signal: Fix racy access to __task_cred in kill_pid_info_as_uid()

kill_pid_info_as_uid() accesses __task_cred() without being in a RCU
read side critical section. tasklist_lock is not protecting that when
CONFIG_TREE_PREEMPT_RCU=y.

Convert the whole tasklist_lock section to rcu and use
lock_task_sighand to prevent the exit race.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <20091210004703.232302055@linutronix.de>
Acked-by: Oleg Nesterov <oleg@redhat.com>

diff --git a/kernel/signal.c b/kernel/signal.c
index 6b982f2..7331656 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1175,11 +1175,12 @@ int kill_pid_info_as_uid(int sig, struct siginfo *info, struct pid *pid,
        int ret = -EINVAL;
        struct task_struct *p;
        const struct cred *pcred;
+       unsigned long flags;
 
        if (!valid_signal(sig))
                return ret;
 
-       read_lock(&tasklist_lock);
+       rcu_read_lock();
        p = pid_task(pid, PIDTYPE_PID);
        if (!p) {
                ret = -ESRCH;
@@ -1196,14 +1197,16 @@ int kill_pid_info_as_uid(int sig, struct siginfo *info, struct pid *pid,
        ret = security_task_kill(p, info, sig, secid);
        if (ret)
                goto out_unlock;
-       if (sig && p->sighand) {
-               unsigned long flags;
-               spin_lock_irqsave(&p->sighand->siglock, flags);
-               ret = __send_signal(sig, info, p, 1, 0);
-               spin_unlock_irqrestore(&p->sighand->siglock, flags);
+
+       if (sig) {
+               if (lock_task_sighand(p, &flags)) {
+                       ret = __send_signal(sig, info, p, 1, 0);
+                       unlock_task_sighand(p, &flags);
+               } else
+                       ret = -ESRCH;
        }
 out_unlock:
-       read_unlock(&tasklist_lock);
+       rcu_read_unlock();
        return ret;
 }
 EXPORT_SYMBOL_GPL(kill_pid_info_as_uid);