[PATCH] Return EPERM not ECHILD on security_task_wait failure

[Roland McGrath <roland@redhat.com>]

wait* syscalls return -ECHILD even when an individual PID of a live
child was requested explicitly, when security_task_wait denies the
operation.  This means that something like a broken SELinux policy
can produce an unexpected failure that looks just like a bug with
wait or ptrace or something.

This patch makes do_wait return -EPERM instead of -ECHILD if some
children were ruled out solely because security_task_wait failed.

Signed-off-by: Roland McGrath <roland@redhat.com>
---
 kernel/exit.c |   12 +++++++++++-
 1 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index f132349..a41052f 100644  
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1067,7 +1067,7 @@ static int eligible_child(pid_t pid, int
 		return 2;
 
 	if (security_task_wait(p))
-		return 0;
+		return -1;
 
 	return 1;
 }
@@ -1449,6 +1449,7 @@ static long do_wait(pid_t pid, int optio
 	DECLARE_WAITQUEUE(wait, current);
 	struct task_struct *tsk;
 	int flag, retval;
+	int allowed, denied;
 
 	add_wait_queue(&current->signal->wait_chldexit,&wait);
 repeat:
@@ -1457,6 +1458,7 @@ repeat:
 	 * match our criteria, even if we are not able to reap it yet.
 	 */
 	flag = 0;
+	allowed = denied = 0;
 	current->state = TASK_INTERRUPTIBLE;
 	read_lock(&tasklist_lock);
 	tsk = current;
@@ -1472,6 +1474,12 @@ repeat:
 			if (!ret)
 				continue;
 
+			if (unlikely(ret < 0)) {
+				denied = 1;
+				continue;
+			}
+			allowed = 1;
+
 			switch (p->state) {
 			case TASK_TRACED:
 				/*
@@ -1570,6 +1578,8 @@ check_continued:
 		goto repeat;
 	}
 	retval = -ECHILD;
+	if (unlikely(denied) && !allowed)
+		retval = -EPERM;
 end:
 	current->state = TASK_RUNNING;
 	remove_wait_queue(&current->signal->wait_chldexit,&wait);