[PATCH V2] 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 ?

Test-case:

	#include <signal.h>
	#include <mqueue.h>
	#include <unistd.h>
	#include <sys/wait.h>
	#include <assert.h>

	static int notified;

	static void sigh(int sig)
	{
		notified = 1;
	}

	int main(void)
	{
		signal(SIGIO, sigh);

		int fd = mq_open("/mq", O_RDWR|O_CREAT, 0666, NULL);
		assert(fd >= 0);

		struct sigevent se = {
			.sigev_notify	= SIGEV_SIGNAL,
			.sigev_signo	= SIGIO,
		};
		assert(mq_notify(fd, &se) == 0);

		if (!fork()) {
			assert(setuid(1) == 0);
			mq_send(fd, "",1,0);
			return 0;
		}

		wait(NULL);
		mq_unlink("/mq");
		assert(notified);
		return 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 | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 49a05ba3000d..63b164932ffd 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -774,28 +774,40 @@ static void __do_notify(struct mqueue_inode_info *info)
 	 * synchronously. */
 	if (info->notify_owner &&
 	    info->attr.mq_curmsgs == 1) {
-		struct kernel_siginfo sig_i;
 		switch (info->notify.sigev_notify) {
 		case SIGEV_NONE:
 			break;
-		case SIGEV_SIGNAL:
-			/* sends signal */
+		case SIGEV_SIGNAL: {
+			struct kernel_siginfo sig_i;
+			struct task_struct *task;
+
+			/* do_mq_notify() accepts sigev_signo == 0, why?? */
+			if (!info->notify.sigev_signo)
+				break;
 
 			clear_siginfo(&sig_i);
 			sig_i.si_signo = info->notify.sigev_signo;
 			sig_i.si_errno = 0;
 			sig_i.si_code = SI_MESGQ;
 			sig_i.si_value = info->notify.sigev_value;
-			/* map current pid/uid into info->owner's namespaces */
 			rcu_read_lock();
+			/* map current pid/uid into info->owner's namespaces */
 			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());
+			/*
+			 * We can't use kill_pid_info(), this signal should
+			 * bypass check_kill_permission(). It is from kernel
+			 * but si_fromuser() can't know this.
+			 */
+			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);
 			netlink_sendskb(info->notify_sock, info->notify_cookie);
-- 
2.25.1.362.g51ebf55