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Commit 2d39b3cd authored by Jamie Iles's avatar Jamie Iles Committed by Linus Torvalds
Browse files

signal: protect SIGNAL_UNKILLABLE from unintentional clearing. 

Since commit 00cd5c37 ("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: default avatarJamie Iles <jamie.iles@oracle.com>
Acked-by: default avatarOleg 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 20f664aa

include/linux/sched.h

+10 −0
Original line number Diff line number Diff line
@@ -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)

{

kernel/signal.c

+2 −2
Original line number Diff line number Diff line
@@ -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;

		}