
-                up_write(&dest_keyring->sem);
+        up_write(&dest_keyring->sem);
         mutex_unlock(&user->cons_lock);
         *_key = key;
         kleave(" = 0 [%d]", key_serial(key));
@@ -348,21 +370,26 @@ static struct key *construct_key_and_link(struct key_type *type,
         if (!user)
                 return ERR_PTR(-ENOMEM);
 
+        construct_get_dest_keyring(&dest_keyring);
+
         ret = construct_alloc_key(type, description, dest_keyring, flags, user,
                                   &key);
         key_user_put(user);
 
         if (ret == 0) {
-                ret = construct_key(key, callout_info, callout_len, aux);
+                ret = construct_key(key, callout_info, callout_len, aux,
+                                    dest_keyring);
                 if (ret < 0)
                         goto construction_failed;
         }
 
+        key_put(dest_keyring);
         return key;
 
 construction_failed:
         key_negate_and_link(key, key_negative_timeout, NULL, NULL);
         key_put(key);
+        key_put(dest_keyring);
         return ERR_PTR(ret);
 }