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>

1 files changed, 10 insertions, 7 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 6b982f2cf524..73316568a69c 100644
--- 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);