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, 7 insertions, 4 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index c4d6b59553e9..ccc9eb736d35 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -112,14 +112,17 @@ EXPORT_SYMBOL(cad_pid);
 
 void (*pm_power_off_prepare)(void);
 
+/*
+ * set the priority of a task
+ * - the caller must hold the RCU read lock
+ */
 static int set_one_prio(struct task_struct *p, int niceval, int error)
 {
-        uid_t euid = current_euid();
+        const struct cred *cred = current_cred(), *pcred = __task_cred(p);
         int no_nice;
 
-        if (p->cred->uid  != euid &&
-            p->cred->euid != euid &&
-            !capable(CAP_SYS_NICE)) {
+        if (pcred->uid  != cred->euid &&
+            pcred->euid != cred->euid && !capable(CAP_SYS_NICE)) {
                 error = -EPERM;
                 goto out;
         }