signal: protect SIGNAL_UNKILLABLE from unintentional clearing.

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.iles@oracle.com
Signed-off-by: Jamie Iles <jamie.iles@oracle.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4d1905245c7aa50df56acf0f77c77f3347c28c04..ad3ec9ec61f7b6de743b5d6de4225defcc18be99 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -854,6 +854,16 @@ 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)
 {

diff --git a/kernel/signal.c b/kernel/signal.c
index ff046b73ff2d309ce00fbd3bd8949a8f6f5fd297..3603d93a19689be7188a004f2b999b27e0ebdf2f 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -346,7 +346,7 @@ static bool task_participate_group_stop(struct task_struct *task)
         * 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;
@@ -843,7 +843,7 @@ static bool prepare_signal(int sig, struct task_struct *p, bool force)
                         * 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;
                }