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, 5 insertions, 3 deletions

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b23492ee3e50..7555219c535b 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1110,7 +1110,7 @@ asmlinkage long sys_migrate_pages(pid_t pid, unsigned long maxnode,
                 const unsigned long __user *old_nodes,
                 const unsigned long __user *new_nodes)
 {
-        struct cred *cred, *tcred;
+        const struct cred *cred = current_cred(), *tcred;
         struct mm_struct *mm;
         struct task_struct *task;
         nodemask_t old;
@@ -1145,14 +1145,16 @@ asmlinkage long sys_migrate_pages(pid_t pid, unsigned long maxnode,
          * capabilities, superuser privileges or the same
          * userid as the target process.
          */
-        cred = current->cred;
-        tcred = task->cred;
+        rcu_read_lock();
+        tcred = __task_cred(task);
         if (cred->euid != tcred->suid && cred->euid != tcred->uid &&
             cred->uid  != tcred->suid && cred->uid  != tcred->uid &&
             !capable(CAP_SYS_NICE)) {
+                rcu_read_unlock();
                 err = -EPERM;
                 goto out;
         }
+        rcu_read_unlock();
 
         task_nodes = cpuset_mems_allowed(task);
         /* Is the user allowed to access the target nodes? */