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, 10 insertions, 4 deletions

diff --git a/fs/ioprio.c b/fs/ioprio.c
index 5112554fd21..3569e0ad86a 100644
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -31,10 +31,16 @@ static int set_task_ioprio(struct task_struct *task, int ioprio)
 {
         int err;
         struct io_context *ioc;
+        const struct cred *cred = current_cred(), *tcred;
 
-        if (task->cred->uid != current_euid() &&
-            task->cred->uid != current_uid() && !capable(CAP_SYS_NICE))
+        rcu_read_lock();
+        tcred = __task_cred(task);
+        if (tcred->uid != cred->euid &&
+            tcred->uid != cred->uid && !capable(CAP_SYS_NICE)) {
+                rcu_read_unlock();
                 return -EPERM;
+        }
+        rcu_read_unlock();
 
         err = security_task_setioprio(task, ioprio);
         if (err)
@@ -131,7 +137,7 @@ asmlinkage long sys_ioprio_set(int which, int who, int ioprio)
                                 break;
 
                         do_each_thread(g, p) {
-                                if (p->cred->uid != who)
+                                if (__task_cred(p)->uid != who)
                                         continue;
                                 ret = set_task_ioprio(p, ioprio);
                                 if (ret)
@@ -224,7 +230,7 @@ asmlinkage long sys_ioprio_get(int which, int who)
                                 break;
 
                         do_each_thread(g, p) {
-                                if (p->cred->uid != user->uid)
+                                if (__task_cred(p)->uid != user->uid)
                                         continue;
                                 tmpio = get_task_ioprio(p);
                                 if (tmpio < 0)