[Devel] [PATCH 3/3] Signal semantics for pid namespaces

[sukadev at us.ibm.com]

From: Sukadev Bhattiprolu <sukadev at us.ibm.com>
Subject: [RFC][PATCH] Masquerade sender if in ancestor ns

With support for multiple pid namespaces, each pid namespace has a
separate child reaper and this process needs some special handling
of signals. 

	- The child reaper should appear like a normal process to other
	  processes in its ancestor namespaces and so should be killable
	  (or not) in the usual way.

       	- The child reaper should receive, from processes in it's active
          and decendent namespaces, only those signals for which it has
          installed a signal handler.

	- System-wide signals (eg: kill signum -1) from within a child namespace
	  should only affect processes within that namespace and descendant
	  namespaces. They should not be posted to processes in ancestor or
	  sibling namespaces.

	- If the sender of a signal does not have a pid_t in the receiver's
	  namespace (eg: a process in init_pid_ns sends a signal to a process
	  in a descendant namespace), the sender's pid and uid should appear
	  as 0 in the signal's 'siginfo' structure.

	- Existing rules for SIGIO delivery still apply and a process can
	  choose any other process in its namespace and descendant namespaces
	  to receive the SIGIO signal.
	  
	  The following appears to be incorrect in the fcntl() man page for
	  F_SETOWN.

              Sending a signal to  the  owner  process  (group)  specified  by
              F_SETOWN  is  subject  to  the  same  permissions  checks as are
              described for kill(2), where the sending process is the one that
              employs F_SETOWN (but see BUGS below).

	Current behavior is that the SIGIO signal is delivered on behalf of
	the process that caused the event (eg: made data available on the
	file) and not the process that called fcntl().

Changelog:
	- [Oleg Nesterov]: Used the interfaces, is_current_in_ancestor_pid_ns()
	  and is_current_in_same_or_ancestor_pid_ns().
	- [Oleg Nesterov]: Clear info.si_uid also when masquerading sender.

Signed-off-by: Sukadev Bhattiprolu <sukadev at us.ibm.com>
---
 kernel/signal.c |   25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

Index: 2.6.23-rc3-mm1/kernel/signal.c
===================================================================
--- 2.6.23-rc3-mm1.orig/kernel/signal.c	2007-08-31 12:14:33.000000000 -0700
+++ 2.6.23-rc3-mm1/kernel/signal.c	2007-08-31 12:15:36.000000000 -0700
@@ -25,6 +25,7 @@
 #include <linux/capability.h>
 #include <linux/freezer.h>
 #include <linux/pid_namespace.h>
+#include <linux/pid.h>
 #include <linux/nsproxy.h>
 #include <linux/hardirq.h>
 
@@ -48,7 +49,7 @@ static int sig_init_ignore(struct task_s
 	if (likely(!is_container_init(tsk->group_leader)))
 		return 0;
 
-	if (!in_interrupt())
+	if (is_current_in_ancestor_pid_ns(tsk) && !in_interrupt())
 		return 0;
 
 	return 1;
@@ -684,6 +685,20 @@ static void handle_stop_signal(int sig, 
 	}
 }
 
+static void masquerade_sender(struct task_struct *t, struct sigqueue *q)
+{
+	/*
+	 * If the sender does not have a pid_t in the receiver's active
+	 * pid namespace, set si_pid to 0 and pretend signal originated
+	 * from the kernel.
+	 */
+	if (!pid_ns_equal(t)) {
+		q->info.si_pid = 0;
+		q->info.si_uid = 0;
+		q->info.si_code = SI_KERNEL;
+	}
+}
+
 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
 			struct sigpending *signals)
 {
@@ -735,6 +750,7 @@ static int send_signal(int sig, struct s
 			copy_siginfo(&q->info, info);
 			break;
 		}
+		masquerade_sender(t, q);
 	} else if (!is_si_special(info)) {
 		if (sig >= SIGRTMIN && info->si_code != SI_USER)
 		/*
@@ -1168,6 +1184,7 @@ EXPORT_SYMBOL_GPL(kill_pid_info_as_uid);
 static int kill_something_info(int sig, struct siginfo *info, int pid)
 {
 	int ret;
+
 	rcu_read_lock();
 	if (!pid) {
 		ret = kill_pgrp_info(sig, info, task_pgrp(current));
@@ -1177,6 +1194,12 @@ static int kill_something_info(int sig, 
 
 		read_lock(&tasklist_lock);
 		for_each_process(p) {
+			/*
+			 * System-wide signals only apply to pid namespace
+			 * of sender.
+			 */
+			if (!is_current_in_same_or_ancestor_pid_ns(p))
+				continue;
 			if (p->pid > 1 && !same_thread_group(p, current)) {
 				int err = group_send_sig_info(sig, info, p);
 				++count;
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