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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2009-02-17 at 15:45 +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.

This looks good to me.

Does anybody object to the -ENOSPC return value? Should we introduce
-ENOTIME for that?

Michael, Alan?

> Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

Ingo, I'll send you the patch once the error issue is settled.

> 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 +2320,13 @@ 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);
> +extern int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk);
>  #endif
>  #endif
>  
> +extern int task_can_switch_user(struct user_struct *up,
> +					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
> @@ -362,6 +362,24 @@ static void free_user(struct user_struct
>  
>  #endif
>  
> +#if defined(CONFIG_RT_GROUP_SCHED) && defined(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(struct user_struct *up, struct task_struct *tsk)
> +{
> +
> +	return sched_rt_can_attach(up->tg, tsk);
> +
> +}
> +#else
> +int task_can_switch_user(struct user_struct *up, struct task_struct *tsk)
> +{
> +	return 1;
> +}
> +#endif
> +
>  /*
>   * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
>   * caller must undo that ref with free_uid().
> Index: linux-2.6/kernel/sys.c
> ===================================================================
> --- linux-2.6.orig/kernel/sys.c
> +++ linux-2.6/kernel/sys.c
> @@ -560,7 +560,7 @@ error:
>  	abort_creds(new);
>  	return retval;
>  }
> -  
> +
>  /*
>   * change the user struct in a credentials set to match the new UID
>   */
> @@ -572,6 +572,11 @@ static int set_user(struct cred *new)
>  	if (!new_user)
>  		return -EAGAIN;
>  
> +	if (!task_can_switch_user(new_user, current)) {
> +		free_uid(new_user);
> +		return -ENOSPC;
> +	}
> +
>  	if (atomic_read(&new_user->processes) >=
>  				current->signal->rlim[RLIMIT_NPROC].rlim_cur &&
>  			new_user != INIT_USER) {
> @@ -632,10 +637,11 @@ SYSCALL_DEFINE2(setreuid, uid_t, ruid, u
>  			goto error;
>  	}
>  
> -	retval = -EAGAIN;
> -	if (new->uid != old->uid && set_user(new) < 0)
> -		goto error;
> -
> +	if (new->uid != old->uid) {
> +		retval = set_user(new);
> +		if (retval < 0)
> +			goto error;
> +	}
>  	if (ruid != (uid_t) -1 ||
>  	    (euid != (uid_t) -1 && euid != old->uid))
>  		new->suid = new->euid;
> @@ -681,9 +687,10 @@ SYSCALL_DEFINE1(setuid, uid_t, uid)
>  	retval = -EPERM;
>  	if (capable(CAP_SETUID)) {
>  		new->suid = new->uid = uid;
> -		if (uid != old->uid && set_user(new) < 0) {
> -			retval = -EAGAIN;
> -			goto error;
> +		if (uid != old->uid) {
> +			retval = set_user(new);
> +			if (retval < 0)
> +				goto error;
>  		}
>  	} else if (uid != old->uid && uid != new->suid) {
>  		goto error;
> @@ -735,11 +742,13 @@ SYSCALL_DEFINE3(setresuid, uid_t, ruid, 
>  			goto error;
>  	}
>  
> -	retval = -EAGAIN;
>  	if (ruid != (uid_t) -1) {
>  		new->uid = ruid;
> -		if (ruid != old->uid && set_user(new) < 0)
> -			goto error;
> +		if (ruid != old->uid) {
> +			retval = set_user(new);
> +			if (retval < 0)
> +				goto error;
> +		}
>  	}
>  	if (euid != (uid_t) -1)
>  		new->euid = euid;