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, 15 insertions, 8 deletions

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index e97a98981862..95bc22bdd060 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -869,18 +869,25 @@ int fuse_update_attributes(struct inode *inode, struct kstat *stat,
  */
 int fuse_allow_task(struct fuse_conn *fc, struct task_struct *task)
 {
+        const struct cred *cred;
+        int ret;
+
         if (fc->flags & FUSE_ALLOW_OTHER)
                 return 1;
 
-        if (task->cred->euid == fc->user_id &&
-            task->cred->suid == fc->user_id &&
-            task->cred->uid == fc->user_id &&
-            task->cred->egid == fc->group_id &&
-            task->cred->sgid == fc->group_id &&
-            task->cred->gid == fc->group_id)
-                return 1;
+        rcu_read_lock();
+        ret = 0;
+        cred = __task_cred(task);
+        if (cred->euid == fc->user_id &&
+            cred->suid == fc->user_id &&
+            cred->uid  == fc->user_id &&
+            cred->egid == fc->group_id &&
+            cred->sgid == fc->group_id &&
+            cred->gid  == fc->group_id)
+                ret = 1;
+        rcu_read_unlock();
 
-        return 0;
+        return ret;
 }
 
 static int fuse_access(struct inode *inode, int mask)