signal: avoid shared siginfo namespace rewrites

commit d19cdc167e upstream.

send_signal_locked() rewrites sender ids for the target namespace.  Group
sends reuse the same siginfo, so one recipient can affect the next.

Copy the siginfo before changing it.

Link: https://lore.kernel.org/86a8857d58d43ee26a8b365b837fd24830343494.1782159692.git.include@grrlz.net
Fixes: 7a0cf09494 ("signal: Correct namespace fixups of si_pid and si_uid")
Signed-off-by: Bradley Morgan <include@grrlz.net>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Adrian Huang <adrianhuang0701@gmail.com>
Cc: Aleksandr Nogikh <nogikh@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/signal.c

@@ -1214,6 +1214,7 @@ static inline bool has_si_pid_and_uid(struct kernel_siginfo *info)
 int send_signal_locked(int sig, struct kernel_siginfo *info,
 		       struct task_struct *t, enum pid_type type)
 {
+	struct kernel_siginfo rewritten;
 	/* Should SIGKILL or SIGSTOP be received by a pid namespace init? */
 	bool force = false;
 
@@ -1227,6 +1228,9 @@ int send_signal_locked(int sig, struct kernel_siginfo *info,
 		/* SIGKILL and SIGSTOP is special or has ids */
 		struct user_namespace *t_user_ns;
 
+		rewritten = *info;
+		info = &rewritten;
+
 		rcu_read_lock();
 		t_user_ns = task_cred_xxx(t, user_ns);
 		if (current_user_ns() != t_user_ns) {