signals: protect cinit from unblocked SIG_DFL signals

Drop early any SIG_DFL or SIG_IGN signals to container-init from within
the same container.  But queue SIGSTOP and SIGKILL to the container-init
if they are from an ancestor container.

Blocked, fatal signals (i.e when SIG_DFL is to terminate) from within the
container can still terminate the container-init.  That will be addressed
in the next patch.

Note:	To be bisect-safe, SIGNAL_UNKILLABLE will be set for container-inits
   	in a follow-on patch. Until then, this patch is just a preparatory
	step.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 9 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 7b6de962a1af..fb19aae2363b 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -55,20 +55,21 @@ static int sig_handler_ignored(void __user *handler, int sig)
                 (handler == SIG_DFL && sig_kernel_ignore(sig));
 }
 
-static int sig_task_ignored(struct task_struct *t, int sig)
+static int sig_task_ignored(struct task_struct *t, int sig,
+                int from_ancestor_ns)
 {
         void __user *handler;
 
         handler = sig_handler(t, sig);
 
         if (unlikely(t->signal->flags & SIGNAL_UNKILLABLE) &&
-                        handler == SIG_DFL)
+                        handler == SIG_DFL && !from_ancestor_ns)
                 return 1;
 
         return sig_handler_ignored(handler, sig);
 }
 
-static int sig_ignored(struct task_struct *t, int sig)
+static int sig_ignored(struct task_struct *t, int sig, int from_ancestor_ns)
 {
         /*
          * Blocked signals are never ignored, since the
@@ -78,7 +79,7 @@ static int sig_ignored(struct task_struct *t, int sig)
         if (sigismember(&t->blocked, sig) || sigismember(&t->real_blocked, sig))
                 return 0;
 
-        if (!sig_task_ignored(t, sig))
+        if (!sig_task_ignored(t, sig, from_ancestor_ns))
                 return 0;
 
         /*
@@ -634,7 +635,7 @@ static int check_kill_permission(int sig, struct siginfo *info,
  * Returns true if the signal should be actually delivered, otherwise
  * it should be dropped.
  */
-static int prepare_signal(int sig, struct task_struct *p)
+static int prepare_signal(int sig, struct task_struct *p, int from_ancestor_ns)
 {
         struct signal_struct *signal = p->signal;
         struct task_struct *t;
@@ -718,7 +719,7 @@ static int prepare_signal(int sig, struct task_struct *p)
                 }
         }
 
-        return !sig_ignored(p, sig);
+        return !sig_ignored(p, sig, from_ancestor_ns);
 }
 
 /*
@@ -832,7 +833,8 @@ static int __send_signal(int sig, struct siginfo *info, struct task_struct *t,
         trace_sched_signal_send(sig, t);
 
         assert_spin_locked(&t->sighand->siglock);
-        if (!prepare_signal(sig, t))
+
+        if (!prepare_signal(sig, t, from_ancestor_ns))
                 return 0;
 
         pending = group ? &t->signal->shared_pending : &t->pending;
@@ -902,7 +904,15 @@ out_set:
 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
                         int group)
 {
-        return __send_signal(sig, info, t, group, 0);
+        int from_ancestor_ns = 0;
+
+#ifdef CONFIG_PID_NS
+        if (!is_si_special(info) && SI_FROMUSER(info) &&
+                        task_pid_nr_ns(current, task_active_pid_ns(t)) <= 0)
+                from_ancestor_ns = 1;
+#endif
+
+        return __send_signal(sig, info, t, group, from_ancestor_ns);
 }
 
 int print_fatal_signals;
@@ -1336,7 +1346,7 @@ int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group)
                 goto ret;
 
         ret = 1; /* the signal is ignored */
-        if (!prepare_signal(sig, t))
+        if (!prepare_signal(sig, t, 0))
                 goto out;
 
         ret = 0;