linux-kernel - Re: [RESEND RFC PATCH 1/1] Selectively allow CAP_SYS

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Date:   Thu, 21 Nov 2019 15:27:49 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Prakash Sangappa <prakash.sangappa@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        tglx@...utronix.de, peterz@...radead.org, serge@...lyn.com
Subject: Re: [RESEND RFC PATCH 1/1] Selectively allow CAP_SYS_NICE capability inside user namespaces

Prakash Sangappa <prakash.sangappa@...cle.com> writes:

> Allow CAP_SYS_NICE to take effect for processes having effective uid of a
> root user from init namespace.
>
> Signed-off-by: Prakash Sangappa <prakash.sangappa@...cle.com>
> ---
>  kernel/sched/core.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7880f4f..628bd46 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4548,6 +4548,8 @@ int can_nice(const struct task_struct *p, const int nice)
>  	int nice_rlim = nice_to_rlimit(nice);
>  
>  	return (nice_rlim <= task_rlimit(p, RLIMIT_NICE) ||
> +		(ns_capable(__task_cred(p)->user_ns, CAP_SYS_NICE) &&
> +		uid_eq(current_euid(), GLOBAL_ROOT_UID)) ||
>  		capable(CAP_SYS_NICE));
>  }
>  
> @@ -4784,7 +4786,9 @@ static int __sched_setscheduler(struct task_struct *p,
>  	/*
>  	 * Allow unprivileged RT tasks to decrease priority:
>  	 */
> -	if (user && !capable(CAP_SYS_NICE)) {
> +	if (user && !(ns_capable(__task_cred(p)->user_ns, CAP_SYS_NICE) &&
> +		uid_eq(current_euid(), GLOBAL_ROOT_UID)) &&
> +		!capable(CAP_SYS_NICE)) {
>  		if (fair_policy(policy)) {
>  			if (attr->sched_nice < task_nice(p) &&
>  			    !can_nice(p, attr->sched_nice))

I remember looking at this before.  I don't remember if I commented.

1) Having GLOBAL_ROOT_UID in a user namespace is A Bad Idea™.
   Definitely not something we should make special case for.
   That configuration is almost certainly a privilege escalation waiting
   to happen.

2) If I read the other thread correctly there was talk about setting the
   nice levels of processes in other containers.  Ouch!

   The only thing I can think that makes any sense at all is to allow
   setting the nice levels of the processes in your own container.

   I can totally see having a test to see if a processes credentials are
   in the caller's user namespace or a child of caller's user namespace
   and allowing admin level access if the caller has the appropriate
   caps in their user namespace.

   But in this case I don't see anything preventing the admin in a
   container from using the ordinary nice levels on a task.  You are
   unlocking the nice levels reserved for the system administrator
   for special occassions.   I don't see how that makes any sense
   to do from inside a container.

The design goal of user namespaces (assuming a non-buggy kernel) is to
ensure user namespaces give a user no more privileges than the user had
before creating a user namespace.  In this case you are granting a user
who creates a user namespace the ability to change nice levels on all
process in the system (limited to users whose uid happens to be
GLOBAL_ROOT_UID).  But still this is effectively a way to get
CAP_SYS_NICE back if it was dropped.

As a violation of security policy this change simply can not be allowed.
The entire idiom:  "ns_capable(__task_cred(p)->user_ns, ...)" is a check
that provides no security.

Eric