signal: Correct namespace fixups of si_pid and si_uid

The function send_signal was split from __send_signal so that it would
be possible to bypass the namespace logic based upon current[1].  As it
turns out the si_pid and the si_uid fixup are both inappropriate in
the case of kill_pid_usb_asyncio so move that logic into send_signal.

It is difficult to arrange but possible for a signal with an si_code
of SI_TIMER or SI_SIGIO to be sent across namespace boundaries.  In
which case tests for when it is ok to change si_pid and si_uid based
on SI_FROMUSER are incorrect.  Replace the use of SI_FROMUSER with a
new test has_si_pid_and_used based on siginfo_layout.

Now that the uid fixup is no longer present after expanding
SEND_SIG_NOINFO properly calculate the si_uid that the target
task needs to read.

[1] 7978b567d3 ("signals: add from_ancestor_ns parameter to send_signal()")
Cc: stable@vger.kernel.org
Fixes: 6588c1e3ff ("signals: SI_USER: Masquerade si_pid when crossing pid ns boundary")
Fixes: 6b550f9495 ("user namespace: make signal.c respect user namespaces")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

kernel/signal.c

@@ -1056,27 +1056,6 @@ static inline bool legacy_queue(struct sigpending *signals, int sig)
 	return (sig < SIGRTMIN) && sigismember(&signals->signal, sig);
 }
 
-#ifdef CONFIG_USER_NS
-static inline void userns_fixup_signal_uid(struct kernel_siginfo *info, struct task_struct *t)
-{
-	if (current_user_ns() == task_cred_xxx(t, user_ns))
-		return;
-
-	if (SI_FROMKERNEL(info))
-		return;
-
-	rcu_read_lock();
-	info->si_uid = from_kuid_munged(task_cred_xxx(t, user_ns),
-					make_kuid(current_user_ns(), info->si_uid));
-	rcu_read_unlock();
-}
-#else
-static inline void userns_fixup_signal_uid(struct kernel_siginfo *info, struct task_struct *t)
-{
-	return;
-}
-#endif
-
 static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struct *t,
 			enum pid_type type, int from_ancestor_ns)
 {
@@ -1134,7 +1113,11 @@ static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struc
 			q->info.si_code = SI_USER;
 			q->info.si_pid = task_tgid_nr_ns(current,
 							task_active_pid_ns(t));
-			q->info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
+			rcu_read_lock();
+			q->info.si_uid =
+				from_kuid_munged(task_cred_xxx(t, user_ns),
+						 current_uid());
+			rcu_read_unlock();
 			break;
 		case (unsigned long) SEND_SIG_PRIV:
 			clear_siginfo(&q->info);
@@ -1146,13 +1129,8 @@ static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struc
 			break;
 		default:
 			copy_siginfo(&q->info, info);
-			if (from_ancestor_ns)
-				q->info.si_pid = 0;
 			break;
 		}
-
-		userns_fixup_signal_uid(&q->info, t);
-
 	} else if (!is_si_special(info)) {
 		if (sig >= SIGRTMIN && info->si_code != SI_USER) {
 			/*
@@ -1196,6 +1174,28 @@ static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struc
 	return ret;
 }
 
+static inline bool has_si_pid_and_uid(struct kernel_siginfo *info)
+{
+	bool ret = false;
+	switch (siginfo_layout(info->si_signo, info->si_code)) {
+	case SIL_KILL:
+	case SIL_CHLD:
+	case SIL_RT:
+		ret = true;
+		break;
+	case SIL_TIMER:
+	case SIL_POLL:
+	case SIL_FAULT:
+	case SIL_FAULT_MCEERR:
+	case SIL_FAULT_BNDERR:
+	case SIL_FAULT_PKUERR:
+	case SIL_SYS:
+		ret = false;
+		break;
+	}
+	return ret;
+}
+
 static int send_signal(int sig, struct kernel_siginfo *info, struct task_struct *t,
 			enum pid_type type)
 {
@@ -1205,7 +1205,20 @@ static int send_signal(int sig, struct kernel_siginfo *info, struct task_struct
 	from_ancestor_ns = si_fromuser(info) &&
 			   !task_pid_nr_ns(current, task_active_pid_ns(t));
 #endif
+	if (!is_si_special(info) && has_si_pid_and_uid(info)) {
+		struct user_namespace *t_user_ns;
 
+		rcu_read_lock();
+		t_user_ns = task_cred_xxx(t, user_ns);
+		if (current_user_ns() != t_user_ns) {
+			kuid_t uid = make_kuid(current_user_ns(), info->si_uid);
+			info->si_uid = from_kuid_munged(t_user_ns, uid);
+		}
+		rcu_read_unlock();
+
+		if (!task_pid_nr_ns(current, task_active_pid_ns(t)))
+			info->si_pid = 0;
+	}
 	return __send_signal(sig, info, t, type, from_ancestor_ns);
 }