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>

1 files changed, 6 insertions, 4 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
          */