CRED: Use RCU to access another task's creds and to release a task's own creds

Use RCU to access another task's creds and to release a task's own creds.
This means that it will be possible for the credentials of a task to be
replaced without another task (a) requiring a full lock to read them, and (b)
seeing deallocated memory.

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

2 files changed, 20 insertions, 14 deletions

diff --git a/security/keys/permission.c b/security/keys/permission.c
index baf3d5f31e71..13c36164f284 100644
--- a/security/keys/permission.c
+++ b/security/keys/permission.c
@@ -22,13 +22,16 @@ int key_task_permission(const key_ref_t key_ref,
                         struct task_struct *context,
                         key_perm_t perm)
 {
-        struct cred *cred = context->cred;
+        const struct cred *cred;
         struct key *key;
         key_perm_t kperm;
         int ret;
 
         key = key_ref_to_ptr(key_ref);
 
+        rcu_read_lock();
+        cred = __task_cred(context);
+
         /* use the second 8-bits of permissions for keys the caller owns */
         if (key->uid == cred->fsuid) {
                 kperm = key->perm >> 16;
@@ -43,10 +46,7 @@ int key_task_permission(const key_ref_t key_ref,
                         goto use_these_perms;
                 }
 
-                spin_lock(&cred->lock);
                 ret = groups_search(cred->group_info, key->gid);
-                spin_unlock(&cred->lock);
-
                 if (ret) {
                         kperm = key->perm >> 8;
                         goto use_these_perms;
@@ -57,6 +57,8 @@ int key_task_permission(const key_ref_t key_ref,
         kperm = key->perm;
 
 use_these_perms:
+        rcu_read_lock();
+
         /* use the top 8-bits of permissions for keys the caller possesses
          * - possessor permissions are additive with other permissions
          */
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index ce8ac6073d57..212601ebaa46 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -412,10 +412,13 @@ key_ref_t search_process_keyrings(struct key_type *type,
                                   struct task_struct *context)
 {
         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
@@ -428,9 +431,9 @@ key_ref_t search_process_keyrings(struct key_type *type,
         err = ERR_PTR(-EAGAIN);
 
         /* search the thread keyring first */
-        if (context->cred->thread_keyring) {
+        if (cred->thread_keyring) {
                 key_ref = keyring_search_aux(
-                        make_key_ref(context->cred->thread_keyring, 1),
+                        make_key_ref(cred->thread_keyring, 1),
                         context, type, description, match);
                 if (!IS_ERR(key_ref))
                         goto found;
@@ -495,9 +498,9 @@ key_ref_t search_process_keyrings(struct key_type *type,
                 }
         }
         /* or search the user-session keyring */
-        else if (context->cred->user->session_keyring) {
+        else if (cred->user->session_keyring) {
                 key_ref = keyring_search_aux(
-                        make_key_ref(context->cred->user->session_keyring, 1),
+                        make_key_ref(cred->user->session_keyring, 1),
                         context, type, description, match);
                 if (!IS_ERR(key_ref))
                         goto found;
@@ -519,20 +522,20 @@ key_ref_t search_process_keyrings(struct key_type *type,
          * search the keyrings of the process mentioned there
          * - we don't permit access to request_key auth keys via this method
          */
-        if (context->cred->request_key_auth &&
+        if (cred->request_key_auth &&
             context == current &&
             type != &key_type_request_key_auth
             ) {
                 /* defend against the auth key being revoked */
-                down_read(&context->cred->request_key_auth->sem);
+                down_read(&cred->request_key_auth->sem);
 
-                if (key_validate(context->cred->request_key_auth) == 0) {
-                        rka = context->cred->request_key_auth->payload.data;
+                if (key_validate(cred->request_key_auth) == 0) {
+                        rka = cred->request_key_auth->payload.data;
 
                         key_ref = search_process_keyrings(type, description,
                                                           match, rka->context);
 
-                        up_read(&context->cred->request_key_auth->sem);
+                        up_read(&cred->request_key_auth->sem);
 
                         if (!IS_ERR(key_ref))
                                 goto found;
@@ -549,7 +552,7 @@ key_ref_t search_process_keyrings(struct key_type *type,
                                 break;
                         }
                 } else {
-                        up_read(&context->cred->request_key_auth->sem);
+                        up_read(&cred->request_key_auth->sem);
                 }
         }
 
@@ -557,6 +560,7 @@ 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() */