4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jamie Iles <jamie.i...@oracle.com>


[ Upstream commit 2d39b3cd34e6d323720d4c61bd714f5ae202c022 ]

Since commit 00cd5c37afd5 ("ptrace: permit ptracing of /sbin/init") we
can now trace init processes.  init is initially protected with
SIGNAL_UNKILLABLE which will prevent fatal signals such as SIGSTOP, but
there are a number of paths during tracing where SIGNAL_UNKILLABLE can
be implicitly cleared.

This can result in init becoming stoppable/killable after tracing.  For
example, running:

  while true; do kill -STOP 1; done &
  strace -p 1

and then stopping strace and the kill loop will result in init being
left in state TASK_STOPPED.  Sending SIGCONT to init will resume it, but
init will now respond to future SIGSTOP signals rather than ignoring
them.

Make sure that when setting SIGNAL_STOP_CONTINUED/SIGNAL_STOP_STOPPED
that we don't clear SIGNAL_UNKILLABLE.

Link: http://lkml.kernel.org/r/20170104122017.25047-1-jamie.i...@oracle.com
Signed-off-by: Jamie Iles <jamie.i...@oracle.com>
Acked-by: Oleg Nesterov <o...@redhat.com>
Cc: Alexander Viro <v...@zeniv.linux.org.uk>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Andrew Morton <a...@linux-foundation.org>
Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.le...@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 include/linux/sched.h |   10 ++++++++++
 kernel/signal.c       |    4 ++--
 2 files changed, 12 insertions(+), 2 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -801,6 +801,16 @@ struct signal_struct {
 
 #define SIGNAL_UNKILLABLE      0x00000040 /* for init: ignore fatal signals */
 
+#define SIGNAL_STOP_MASK (SIGNAL_CLD_MASK | SIGNAL_STOP_STOPPED | \
+                         SIGNAL_STOP_CONTINUED)
+
+static inline void signal_set_stop_flags(struct signal_struct *sig,
+                                        unsigned int flags)
+{
+       WARN_ON(sig->flags & (SIGNAL_GROUP_EXIT|SIGNAL_GROUP_COREDUMP));
+       sig->flags = (sig->flags & ~SIGNAL_STOP_MASK) | flags;
+}
+
 /* If true, all threads except ->group_exit_task have pending SIGKILL */
 static inline int signal_group_exit(const struct signal_struct *sig)
 {
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -346,7 +346,7 @@ static bool task_participate_group_stop(
         * fresh group stop.  Read comment in do_signal_stop() for details.
         */
        if (!sig->group_stop_count && !(sig->flags & SIGNAL_STOP_STOPPED)) {
-               sig->flags = SIGNAL_STOP_STOPPED;
+               signal_set_stop_flags(sig, SIGNAL_STOP_STOPPED);
                return true;
        }
        return false;
@@ -845,7 +845,7 @@ static bool prepare_signal(int sig, stru
                         * will take ->siglock, notice SIGNAL_CLD_MASK, and
                         * notify its parent. See get_signal_to_deliver().
                         */
-                       signal->flags = why | SIGNAL_STOP_CONTINUED;
+                       signal_set_stop_flags(signal, why | 
SIGNAL_STOP_CONTINUED);
                        signal->group_stop_count = 0;
                        signal->group_exit_code = 0;
                }


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