Re: RT scheduling and a way to make a process hang, unkillable

[Dhaval Giani]

On Mon, Feb 16, 2009 at 12:08:36PM +0100, Peter Zijlstra wrote:
> On Mon, 2009-02-16 at 16:06 +0530, Dhaval Giani wrote:
> 
> > sched: Don't allow setuid to succeed if the user does not have rt bandwidth
> > 
> > Corey Hickey reported that on using setuid to change the uid of a
> > rt process, the process would be unkillable and not be running.
> > This is because there was no rt runtime for that user group. Add
> > in a check to see if a user can attach an rt task to its task group.
> > 
> > Disclaimer: Not sure about the return values, and if setuid allows
> > return values other than EPERM and EAGAIN.
> > 
> > Not-Yet-Signed-off-by: Dhaval Giani <dhaval@xxxxxxxxxxxxxxxxxx>
> > 
> > Index: linux-2.6/include/linux/sched.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/sched.h
> > +++ linux-2.6/include/linux/sched.h
> > @@ -2320,9 +2320,12 @@ extern long sched_group_rt_runtime(struc
> >  extern int sched_group_set_rt_period(struct task_group *tg,
> >  				      long rt_period_us);
> >  extern long sched_group_rt_period(struct task_group *tg);
> > +int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk);
> >  #endif
> >  #endif
> >  
> > +int rt_task_can_switch_user(uid_t uid, struct task_struct *tsk);
> > +
> >  #ifdef CONFIG_TASK_XACCT
> >  static inline void add_rchar(struct task_struct *tsk, ssize_t amt)
> >  {
> > Index: linux-2.6/kernel/sched.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/sched.c
> > +++ linux-2.6/kernel/sched.c
> > @@ -9466,6 +9466,16 @@ static int sched_rt_global_constraints(v
> >  
> >  	return ret;
> >  }
> > +
> > +int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk)
> > +{
> > +	/* Don't accept realtime tasks when there is no way for them to run */
> > +	if (rt_task(tsk) && tg->rt_bandwidth.rt_runtime == 0)
> > +		return -EINVAL;
> > +
> > +	return 0;
> > +}
> > +
> >  #else /* !CONFIG_RT_GROUP_SCHED */
> >  static int sched_rt_global_constraints(void)
> >  {
> > @@ -9559,8 +9569,7 @@ cpu_cgroup_can_attach(struct cgroup_subs
> >  		      struct task_struct *tsk)
> >  {
> >  #ifdef CONFIG_RT_GROUP_SCHED
> > -	/* Don't accept realtime tasks when there is no way for them to run */
> > -	if (rt_task(tsk) && cgroup_tg(cgrp)->rt_bandwidth.rt_runtime == 0)
> > +	if (sched_rt_can_attach(cgroup_tg(cgrp), tsk))
> >  		return -EINVAL;
> >  #else
> >  	/* We don't support RT-tasks being in separate groups */
> > Index: linux-2.6/kernel/user.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/user.c
> > +++ linux-2.6/kernel/user.c
> > @@ -216,8 +216,28 @@ static ssize_t cpu_rt_period_store(struc
> >  
> >  static struct kobj_attribute cpu_rt_period_attr =
> >  	__ATTR(cpu_rt_period, 0644, cpu_rt_period_show, cpu_rt_period_store);
> > +
> >  #endif
> >  
> > +#ifdef CONFIG_RT_GROUP_SCHED && CONFIG_USER_SCHED
> > +/*
> > + * We need to check if a setuid can take place. This function should be called
> > + * before successfully completing the setuid.
> > + */
> > +
> > +int rt_task_can_switch_user(uid_t uid, struct task_struct *tsk)
> > +{
> > +	struct user_struct *up = find_user(uid);
> > +
> > +	return sched_rt_can_attach(up->tg, tsk);
> > +}
> 
> Should that not be !sched_rt_can_attach(), or preferably change
> sched_rt_can_attach() to return a boolean, like the name implies.
> 

Yeah, made it boolean. how does the following look?

--
sched: Don't allow setuid to succeed if the user does not have rt bandwidth

Corey Hickey reported that on using setuid to change the uid of a
rt process, the process would be unkillable and not be running.
This is because there was no rt runtime for that user group. Add
in a check to see if a user can attach an rt task to its task group.

Disclaimer: Not sure about the return values, and if setuid allows
return values other than EPERM and EAGAIN.

Changes from v1:
1. Peter suggested that rt_task_can_change_user should be renamed to
task_can_change_user
2. Changed sched_rt_can_attach to boolean.

Signed-off-by: Dhaval Giani <dhaval@xxxxxxxxxxxxxxxxxx>


Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -2320,9 +2320,12 @@ extern long sched_group_rt_runtime(struc
 extern int sched_group_set_rt_period(struct task_group *tg,
 				      long rt_period_us);
 extern long sched_group_rt_period(struct task_group *tg);
+int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk);
 #endif
 #endif
 
+int task_can_switch_user(uid_t uid, struct task_struct *tsk);
+
 #ifdef CONFIG_TASK_XACCT
 static inline void add_rchar(struct task_struct *tsk, ssize_t amt)
 {
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -9466,6 +9466,16 @@ static int sched_rt_global_constraints(v
 
 	return ret;
 }
+
+int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk)
+{
+	/* Don't accept realtime tasks when there is no way for them to run */
+	if (rt_task(tsk) && tg->rt_bandwidth.rt_runtime == 0)
+		return 0;
+
+	return 1;
+}
+
 #else /* !CONFIG_RT_GROUP_SCHED */
 static int sched_rt_global_constraints(void)
 {
@@ -9559,8 +9569,7 @@ cpu_cgroup_can_attach(struct cgroup_subs
 		      struct task_struct *tsk)
 {
 #ifdef CONFIG_RT_GROUP_SCHED
-	/* Don't accept realtime tasks when there is no way for them to run */
-	if (rt_task(tsk) && cgroup_tg(cgrp)->rt_bandwidth.rt_runtime == 0)
+	if (!sched_rt_can_attach(cgroup_tg(cgrp), tsk))
 		return -EINVAL;
 #else
 	/* We don't support RT-tasks being in separate groups */
Index: linux-2.6/kernel/user.c
===================================================================
--- linux-2.6.orig/kernel/user.c
+++ linux-2.6/kernel/user.c
@@ -216,8 +216,28 @@ static ssize_t cpu_rt_period_store(struc
 
 static struct kobj_attribute cpu_rt_period_attr =
 	__ATTR(cpu_rt_period, 0644, cpu_rt_period_show, cpu_rt_period_store);
+
 #endif
 
+#ifdef CONFIG_RT_GROUP_SCHED && CONFIG_USER_SCHED
+/*
+ * We need to check if a setuid can take place. This function should be called
+ * before successfully completing the setuid.
+ */
+
+int task_can_switch_user(uid_t uid, struct task_struct *tsk)
+{
+	struct user_struct *up = find_user(uid);
+
+	return sched_rt_can_attach(up->tg, tsk);
+}
+#else
+int task_can_switch_user(uid_t uid, struct task_struct *tsk)
+{
+	return 1;
+}
+
+#endif
 /* default attributes per uid directory */
 static struct attribute *uids_attributes[] = {
 #ifdef CONFIG_FAIR_GROUP_SCHED
Index: linux-2.6/kernel/sys.c
===================================================================
--- linux-2.6.orig/kernel/sys.c
+++ linux-2.6/kernel/sys.c
@@ -579,6 +579,9 @@ static int set_user(struct cred *new)
 		return -EAGAIN;
 	}
 
+	if (!task_can_switch_user(new->uid, current))
+		return -EAGAIN;
+
 	free_uid(new->user);
 	new->user = new_user;
 	return 0;
-- 
regards,
Dhaval
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