signals: fix /sbin/init protection from unwanted signals

PATCH New Oleg Nesterov 1 participant 1 message Mar 23, 2008 Scheduler

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Oleg Nesterov

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2008-03-23 13:02 UTC #0 lore raw

PATCH 1/3 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 Oleg Nesterov <[email protected]>

Acked-by "Eric W. Biederman" <[email protected]>

 include/linux/sched.h |    2 ++
 init/main.c           |    2 ++
 kernel/signal.c       |    9 ++++++---
 3 files changed, 10 insertions(+), 3 deletions(-)

include/linux/sched.h renamed from include/linux/sched.h~1_INIT_SIGS +2 -0

@@ -562,6 +562,8 @@ 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 renamed from init/main.c~1_INIT_SIGS +2 -0

@@ -787,6 +787,8 @@ 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 renamed from kernel/signal.c~1_INIT_SIGS +6 -3

@@ -713,7 +713,8 @@ static void complete_signal(int sig, str
 	 * 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))) {
 		/*

@@ -1600,7 +1601,8 @@ 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;
 		/*

@@ -1729,7 +1731,8 @@ 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)) {