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Commit fae5fa44 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
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signals: fix /sbin/init protection from unwanted signals 



The global init has a lot of long standing problems with the unhandled fatal
signals.

	- The "is_global_init(current)" check in get_signal_to_deliver()
	  protects only the main thread. Sub-thread can dequee the fatal
	  signal and shutdown the whole thread group except the main thread.
	  If it dequeues SIGSTOP /sbin/init will be stopped, this is not
	  right too. Note that we can't use is_global_init(->group_leader),
	  this breaks exec and this can't solve other problems we have.

	- Even if afterwards ignored, the fatal signals sets SIGNAL_GROUP_EXIT
	  on delivery. This breaks exec, has other bad implications, and this
	  is just wrong.

Introduce the new SIGNAL_UNKILLABLE flag to fix these problems.  It also helps
to solve some other problems addressed by the subsequent patches.

Currently we use this flag for the global init only, but it could also be used
by kthreads and (perhaps) by the sub-namespace inits.

Signed-off-by: default avatarOleg Nesterov <oleg@tv-sign.ru>
Acked-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 19319103

include/linux/sched.h

+2 −0
Original line number Diff line number Diff line
@@ -561,6 +561,8 @@ struct signal_struct { 
#define SIGNAL_CLD_CONTINUED	0x00000020

#define SIGNAL_CLD_MASK		(SIGNAL_CLD_STOPPED|SIGNAL_CLD_CONTINUED)



#define SIGNAL_UNKILLABLE	0x00000040 /* for init: ignore fatal signals */



/* If true, all threads except ->group_exit_task have pending SIGKILL */

static inline int signal_group_exit(const struct signal_struct *sig)

{

init/main.c

+2 −0
Original line number Diff line number Diff line
@@ -802,6 +802,8 @@ static int noinline init_post(void) 
	(void) sys_dup(0);

	(void) sys_dup(0);



	current->signal->flags |= SIGNAL_UNKILLABLE;



	if (ramdisk_execute_command) {

		run_init_process(ramdisk_execute_command);

		printk(KERN_WARNING "Failed to execute %s\n",

kernel/signal.c

+6 −3
Original line number Diff line number Diff line
@@ -728,7 +728,8 @@ static void complete_signal(int sig, struct task_struct *p, int group) 
	 * Found a killable thread.  If the signal will be fatal,

	 * then start taking the whole group down immediately.

	 */

	if (sig_fatal(p, sig) && !(signal->flags & SIGNAL_GROUP_EXIT) &&

	if (sig_fatal(p, sig) &&

	    !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&

	    !sigismember(&t->real_blocked, sig) &&

	    (sig == SIGKILL || !(t->ptrace & PT_PTRACED))) {

		/*
@@ -1615,7 +1616,8 @@ static int do_signal_stop(int signr) 
	} else {

		struct task_struct *t;



		if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED) ||

		if (unlikely((sig->flags & (SIGNAL_STOP_DEQUEUED | SIGNAL_UNKILLABLE))

					 != SIGNAL_STOP_DEQUEUED) ||

		    unlikely(signal_group_exit(sig)))

			return 0;

		/*
@@ -1761,7 +1763,8 @@ relock: 
		/*

		 * Global init gets no signals it doesn't want.

		 */

		if (is_global_init(current))

		if (unlikely(signal->flags & SIGNAL_UNKILLABLE) &&

		    !signal_group_exit(signal))

			continue;



		if (sig_kernel_stop(signr)) {