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, 20 insertions, 12 deletions

diff --git a/arch/ia64/kernel/perfmon.c b/arch/ia64/kernel/perfmon.c
index dd38db46a77a..0e499757309b 100644
--- a/arch/ia64/kernel/perfmon.c
+++ b/arch/ia64/kernel/perfmon.c
@@ -2399,25 +2399,33 @@ error_kmem:
 static int
 pfm_bad_permissions(struct task_struct *task)
 {
+        const struct cred *tcred;
         uid_t uid = current_uid();
         gid_t gid = current_gid();
+        int ret;
+
+        rcu_read_lock();
+        tcred = __task_cred(task);
 
         /* inspired by ptrace_attach() */
         DPRINT(("cur: uid=%d gid=%d task: euid=%d suid=%d uid=%d egid=%d sgid=%d\n",
                 uid,
                 gid,
-                task->euid,
-                task->suid,
-                task->uid,
-                task->egid,
-                task->sgid));
-
-        return (uid != task->euid)
-            || (uid != task->suid)
-            || (uid != task->uid)
-            || (gid != task->egid)
-            || (gid != task->sgid)
-            || (gid != task->gid)) && !capable(CAP_SYS_PTRACE);
+                tcred->euid,
+                tcred->suid,
+                tcred->uid,
+                tcred->egid,
+                tcred->sgid));
+
+        ret = ((uid != tcred->euid)
+               || (uid != tcred->suid)
+               || (uid != tcred->uid)
+               || (gid != tcred->egid)
+               || (gid != tcred->sgid)
+               || (gid != tcred->gid)) && !capable(CAP_SYS_PTRACE);
+
+        rcu_read_unlock();
+        return ret;
 }
 
 static int