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, 21 insertions, 10 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 733c59e645a..92992e287b1 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -345,7 +345,9 @@ static inline struct task_group *task_group(struct task_struct *p)
         struct task_group *tg;
 
 #ifdef CONFIG_USER_SCHED
-        tg = p->cred->user->tg;
+        rcu_read_lock();
+        tg = __task_cred(p)->user->tg;
+        rcu_read_unlock();
 #elif defined(CONFIG_CGROUP_SCHED)
         tg = container_of(task_subsys_state(p, cpu_cgroup_subsys_id),
                                 struct task_group, css);
@@ -5121,6 +5123,22 @@ __setscheduler(struct rq *rq, struct task_struct *p, int policy, int prio)
         set_load_weight(p);
 }
 
+/*
+ * check the target process has a UID that matches the current process's
+ */
+static bool check_same_owner(struct task_struct *p)
+{
+        const struct cred *cred = current_cred(), *pcred;
+        bool match;
+
+        rcu_read_lock();
+        pcred = __task_cred(p);
+        match = (cred->euid == pcred->euid ||
+                 cred->euid == pcred->uid);
+        rcu_read_unlock();
+        return match;
+}
+
 static int __sched_setscheduler(struct task_struct *p, int policy,
                                 struct sched_param *param, bool user)
 {
@@ -5128,7 +5146,6 @@ static int __sched_setscheduler(struct task_struct *p, int policy,
         unsigned long flags;
         const struct sched_class *prev_class = p->sched_class;
         struct rq *rq;
-        uid_t euid;
 
         /* may grab non-irq protected spin_locks */
         BUG_ON(in_interrupt());
@@ -5181,9 +5198,7 @@ recheck:
                         return -EPERM;
 
                 /* can't change other user's priorities */
-                euid = current_euid();
-                if (euid != p->cred->euid &&
-                    euid != p->cred->uid)
+                if (!check_same_owner(p))
                         return -EPERM;
         }
 
@@ -5394,7 +5409,6 @@ long sched_setaffinity(pid_t pid, const cpumask_t *in_mask)
         cpumask_t cpus_allowed;
         cpumask_t new_mask = *in_mask;
         struct task_struct *p;
-        uid_t euid;
         int retval;
 
         get_online_cpus();
@@ -5415,11 +5429,8 @@ long sched_setaffinity(pid_t pid, const cpumask_t *in_mask)
         get_task_struct(p);
         read_unlock(&tasklist_lock);
 
-        euid = current_euid();
         retval = -EPERM;
-        if (euid != p->cred->euid &&
-            euid != p->cred->uid &&
-            !capable(CAP_SYS_NICE))
+        if (!check_same_owner(p) && !capable(CAP_SYS_NICE))
                 goto out_unlock;
 
         retval = security_task_setscheduler(p, 0, NULL);