uglify kill_pid_info() to fix kill() vs exec() race

kill_pid_info()->pid_task() could be the old leader of the execing process.
In that case it is possible that the leader will be released before we take
siglock. This means that kill_pid_info() (and thus sys_kill()) can return a
false -ESRCH.

Change the code to retry when lock_task_sighand() fails. The endless loop is
not possible, __exit_signal() both clears ->sighand and does detach_pid().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 3 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 3d3adb94561..b0b43a4ad8d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1050,17 +1050,26 @@ int kill_pgrp_info(int sig, struct siginfo *info, struct pid *pgrp)
 
 int kill_pid_info(int sig, struct siginfo *info, struct pid *pid)
 {
-        int error;
+        int error = -ESRCH;
         struct task_struct *p;
 
         rcu_read_lock();
         if (unlikely(sig_needs_tasklist(sig)))
                 read_lock(&tasklist_lock);
 
+retry:
         p = pid_task(pid, PIDTYPE_PID);
-        error = -ESRCH;
-        if (p)
+        if (p) {
                 error = group_send_sig_info(sig, info, p);
+                if (unlikely(error == -ESRCH))
+                        /*
+                         * The task was unhashed in between, try again.
+                         * If it is dead, pid_task() will return NULL,
+                         * if we race with de_thread() it will find the
+                         * new leader.
+                         */
+                        goto retry;
+        }
 
         if (unlikely(sig_needs_tasklist(sig)))
                 read_unlock(&tasklist_lock);