[PATCH] ipc/mqueue.c: change __do_notify() to bypass check_kill_permission()

[Oleg Nesterov <oleg@redhat.com>]

Commit cc731525f26a ("signal: Remove kernel interal si_code magic")
changed the value of SI_FROMUSER(SI_MESGQ), this means that mq_notify()
no longer works if the sender doesn't have rights to send a signal.

Change __do_notify() to use do_send_sig_info() instead of kill_pid_info()
to avoid check_kill_permission().

This needs the additional notify.sigev_signo != 0 check, shouldn't we
change do_mq_notify() to deny sigev_signo == 0 ?

Reported-by: Yoji <yoji.fujihar.min@gmail.com>
Fixes: cc731525f26a ("signal: Remove kernel interal si_code magic")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 ipc/mqueue.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 49a05ba3000d..3145fae162c1 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -775,12 +775,15 @@ static void __do_notify(struct mqueue_inode_info *info)
 	if (info->notify_owner &&
 	    info->attr.mq_curmsgs == 1) {
 		struct kernel_siginfo sig_i;
+		struct task_struct *task;
 		switch (info->notify.sigev_notify) {
 		case SIGEV_NONE:
 			break;
 		case SIGEV_SIGNAL:
+			/* do_mq_notify() accepts sigev_signo == 0, why?? */
+			if (!info->notify.sigev_signo)
+				break;
 			/* sends signal */
-
 			clear_siginfo(&sig_i);
 			sig_i.si_signo = info->notify.sigev_signo;
 			sig_i.si_errno = 0;
@@ -790,11 +793,13 @@ static void __do_notify(struct mqueue_inode_info *info)
 			rcu_read_lock();
 			sig_i.si_pid = task_tgid_nr_ns(current,
 						ns_of_pid(info->notify_owner));
-			sig_i.si_uid = from_kuid_munged(info->notify_user_ns, current_uid());
+			sig_i.si_uid = from_kuid_munged(info->notify_user_ns,
+						current_uid());
+			task = pid_task(info->notify_owner, PIDTYPE_PID);
+			if (task)
+				do_send_sig_info(info->notify.sigev_signo,
+						&sig_i, task, PIDTYPE_TGID);
 			rcu_read_unlock();
-
-			kill_pid_info(info->notify.sigev_signo,
-				      &sig_i, info->notify_owner);
 			break;
 		case SIGEV_THREAD:
 			set_cookie(info->notify_cookie, NOTIFY_WOKENUP);
-- 
2.25.1.362.g51ebf55