Re: [local DoS] Re: Linux 2.6.24-rc4

[Ingo Molnar <mingo@elte.hu>]

* Luiz Fernando N. Capitulino <lcapitulino@mandriva.com.br> wrote:

> | The problem is on SMP: if sched_rr_get_interval() gets a task from 
> | an otherwise idle runqueue, then rq->load.weight is 0. Normally 
> | sched_slice() is only used on a busy runqueue. So the correct fixup 
> | site is not in sched_slice() but in sys_sched_rr_get_interval() - 
> | i'm working on the right fix, i hope to be able to send a pull 
> | request in a few minutes.
> 
>  Ingo, I can reproduce this w/o SMP support as well.

hm, if you run this as an RT task, right? Or can you trigger it via pure 
SCHED_OTHER tasks as well? Below is my candidate fix.

	Ingo

--------------->
Subject: sched: fix crash in sys_sched_rr_get_interval()
From: Ingo Molnar <mingo@elte.hu>

Luiz Fernando N. Capitulino reported that sched_rr_get_interval()
crashes for SCHED_OTHER tasks that are on an idle runqueue.

The fix is to return a 0 timeslice for tasks that are on an idle
runqueue. (and which are not running, obviously)

Reported-by: Luiz Fernando N. Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/sched.c |   14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -4850,17 +4850,21 @@ long sys_sched_rr_get_interval(pid_t pid
 	if (retval)
 		goto out_unlock;
 
-	if (p->policy == SCHED_FIFO)
-		time_slice = 0;
-	else if (p->policy == SCHED_RR)
+	/*
+	 * Time slice is 0 for SCHED_FIFO tasks and for SCHED_OTHER
+	 * tasks that are on an otherwise idle runqueue:
+	 */
+	time_slice = 0;
+	if (p->policy == SCHED_RR) {
 		time_slice = DEF_TIMESLICE;
-	else {
+	} else {
 		struct sched_entity *se = &p->se;
 		unsigned long flags;
 		struct rq *rq;
 
 		rq = task_rq_lock(p, &flags);
-		time_slice = NS_TO_JIFFIES(sched_slice(cfs_rq_of(se), se));
+		if (rq->cfs.load.weight)
+			time_slice = NS_TO_JIFFIES(sched_slice(&rq->cfs, se));
 		task_rq_unlock(rq, &flags);
 	}
 	read_unlock(&tasklist_lock);