signal: protect SIGNAL_UNKILLABLE from unintentional clearing.

Since commit 00cd5c3 ("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/[email protected] Signed-off-by: Jamie Iles <[email protected]> Acked-by: Oleg Nesterov <[email protected]> Cc: Alexander Viro <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Peter Zijlstra <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
cail · Jan 11, 2017 · 2d39b3c · 2d39b3c
1 parent 20f664a
commit 2d39b3c
Show file tree

Hide file tree

Showing 2 changed files with 12 additions and 2 deletions.
diff --git 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
@@ -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;
 		}